Identity

Nomenclature

The genus Vertebrata was first established by Samuel Frederick Gray (1821). After the publication of Greville (1824), the genus was abandoned but reestablished by Christensen (1967) following a suggestion by Kylin (1956). Vertebrata lanosa (Linnaeus) T.A. Christensen was selected as the lectotype of the genus.

Nomenclatural synonyms of V. lanosa and V. fucoides

Of all known representatives of the genus Vertebrata, V. lanosa and V. fucoides are currently the most thoroughly studied and our review mostly focuses on these two species. Here we provide a brief history of their scientific names and a list of synonyms.

Vertebrata lanosa was first described by Linnaeus as Fucus lanosus (Linnaeus 1767). Most probably, it was called Fucus due to its ecological preferences, as this alga is an obligate epiphyte of fucoid algae. Later, the same species was described as Conferva omissa in Norway and as Ceramium fastigiatum in Germany (Gunnerus 1772; Roth 1800). A series of revisions followed, and for a long time the alga was mostly referred to either as Polysiphonia fastigiata or as Polysiphonia lanosa, based on the initial species epithet, “lanosus” (Greville 1824; Tandy 1931). The currently accepted name, V. lanosa, appeared after the reestablishment of Vertebrata in 1967 (Christensen 1967), though it has been actively used in the scientific literature only during the two last decades.

The history of V. fucoides is even more complicated than that of V. lanosa. This alga was described in 1762–1812 on the coasts of England, Germany, and Sweden under four different names: Conferva fucoides, Conferva nigrescens, Conferva atrorubens, and Ceramium violaceum (Hudson 1762, 1778; Roth 1797; Wahlenberg 1812). In 1817, C. violaceum and C. nigrescens were combined and transferred to the genus Hutchinsia as H. violacea (Agardh 1817). Later, Hutchinsia was disbanded because of the existence of the same name among the higher plants. The corresponding angiosperm genus Hutchinsia is currently named Hornungia Reichenbach. (Brassicaceae) (WFO 2022). All the Vertebrata-related species were combined and included into the new genus Polysiphonia, as P. fucoides or P. nigrescens with several subspecies (Greville 1824; Agardh 1863). The alga was mentioned in the scientific literature under these two names for a long time, until it was finally transferred to Vertebrata as V. fucoides (Hudson) Kuntze 1891 (Díaz-Tapia et al. 2017a).

The following synonyms are currently listed in AlgaeBase for V. lanosa and V. fucoides (Guiry and Guiry 2021):

Homotypic synonyms of V. lanosa:

Fucus lanosus Linnaeus 1767

Polysiphonia lanosa (Linnaeus) Tandy 1931

Heterotypic synonyms of V. lanosa:

Conferva omissa Gunnerus 1772

Ceramium fastigiatum Roth 1800

Hutchinsia fastigiata (Roth) C.Agardh 1817

Vertebrata fastigiata S.F.Gray 1821

Polysiphonia fastigiata (Roth) Greville 1824

Homotypic synonyms of V. fucoides:

Conferva fucoides Hudson 1762

Polysiphonia fucoides (Hudson) Greville 1824

Polysiphonia nigrescens var. fucoides (Hudson) Harvey 1853

Polysiphonia urceolata f. fucoides (Hudson) J.Agardh 1863

Heterotypic synonyms of V. fucoides:

Conferva nigrescens Hudson 1778

Ceramium violaceum Roth 1797

Conferva atrorubens Wahlenberg 1812

Hutchinsia violacea var. nigrescens (Hudson) C.Agardh 1817

Hutchinsia violacea (Roth) C.Agardh 1817

Hutchinsia nigrescens (Hudson) Lyngbye 1819

Hutchinsia nigrescens var. pectinata C.Agardh 1824

Ceramium violaceum var. nigrescens (Hudson) Wahlenberg 1826

Polysiphonia violacea (Roth) Sprengel 1827

Polysiphonia nigrescens (Hudson) Greville ex Harvey 1833

Polysiphonia affinis Moore 1837

Polysiphonia atropurpurea Moore 1837

Polysiphonia senticosa Suhr ex Kützing 1849

Common names

To our knowledge, only V. lanosa has common names, of which the most widely used is “wrack siphon weed” (Bunker et al. 2017; Mathieson and Dawes 2017). Due to its truffle-like taste and odor, this alga is also called “sea truffle” or “ocean truffle” (Bjordal et al. 2020).

Taxonomy

The evolutionary origin of Vertebrata is estimated as 91–139 million years ago (Díaz-Tapia et al. 2017a). According to current taxonomy (Guiry and Guiry 2021) this genus is placed in the family Rhodomelaceae, order Ceramiales, class Florideophyceae, and phylum Rhodophyta (red algae). After the reestablishment of the genus in 1967 (Christensen 1967), a series of phylogenetic studies, considering both anatomical and molecular data, was carried out to clarify the list of Vertebrata species (Choi et al. 2001; Díaz-Tapia et al. 2017a, b; Savoie and Saunders 2019). As a result, the genus Vertebrata was substantially expanded and acknowledged to be highly supported in phylogenetic analyses. Now it comprises 30 species (Guiry and Guiry 2021). Though these species are morphologically variable, they are united by having more than six pericentral cells and multinucleate trichoblast cells, as well as by rbcL and 18S DNA sequences (Díaz-Tapia et al. 2017a; Savoie and Saunders 2019). The comprehensive phylogenetic analysis of the family Rhodomelaceae by Díaz-Tapia et al. (2017b) has led to the segregation of the tribe Streblocladieae from the tribe Polysiphonieae for Vertebrata and seven other species (Savoie and Saunders 2019).

Morphology, anatomy, and cytology

The genus Vertebrata has filamentous segmented, i.e., polysiphonous, thalli up to 30 cm high. Abundant pseudodichotomous branching leads to formation of rather dense tufts, which can be almost spherical in V. lanosa to more elongated in V. fucoides. The main axes and branches of V. fucoides bear relatively long branched trichoblasts. Plants attach to the substratum by rhizoids, and different parts of the thallus can be erect, decumbent, or prostrate. The color of these algae varies from purple to brownish red or black in V. lanosa and to dark brown or black in V. fucoides (Zinova 1955; Kim et al. 2002; Díaz-Tapia et al. 2017a).

Thalli of Vertebrata are radially symmetrical; their axial elements (120-500 μm in diameter) are composed of multiple stacked segments having a central multinucleate (up to 32 nuclei in V. lanosa) cell surrounded by 6-24 pericentral cells (Fig. 1). Each of these elongated cells contains a large central vacuole surrounded by a thin peripheral layer of cytoplasm. Typically, there is a linear relationship between the size of pericentral cells and the number of nuclei per cell. All cells in this polysiphonous system are interlinked by primary or secondary pit connections (Goff and Coleman 1986). In V. fucoides the base of the main axis and principal branches can be covered with corticating cells (Zinova 1955), whereas in some other species (including V. lanosa) the cortication is absent (Kim et al. 2002; Díaz-Tapia et al. 2017a). Trichoblasts are uniseriate (Fig. 1). All Vertebrata species are multinuclear in the proximal trichoblast cells, which may contain more than eight uniformly distributed nuclei. Closer to trichoblast apices the number of nuclei per cell decreases, so that apical cells can be uninucleate (Díaz-Tapia et al. 2017a).

Fig. 1
figure 1

Scheme of the vegetative structures of Vertebrata thallus (on the example of V. fucoides). AC, apical cell; AS, axial segment; CC, central cell; N, nucleus; PC, pericentral cell; T, trichoblast; V, vacuole

Full size image

Tetrasporophytic plants may bear tetrasporangia and produce tetraspores (Fig. 2). Tetrasporangia are formed in the pericentral cells of the axial elements of the last two orders of branching. The fertile branchlets typically have fewer pericentral cells (8 – in V. lanosa), compared to the sterile axes. In each segment of the branchlet only one pericentral cell bears tetrasporangium, and tetrasporangia of several neighboring segments may be straight or spirally (more frequently) arranged. Tetrasporangia of V. lanosa are 100–120 μm in diameter and spherical, whereas those of V. fucoides are smaller (52–78 μm in diameter) and ovate. Each mature tetrasporangium contains 4 tetraspores resulting from a tetrahedral division (Fritsch 1945; Zinova 1955; Kim et al. 2002; Díaz-Tapia and Bárbara 2013).

Fig. 2
figure 2

Scheme of the reproductive structures and life history of Vertebrata (on the example of V. fucoides). CB, carpogonial branch; CC, cystocarp; CS, carpospore; CSR, carposporangium; FG, female gametophyte; MG, male gametophyte; P, procarp; S, spermatium; SS, spermatangium sorus; T, tetrasporophyte; TR, trichogyne; TS, tetraspore; TSR, tetrasporangium

Full size image

Male gametophytes form multiple ovate spermatangia at the tips of polysiphonous branches (V. lanosa) or at the trichoblasts (V. fucoides). Spermatangia are gathered into sori having a spikelet-like shape, 150–340 μm long and 30–80 μm in diameter (Fig. 2). In some Vertebrata species (e.g., V. fucoides) the apical cells of the sori are sterile. Each mature spermatangium produces a single spermatium, pushing out its protoplast through the rupture of the cell wall (Kugrens 1980; Kim et al. 2002; Díaz-Tapia and Bárbara 2013).

Female gametophytes of Vertebrata develop procarps at the base of trichoblasts (Zinova 1955). Each procarp contains a four-celled carpogonial branch (with the apical cell being a carpogonium), surrounded by several groups of sterile cells. The carpogonia are elongated and possess relatively long trichogynes constricted at the base. After fertilization ovate cystocarps are formed (Fig. 2). Their size varies among species: e.g., for V. lanosa it is 200–300 μm high and 100–200 μm in diameter, and for V. fucoides – 400–600 μm high and 320–500 μm in diameter. The mature cystocarps are covered by pericarp and contain multiple gonimoblast filaments bearing carposporangia with single carpospores (Fritsch 1945; Wetherbee 1980; Kim et al. 2002; Díaz-Tapia and Bárbara 2013). Newly released carpospores are clavate or spindle-shaped, but in 3.5–4 minutes they become spherical (~30 μm in diameter) (Boney 1967).

Thalli of Vertebrata grow by the divisions of dome-shaped apical cells. In both gametophyte and tetrasporophyte the apical cells are uninucleate, but highly polyploid (with DNA C-value up to 128). Measurements of DNA content in the different cells of several Polysiphonia and Vertebrata species showed that their growth and development of polysiphonous structure is accompanied by progressive reduction of DNA amount per nucleus, while the number of nuclei per cell increases (Fig. 1). Thus, the cells of the oldest segments contain maximal number of nuclei, all having a DNA quantity corresponding to the haploid (for gametophyte) or diploid (for tetrasporophyte) state (Goff and Coleman 1986). Biological reasons of such uncoupling of DNA synthesis and cell/nucleus division may include, first, acceleration of thallus growth due to cell elongation and then controlling of peripheral cytoplasm layer of resulting huge cells by several uniformly distributed nuclei.

The haploid set of chromosomes of V. lanosa is 26 and that of V. fucoides is 30 (Austin 1956, 1959). For Rhodophyta this is a relatively high number, presumably emerged due to polyploidization accompanied with some other chromosome rearrangements (Kapraun and Freshwater 2012). Both karyotypes contain nearly equal numbers of small (0.2–0.4 μm) and large (0.8–1.2 μm) chromosomes (Kapraun 1993). For the most studied Vertebrata species, V. lanosa, the plastid and mitochondrial genomes have been sequenced. The plastid genome of this alga, containing 167 kbp with 193 protein-coding genes, is among the smallest known in red algae, which is apparently a specific feature of the Rhodomelaceae (Salomaki et al. 2015; Verbruggen and Costa 2015). The mitochondrial genome of V. lanosa is similar to other published red algal mitochondrial genomes: it is ~25 kbp long and contains 23 protein-coding genes, 2 rRNAs and 19 tRNAs with 10.4% of the DNA being non-coding (Salomaki and Lane 2017).

Life history

Both V. lanosa and V. fucoides are perennial species having a triphasic life cycle, typical of the Florideophyceae, with two diploid spore-producing generations and a haploid gamete-producing generation (Fig. 2). Haploid male and female gametophytes produce spermatia and carpogonia, respectively. The released spermatia, carried passively by water motion, reach carpogonia, which are anchored on the female gametophytes. Fertilization gives rise to the first diploid generation, the carposporophyte. The carposporophytes proliferate on the female gametophyte, form the cystocarps with carposporangia, and finally produce diploid carpospores. The released spores attach to a substratum, germinate, and form the second diploid generation, the tetrasporophyte. The tetrasporophyte plants produce haploid tetraspores, which germinate, establishing the gametophyte generation. The vegetative structures of the tetrasporophytic and gametophytic thalli are morphologically indistinguishable (Goff and Coleman 1986; Lee 2008). Polyploidy is presumed to be established in both carpospores and tetraspores soon after their attachment to a substratum (Goff and Coleman 1986).

The reproductive biology of V. lanosa was studied in detail for several populations inhabiting the Atlantic coast of Canada (Garbary et al. 1991; Kaczmarska and Dowe 1997). It was shown that reproduction of this alga exhibits strong seasonality. First, in spring, the male and female gametophytes mature synchronously, so that fertilization usually occurs in May. The spermatia to carpogonia ratio is relatively high, on average 4000 to 1, thus providing efficient fertilization (up to 90% success). The first young cystocarps appear in June, and the release of carpospores lasts from July until October. The tetrasporophytes form sporangia and produce tetraspores from late May–early June until October. All free-living life-history phases (females, males and tetrasporophytes) are present in almost equal abundance during the growing season, and most mature plants maintain reproductive activity (Kaczmarska and Dowe 1997). To the south of the Canadian coast, on the shores of New Hampshire V. lanosa and V. fucoides are reproductive throughout the year (Hehre and Mathieson 1970).

Distribution and ecology

Ocean Biodiversity Information System (OBIS, https://obis.org), World Register of Marine Species (WoRMS, https://www.marinespecies.org), and AlgaeBase (https://www.algaebase.org) online resources were used to obtain the basic information about the geographical distribution of Vertebrata species (Fig. 3).

Fig. 3
figure 3

Distribution of Vertebrata lanosa and V. fucoides (based on the online resources: https://www.algaebase.org, https://www.marinespecies.org, https://obis.org). For the findings of V. lanosa in the Red Sea, the Persian Gulf, and along the Indian coast, see comments in the text

Full size image

Vertebrata lanosa and V. fucoides are the North Atlantic species and both are ubiquitous in this region (Fig. 3). Besides Atlantic coasts, Vertebrata species also grow abundantly on the Pacific coast of Canada and USA, in the Canadian Arctic (the Baffin and Beaufort Seas), and in the Norwegian and Russian Arctic (the Barents and White Seas). Data on the findings of V. fucoides to the east of the Barents Sea are ambiguous: though WoRMS indicates this species as growing in the Kara and Laptev Seas, it is contradicted by the study of Vinogradova (2011). Meanwhile this species is well represented throughout the White and Barents Seas (Garbary and Tarakhovskaya 2013; Mikhaylova 2021).

Apart from the North Atlantic and Arctic seas, according to OBIS and AlgaeBase, the populations of V. lanosa were also found in the Red Sea (https://obis.org/occurrence/b686d439-cd81-49f7-bc32-27cb1e68c9bc; accessed 12.05.2022), the Persian Gulf (https://obis.org/occurrence/239c5997-135b-4fb5-b21a-45762486244a; accessed 12.05.2022), along the western coast of the Indian Peninsula (https://obis.org/occurrence/80f39f9f-285f-4d4a-ae4f-837368df4941; accessed 12.05.2022), around Java and Sumatra, and in the South China Sea (Atmadja and Prud'homme van Reine 2010; Belous et al. 2021). Vertebrata fucoides occurs near Indonesia and the Philippines, spreading to the north-east up to the East China Sea (Phang et al. 2016). This alga also has been found in the Indian Ocean, near Madagascar, and along the Atlantic coasts of Africa and South America (Coll and Oliveira 1999) (Fig. 3). Apparently, the discontinuous pattern of distribution of both Vertebrata species may reflect lack of detailed studies, as most of the comprehensive investigations concern the North Atlantic and Arctic coasts. All the other data, in particular, the singular findings of V. lanosa in the Red Sea, the Persian Gulf, and along the Indian coast need confirmation using molecular approach. Further investigations are required in this field.

Vertebrata species occupy various marine habitats: they can grow as free-living plants on stones, rocks, shells, and different artificial substrata, or colonize other algae either as obligate epiphytes or as parasites. Some species occur mostly in the intertidal zone (V. lanosa), while the others prefer subtidal habitats (V. constricta).

Vertebrata fucoides is a cosmopolitan species growing within a broad range of temperature (0–28 °C), salinity (6.0–32.5‰), and vertical distribution. In different locations it may occur in the low-intertidal zone, in rock pools, in shallow or deep (15–17 m) subtidal water along open, wave-exposed coasts and also in sheltered habitats (Hehre and Mathieson 1970; Fralick and Mathieson 1975). The photosynthetic rate of V. fucoides remains stable under varying light intensities, thus maintaining the ability of this alga to survive in the habitats with different light availability (King and Schramm 1976). The saturation irradiance (140-200 μmol photons m-2 s-1) is relatively high compared to other ceramialean algae (Kain and Norton 1990). This species can be a lithophyte, but also frequently grows epiphytically on the thalli of different brown and red algae, e.g., fucoids, Saccharina latissima, and Ceramium virgatum (Zinova 1955; Fralick and Mathieson 1975; Adams 1991; the authors’ personal observations). Long-term experiments with varying sediment loads showed that V. fucoides is among the species most tolerant to deposited sediment accumulation (Eriksson and Johansson 2005).

Compared to V. fucoides, V. lanosa is more resistant to desiccation, but has a narrower range of temperature and salinity tolerance, as well as vertical distribution (Kain and Norton 1990). This is largely attributed to its growing almost exclusively as an epiphyte on the thalli of the brown alga Ascophyllum nodosum (much rarer, Fucus vesiculosus) (Fig. 4). Thus, the distribution of V. lanosa is restricted not only to the abiotic factors, but also to the range of the host macrophyte. V. lanosa reaches maximum biomass when growing on the open highly wave-exposed coast in relatively cold (~10 °C) water with full salinity (26–32.5‰) (Fralick and Mathieson 1975; Cardinal and Lesage 1992; Bjordal et al. 2020). As a cold water and stenohaline species, V. lanosa may be more sensitive to the environmental changes, including the anthropogenic impact (thermal discharges from power plants, dredging activity etc.), compared to V. fucoides (Garbary et al. 1991).

Fig. 4
figure 4

Abundant growth of Vertebrata lanosa on the thalli of Ascophyllum nodosum (photo by E. Tarakhovskaya). Scale 10 cm

Full size image

The details of the interactions between A. nodosum and V. lanosa have been extensively studied (e.g., Fritsch 1945; Rawlence and Taylor 1970; Pearson and Evans 1990; Garbary et al. 2005, 2014; Longtin and Scrosati 2009). Some features of this long-term and intimate association imply that V. lanosa may be regarded as a parasite. Distribution of V. lanosa on thalli of Ascophyllum is not even: the epiphyte grows most abundantly on the apical and median parts of the host plant (Cardinal and Lesage 1992). Vertebrata starts releasing carpospores shortly after Ascophyllum receptacle shedding, and many spores settle and attach to the abscission sites (Garbary et al. 1991). Apparently, some chemical signaling is involved in the attachment regulation, as it was shown that fucoid exudates promoted the rhizoid growth of V. lanosa (Pearson and Evans 1991). During attachment, rhizoids penetrate deep through the epidermal layer of the Ascophyllum thallus, into the cortex and medulla tissues. Thus, the epiphyte avoids removal drawn by regular shedding of Ascophyllum epidermal cell wall (Rawlence 1972; Halat et al. 2015). Ascophyllum cells can be damaged considerably by rhizoid penetration (Rawlence and Taylor 1970). It also has been reported that phlorotannins accumulate in the host cells adjacent to the rhizoid: a reaction typical for the wounded tissues of brown algae (Garbary et al. 2005; Lemesheva and Tarakhovskaya 2018). It is still unclear to which extent the metabolism of Vertebrata is integrated with that of its host macrophyte. In culture conditions, sporelings and apical fragments of V. lanosa survive longer in presence of fragments of fucoid thalli thus implying that fucoid exudates contribute to physiological performance of Vertebrata (Lining and Garbary 1992). Obviously, V. lanosa is photosynthetically competent. It was shown that this alga has higher rates of both photosynthesis and respiration compared to V. fucoides inhabiting similar sites (Fralick and Mathieson 1975). This enhanced metabolic activity may be attributed to a benefit of growing in association, as some photosynthetic parameters (electron transport rate and pigment content) of V. lanosa decline when it is not attached to the host macrophyte (Garbary et al. 2014). Meanwhile, multiple studies of putative photosynthate and mineral ion exchange between V. lanosa and Ascophyllum have given ambiguous results. The translocation is apparently carried out in both directions, and its reported rates vary considerably, in some cases being no higher than can be accounted for by simple diffusion (Harlin and Craigie 1975; Penot et al. 1993; Ciciotte and Thomas 1997). Notably, V. lanosa is capable of translocating photosynthates within its own thallus, which means that it might effectively distribute the substances derived from Ascophyllum (Turner and Evans 1978). Garbary et al. (2005) suggested that an obligate fungal endosymbiont of Ascophyllum, Mycophycias ascophylli, may contribute to establishment of the Vertebrata – brown macrophyte association, in particular to the restriction of the growth of Vertebrata rhizoids and metabolite exchange. Further studies are needed to reveal the physiological and biochemical factors contributing to host specificity of V. lanosa.

Living as a hemi-parasite of Ascophyllum, V. lanosa, in turn, hosts the red parasitic alga Choreocolax polysiphoniae, also belonging to the Rhodomelaceae. This parasite forms multicellular unpigmented or slightly pigmented cushion-like structures on thalli of V. lanosa (Salomaki et al. 2015; Karcz and Garbary 2021). It was shown that even when Choreocolax cells possess plastids, the photosynthetic activity is suppressed, and the parasite receives photosynthates (in the form of digeneaside) from V. lanosa (Callow et al. 1979).

Parasitic organisms, infecting closely related species, are frequent among red algae (Goff et al. 1996). Different Vertebrata species can also form such parasite-host systems. One of these systems with V. aterrima, as a host, and V. aterrimophila, as a parasite, has been described in detail recently (Preuss and Zuccarello 2019). It was shown that the parasite grows superficially, never penetrating deep into the host tissues. The spores and vegetative cells of V. aterrimophila can form secondary pit connections with pericentral host cells, and the infection leads to the host cell transformation (increase of the nuclear size, carbohydrate accumulation etc.). Though thalli of the parasite species may be pigmented and exhibit faint autofluorescence, increase of the carbohydrate amount in the adjacent host cells implies that nutrient transport from V. aterrima to V. aterrimophila may take place (Preuss and Zuccarello 2019).

Chemical composition

Mineral composition

Water content of V. lanosa thalli is about 83% FW, similar to tissues of other Ceramiales (77–87% FW; Yanshin et al. 2021). Both V. lanosa and V. fucoides have a relatively high ash content and a well-balanced macronutrient composition (Table 1). Compared to the other red algae, V. lanosa contains high amount of potassium (47 μg g-1 DW vs. 0.5–20 μg g-1 DW in the other studied species; Ito and Hori 1989; Reed 1990; Frikha et al. 2011; Duinker et al. 2020; Yanshin et al. 2021). An active K+ influx mechanism was suggested for this alga (Reed 1990). With respect to microelements, V. lanosa is especially rich in zinc and selenium (81–185 and 0.53-0.83 μg g-1 DW, respectively), compared to other red, brown, and green macrophytes (Stengel et al. 2004; Mæhre et al. 2014; Kleiven et al. 2019; Duinker et al. 2020). Perennial red algae with filamentous thalli, including Vertebrata species, typically have relatively high content of metals (Zn, Cu, Mn etc.) (Stengel et al. 2004; Ryan et al. 2012; Duinker et al. 2020). This is explained by their enhanced nutrient uptake rates due to the high surface area to volume ratios, and by the presence of the sulfated polysaccharides in their cell walls, as sulfate groups provide binding sites for the metal ions (Wallentinus 1984; Stengel et al. 2004).

Table 1 Content of water, ash, macroelements, and trace elements in Vertebrata species
Full size table

Protein and free amino acids

Thalli of V. lanosa and V. fucoides contain relatively high amounts of protein (115.6 and 182.5 mg g-1 DW, respectively) (Table 2). Among marine macrophytes, red algae are known to have the highest protein content. The most protein-rich species are Porphyra tenera, Palmaria palmata, and different ceramialean algae, including Vertebrata and Polysiphonia species (Ramos et al. 2000; Dere et al. 2003; Mæhre et al. 2014; Ignatova and Podkorytova 2021; Yanshin et al. 2021). The major amino acids in the V. lanosa protein are glutamic acid (16.3 mg g-1 DW), lysine (12.6 mg g-1 DW), and aspartic acid (12.3 mg g-1 DW) (Mæhre et al. 2014). A relatively high amount of lysine in this alga is of especial interest, as this amino acid is often limiting in the algal protein (Černá 2011). Vertebrata protein has a high value (0.87) of the chemical amino acid score – a parameter, characterizing the relative content of essential amino acids in the food protein, compared to the standard protein ovalbumin. Multiple literature data imply that the protein of the red algae has more balanced amino acid composition than that of the brown and green macrophytes (reviewed in: Fleurence 1999; Pangestuti and Kim 2015).

Table 2 Biochemical composition of Vertebrata species
Full size table

Dominating free amino acids in V. lanosa are proline, phenylalanine, and glutamic acid. Also, this species contains a considerable amounts of glycine betaine and biologically active sulphonic acid, taurine (Reed 1990; Mæhre et al. 2014; Valverde et al. 2015). Relatively high contents of phenylalanine, proline, and taurine were previously reported from other ceramialean species (Ramos et al. 2000; Yanshin et al. 2021).

Carbohydrates

The total carbohydrate content in Vertebrata species (Table 2) is relatively high, compared to the data reported for the other ceramialean algae (32.33–44.38% DW) (Yanshin et al. 2021). The amount of soluble fiber in V. fucoides is within the range of the corresponding values for other red macrophytes (107-179 mg g-1 DW) (Lahaye 1991; Mišurcová 2012). The main constituents of the soluble fiber are cell wall matrix polysaccharides, which in the red algae are presented by sulfated galactans of different structure (Lahaye 1991; Pierre et al. 2015). The sulfated galactans of several Vertebrata species (V. fucoides, V. lanosa, V. aterrima) were thoroughly investigated. These polysaccharides belong to the agaran type and are classified as agaroids as they contain relatively low amounts of anhydrogalactose and are highly sulfated (Pierre et al. 2015). The main backbone of the molecules is composed of alternating 3-linked β-D-galactopyranosyl and 4-linked α-L-galactopyranosyl units; some of the latter may be in the 3,6-anhydro form. The minor constituents are 6-O-methylgalactose (2–6%), xylose (3–11%), 3-O-methyl-/4-O-methyl-galactose (1–2%), and glucose (1–4%). Galactose and anhydrogalactose units may be substituted on C-6 position with sulfate or methyl groups (Batey and Turvey 1975; Miller 2003; Prado et al. 2008). Complex agaroids of similar structure, resembling porphyrans, also occur in the other Ceramiales species (Craigie 1990; Pierre et al. 2015). According to Prado et al. (2008) different fractions of galactans isolated from V. fucoides have the sulfation level of 18.3–25.6%, thus suggesting that this alga could be effective for removing heavy metals from aqueous environments.

The low-molecular-weight carbohydrates of Vertebrata are still poorly studied. Thalli of V. lanosa contain digeneaside (Table 2) at concentrations close to those reported for other representatives of the Rhodomelaceae (122–788 μm g-1 DW; Karsten et al. 2007; Scholz et al. 2016). Digeneaside (2-O-α-D-mannopyranosyl-D-glycerate) is the main photosynthesis product and soluble storage compound in most ceramialean algae (Eggert and Karsten 2010). Interestingly, V. lanosa (as Polysiphonia fastigiata) was the first alga where this compound was detected (Colin and Augier 1939). Digeneaside and its synthetic derivatives have recently gained much attention due to their ability to protect and stabilize proteins (Hamon et al. 2017). Thus, these compounds are considered as drug candidates against neurodegenerative disorders, such as the Alzheimer and Parkinson diseases (Ryu et al. 2008; Faria et al. 2013). In addition, digeneaside can stimulate the human immune system, promoting the antitumoral activity of monocytes and neutrophils in in vitro tests (Hamon et al. 2017). Besides digeneaside, V. lanosa contains considerable amounts of several polyols, such as mannitol, laminitol, and mytilitol (Wickberg 1957). A more detailed spectrum of Vertebrata low-molecular-weight carbohydrates still needs investigation.

Lipids and fatty acids

A complex lipid profile was revealed in V. lanosa (Pettitt et al. 1989). This alga contains, on average, 20 mg g-1 DW of total lipids (Table 2). Among phospholipids, the dominating compounds are phosphatidylglycerol, phosphatidylcholine, and phosphatidylsulphocholine. The major glycolipids are monoglycosyldiacylglycerol, diglycosyldiacylglycerol, and sulfoquinovosyldiacylglycerol. All of these have galactose as a dominant sugar residue, with lesser amounts of glucose and mannose. The fatty acids profile is typical for red algae: the major representatives are palmitic, oleic, arachidonic, and eicosapentaenoic acids. Together, these account for more than 80% of the total fatty acid complement (Pettitt et al. 1989; Mæhre et al. 2014). Polyunsaturated arachidonic and eicosapentaenoic acids dominate in all lipid classes. The content of eicosapentaenoic acid is particularly high in major glycolipids and phosphatidylcholine. For more detailed lipid composition refer to Pettitt et al. (1989), Pettitt and Harwood (1989), and Mæhre et al. (2014).

Pigments

As in all red algae, Vertebrata contain three classes of photosynthetic pigments: chlorophylls, carotenoids, and phycobilins. The only chlorophyll present in these algae is chlorophyll a, and its content varies within a broad range in different Vertebrata species, being the lowest in V. fucoides and the highest in V. lanosa (Table 2). The dominant phycobilin is phycoerythrin, which accumulates in concentrations similar to chlorophyll a. Compared to other red algae, Vertebrata species contain relatively low amounts of both these pigments (Bianchi et al. 1997; Lalegerie et al. 2019; Yanshin et al. 2021). The only carotene found in Vertebrata is β-carotene and the major xanthophyll is zeaxanthin. Besides zeaxanthin, the other xanthophylls, such as lutein, antheraxanthin, and fucoxanthin may occur in the thalli of different species. The relative content of lutein varies considerably, from zero in V. lanosa up to 20% of total carotenoids in V. fucoides (Bianchi et al. 1997; Schubert and García-Mendoza 2006; Lalegerie et al. 2019). V. hendryi contains antheraxanthin (3.1% of total carotenoids; Schubert and García-Mendoza 2006). Such a pigment profile with β-carotene and zeaxanthin as the dominating carotenoids is typical for Rhodomelaceae, thus setting this family apart from other Ceramiales, that mostly accumulate lutein (Bjørnland and Aguilar-Martinez 1976; Marquardt and Hanelt 2004; Schubert and García-Mendoza 2006). Fucoxanthin, a xanthophyll, which is unusual for the red algae, was initially reported as a minor pigment of V. fucoides (Carter et al. 1939). Though this finding might have been an artifact caused by the contamination of the field samples with fucoxanthin-containing microalgae, later reports confirmed the presence of this pigment in some other rhodomelacean species, including cultivated ones (i.e., Leptosiphonia brodiei, and several species of Laurencia; Bjørnland and Aguilar-Martinez 1976; Arnesen et al. 1979). This issue needs further careful examination, the same as the other biochemical peculiarity of the Rhodomelaceae, floridorubin.

Floridorubin (Fig. 5) was first reported as a water-soluble red pigment with green fluorescence, occurring in the plastids of Rytiphlaea tinctoria (Feldmann and Tixier 1947). Further, this pigment was isolated from two other rhodomelacean species, V. fucoides (O’hEocha 1961, as cited in: Rowan 1989) and Epiglossum proliferum (as Lenormandia prolifera; Pedersen et al. 1979). The structural studies showed that floridorubin occurs in the algal cells not as a single molecule, but as a complex mixture of bromophenol polymers (Chevolot-Magueur et al. 1976). Interestingly, the best known floridorubin producer, R. tinctoria, lacks phycoerythrin, and it was suggested that floridorubin may have a role in photosynthesis of this alga, functioning as a light-harvesting pigment with the absorption maximum at ~530 nm (Chevolot-Magueur et al. 1974).

Fig. 5
figure 5

Secondary metabolites of Vertebrata: bromophenolic compounds, mycosporine-like amino acids, and organosulfur compounds (Wickberg 1957; Glombitza and Stoffelen 1972; Chevolot-Magueur et al. 1976; Glombitza et al. 1985; Scholz et al. 2016; Lalegerie et al. 2019; Lever et al. 2019; Hofer et al. 2019)

Full size image

Phenolic and bromophenolic compounds

A wide spectrum of phenolic acids is present in V. fucoides. The dominating compounds of this class are caffeic and protocatechuic acids, the others are gentisic, hydroxybenzoic, chlorogenic, vanillic, and syringic acids (Babakhani et al. 2016).

Besides floridorubin, mentioned in the previous section, different rhodomelacean species synthesize and accumulate other simple bromophenols. Vertebrata, Polysiphonia, Rhodomela and Symphyocladia are known as the best producers of these compounds (Wang et al. 2013). Vertebrata lanosa, V. fucoides, V. decipiens, V. nigra, V. fruticulosa, and V. thuyoides contain bromocatechol, lanosol and its derivatives (aldehyde, methyl, n-propyl, and sulfate esters), rhodomelol, methylrhodomelol etc. (Fig. 5) (Glombitza and Stoffelen 1972; Stoffelen et al. 1972; Pedersen et al. 1974; Glombitza et al. 1985; Dembitsky and Tolstikov 2003; Shoeib et al. 2004; Olsen et al. 2013; Lever et al. 2019). Recently, two more complex bromophenolic metabolites were revealed in the extracts of V. lanosa and V. decipiens and named vertebratol and polysiphonol, respectively (Fig. 5) (Hofer et al. 2019; Lever et al. 2019). Unique phenolic compounds containing both bromine and chlorine atoms (bromochlorophenols) were isolated from V. fucoides (Pedersen 1978). The quantitative studies of these metabolites are still rare, though the data of Hofer et al. (2019) claimed relatively high contents of different bromophenols in the cells of V. lanosa (Table 2). The dominating bromophenol species were vertebratol (up to 0.678 mg g-1 DW), lanosol (up to 0.313 mg g-1 DW), lanosol methyl ester, and 5-((2,3-dibromo-4,5-dihydroxybenzyloxy)methyl)-3,4-dibromobenzene-1,2-diol (Hofer et al. 2019). The biological reasons for synthesizing large amounts of bromophenolic metabolites by these red algae are currently not elucidated in detail. Given the toxicity of the compounds, they supposedly contribute to the chemical defense against epiphytes, grazers, and microbial infestations (Shoeib et al. 2005). It has been reported that the content of bromophenols varies considerably depending on the collection site, year, and the season, thus implying that specific triggers might stimulate the formation of these metabolites in rhodomelacean species (Shoeib et al. 2005; Wang et al. 2013; Hofer et al. 2019). Data on localization of brominated phenols in red algal cells are still limited. In V. fucoides and L. prolifera bromine (presumably, in the form of simple bromophenols or floridorubin-type polymers) is located in the chloroplasts and, to the lesser extent, in the outer layers of the cell walls and the cuticle of the peripheric cells (Pedersen et al. 1979, 1981).

Mycosporine-like amino acids

Vertebrata lanosa synthesizes and accumulates a wide spectrum of mycosporine-like amino acids (MAAs), the specific metabolites of some red algae, microalgae, cyanobacteria, and fungi (Carreto and Carignan 2011; Lalegerie et al. 2019). The principal MAAs of V. lanosa are palythine, porphyra-334, mycosporine-glycine, palythene, and usujirene (Fig. 5) (Scholz et al. 2016; Lalegerie et al. 2019). MAAs are potent UV-absorbing compounds, antioxidants, and the metabolites, involved in responses to osmotic and dehydration stress (Oren and Gunde-Cimerman 2007; Geraldes and Pinto 2021). Thus, accumulation of MAAs in the cells of the intertidal algae, such as V. lanosa, regularly exposed to solar radiation and dehydration, may contribute considerably to their survival.

Organosulfur compounds

Among red algae, Vertebrata species are major producers of dimethylsulfoniopropionic acid (DMSP, Fig. 5) (Van Alstyne and Puglisi 2007). The content of DMSP in V. lanosa reaches 97 μmol g–1 FW (Reed 1983a, Table 2). Several other rhodomelacean algae (Rhodomela confervoides, Halopithys incurva, Eutrichosiphonia paniculata, Polysiphonia urceolata) also accumulate considerable amounts of DMSP (Reed 1983a; Karsten et al. 1994). This organosulfur metabolite is of particular interest from an ecological perspective, as a biochemical precursor and a predominant biological source of dimethyl sulfide (DMS), one of the major volatile sulfur compounds, transferred from marine environments to the atmosphere (Bates et al. 1992). The first studies, mentioning DMS emission from algal tissues and the role of DMSP as the DMS precursor were carried out on V. lanosa and V. fucoides (Hass 1935; Challenger and Simpson, 1948). Later it was shown that DMSP is synthesized from methionine, and a specific enzyme, DMSP lyase, catalyzes DMSP cleavage resulting in the formation of DMS and acrylic acid (Cantoni and Anderson 1956; Van Alstyne et al. 2001; Van Alstyne 2008). The rate of the enzymatic DMS production in the cells of V. hendryi was estimated as 300 μmol g–1 FW min–1 (Van Alstyne et al. 2001). The bacterial microflora of V. lanosa is enriched in H2S-producing microorganisms (Chan and McManus 1969). This may be the result of excretion of sulfur-containing compounds by Vertebrata thalli.

The other specific sulfur-containing metabolite found in V. lanosa is cysteinolic acid (Fig. 5) (Wickberg 1957). Both this compound and DMSP are supposed to be involved in osmoregulation, being the major compatible solutes in the cells of different marine microalgae (Fenizia et al. 2021). Reed (1983b) reported that DMSP functions as a principal osmolyte in V. lanosa, accumulating in the cells depending on water salinity. Moreover, DMSP and its breakdown products may contribute to cryoprotection, antiherbivore defense, and scavenging of the reactive oxygen species (ROS) in the algal cells (Van Alstyne and Puglisi 2007). DMSP, DMS, acrylic acid, and some other DMSP derivatives (dimethylsulfoxide, methanesulfinic acid) can scavenge hydroxyl radicals and other ROS. As a result, these compounds serve as an antioxidant system, regulated by DMSP lyase activity (Sunda et al. 2002).

Recently, due to the organosulfur compounds, giving its thalli strong taste and odor, V. lanosa has gained especial interest in gastronomy, as a “sea truffle” (Bjordal et al. 2020). Truffles are highly appreciated, especially in French and Italian cuisine, due to their characteristic sulfur aroma. The most important odor compounds of black, white, and summer truffles, as well as of the truffle flavored oils, include DMS, dimethyl disulfide, dimethylthiomethane (“truffle sulfide”), dimethyl sulfoxide, and dimethyl sulphone (Piloni et al. 2005; Culleré et al. 2010; Campo et al. 2018). Thus, the truffle-like scent and flavor of V. lanosa is most probably determined by their ability to synthesize and accumulate organosulfur metabolites.

Utilization

Food production

The demand for marine red algae in food production has been growing rapidly in recent decades. As algal food products have proven to be highly effective for well-balanced diets and healthy nutrition, they are now increasingly utilized not only in Asian countries, traditionally consuming sea vegetables, but also in the Nordic, Mediterranean, and other world cuisines (Mouritsen et al. 2019). The data on biochemical composition of Vertebrata show that this alga may be a valuable source of protein (Mæhre et al. 2014). In theory, it can completely cover the human requirements for protein, though it would mean daily consumption of about 350 g of dried algae, which is too large an amount. However, Vertebrata may be used as a food additive supplementing diet with essential amino acids. Given the limited digestibility of some algal proteins (on average, 50–70%, compared to casein), a relatively high content of free amino acids in Vertebrata thalli is advantageous (Wong and Cheung 2001). Generally, among marine macrophytes, red algae are characterized by the highest protein content, and many protein-rich species belong to the order Ceramiales (V. fucoides, V. lanosa, Ceramium virgatum, Savoiea arctica etc.) (Mæhre et al. 2014; Ignatova and Podkorytova 2021; Yanshin et al. 2021). To our knowledge, none of these species is currently extensively exploited, but now the growing interest in algal food sources may bring them to the fore.

In addition to high protein and amino acid contents, V. lanosa also contains high amounts of eicosapentaenoic and arachidonic acids, representing both major classes of polyunsaturated fatty acids, n-3 and n-6, respectively, with the n-3 being the dominating class (Pettitt et al. 1989; Mæhre et al. 2014). These fatty acids are essential nutrients for human health, involved in cardiovascular and central nervous system function, cell signaling, and immune reactions (Shanab et al. 2018; Chaves et al. 2019). During the last decades, physiologists and dietologists have been raising more and more concerns about the increasing ratio of the n-6 to n-3 fatty acids in the every-day diet of people from the Western industrialized societies (Hallahan and Garland 2005; Shanab et al. 2018). Such a tendency, caused by the increased consumption of vegetable oils and limited consumption of seafood, goes in parallel with the rise in various noncommunicable diseases, in particular, mental disorders and cardiovascular pathologies (Hallahan and Garland 2005; Cornish et al. 2015; Chaves et al. 2019). Thus, marine algae such as V. lanosa with its well-balanced PUFA profile may be a valuable contribution to the diet.

Recently, V. lanosa has gained special attention from a gastronomical perspective for its truffle-like aroma and taste (Berglund 2018; Mouritsen et al. 2019; Bjordal et al. 2020). The dried V. lanosa thalli can be used as a spice, and this product is currently commercially available as “sea truffle” or “ocean truffle” in the Norwegian and Icelandic online shops (https://lofotenseaweed.no/our-seaweed/truffle-seaweed, https://www.norwegianseatruffle.com, https://www.thewildroom.com/sea-truffle; accessed on 8 May 2022). Apparently, the smell, resembling that of truffles, is due to the emission of the volatile sulfur compounds from Vertebrata cells (described in the previous section). Interestingly, it was almost a hundred years ago, when Hass (1935) first mentioned “the peculiarly penetrating, somewhat sickly odour, given off from Polysiphonia fastigiata (V. lanosa) on drying”. Though this initial description did not promise a great gastronomical future for this alga, some recent surveys about organoleptic characteristics of vegetable oils supplemented with dried Vertebrata showed that these products have appealing truffle and umami taste and aroma (Berglund 2018).

The growth of commercial interest in V. lanosa will lead to the increasing demand for this resource. Currently, this alga is harvested only from natural populations, as being an obligate epiphyte, V. lanosa is not easy to cultivate (Bjordal et al. 2020). The host macrophyte, A. nodosum, is also a commercially valuable alga (Pereira et al. 2020), so apparently some mechanisms of joint harvest of both the host and the epiphyte should be collaborated upon to ensure the efficient and sustainable exploitation of this resource. The recent ecological studies show that at least in some regions, V. lanosa may be regarded as a harvestable alga suitable for small-scale exploitation (Knighton et al. 2021). More extensive research is needed to better assess the standing stock, regrowth capacity and the cultivation potential of this species. These issues must be considered in addition to its mineral and biochemical composition. Thus, literature data (Table 1) show that both V. lanosa and V. fucoides can accumulate metals (cadmium, lead, arsenic) up to levels exceeding the maximum permissible concentrations accepted for foodstuffs, according to the Codex Alimentarius international standard (www.codexalimentarius.org; accessed on 11 April 2022). As for the seasonality, it was reported that PUFA content in V. lanosa increases significantly in cold water and is 26% higher in winter, than in summer (Pettitt et al. 1989).

Pharmaceutical applications

Bromophenolic metabolites of all studied Vertebrata species are promising compounds for the development of new pharmaceuticals, as they demonstrate various biological activities such as antioxidative, antimicrobial, antiviral, and anticancer (Liu et al. 2011; Wang et al. 2013). Olsen et al. (2013) reported a bromophenol of V. lanosa was a more efficient antioxidant than luteolin and quercetin in cellular antioxidant activity and cellular lipid peroxidation antioxidant activity assays. The bromophenol containing extracts of V. fucoides were found to be potent retardants of lipid and protein oxidation in chilled food products. Treatment with Vertebrata extract resulted in reduced content of peroxides and carbonyl compounds in chilled fish and protected against the loss of α-tocopherol and tryptophane. The antioxidative activity of the extract was similar to that of synthetic antioxidants such as butylated hydroxytoluene (Farvin and Jacobsen 2015; Babakhani et al. 2016). Crude extracts of V. lanosa with high phenolic content also showed considerable antioxidant activity in a variety of in vitro tests (Zubia et al. 2009).

Some of the halogenated metabolites of Vertebrata, Odonthalia, and Rhodomela, as well as their synthetic analogs, demonstrated antimicrobial activity against various Gram-positive and Gram-negative bacteria and fungi with minimum inhibitory concentration values of 0.8–50 μg mL-1 (Xu et al. 2003; Oh et al. 2008; Hofer et al. 2019; Lever et al. 2019). Lanosol possesses antiviral activity against human rhinovirus HRV2 with IC50 of 2.5 μg mL-1 (Park et al. 2012). Extracts of V. lanosa also have antifungal properties, reducing the rate of colony extension in the dermatophyte fungi Microsporum canis and Trichophylon verrucosum (Tariq 1991).

Lanosol and its derivatives from V. lanosa as well as their synthetic isomers may be perspective anticancer compounds, as they showed cytotoxic activity against human colon cancer cell lines DLD-1 and HCT-116 with IC50 values ranging from 0.8 to 20.4 μmol (Shoeib et al. 2004, 2005). Bromophenolic compounds, structurally similar to lanosol, demonstrated cytotoxic effect against the lung adenocarcinoma (A549), stomach cancer (BGC-823), human colon cancer (HCT-8), and hepatoma (Bel-7402) cell lines (Ma et al. 2006).

Bioindication and bioremediation

A high capacity to absorb and accumulate metals, though limiting the nutritional applications of Vertebrata species, makes them useful for biomonitoring of the marine ecosystems and for heavy metal removal. V. lanosa showed extraordinary bioaccumulation ability and very high concentration factor (CF) values for several heavy metals, such as lead (CF 31000), zinc (CF 137600), cadmium (CF 60000), and nickel (CF 20000) (Ryan et al. 2012). Also this alga has demonstrated effective biosorption of chromium, especially, Cr(VI) (Murphy et al. 2008). Several variants of chemical modification of V. lanosa biomass were suggested to further improve its biosorption potential (Murphy et al. 2009). V. fucoides, when used as a bioindicator species, also showed high bioaccumulation capacities, especially for manganese (CF 10576) and copper (CF 3252) (Arici and Bat 2019). Dried biomass of V. fucoides was an efficient sorbent for Cr(III) removal from contaminated solutions with initial chromium concentrations up to 200 mg L-1. The maximum biosorption capacity was estimated as 16.11 mg g-1 (Blanes et al. 2011). Moreover, V. fucoides proved to have high bioaccumulating capacities for gamma emitting radionuclides, such as 137Cs, 54Mn, 60Co, 65Zn, 110mAg, and 113Sn, in both short-term and long-term experiments. These isotopes represent the most common radionuclides present in discharged radioactive waste, and thus V. fucoides can be recommended as a bioindicator of radioactive environmental pollution under both static and accidental circumstances (Zalewska and Saniewski 2011; Zalewska 2014). Compared to other macrophytes, many rhodomelacean species, e.g., O. dentata, R. confervoides, and S. arctica, tend to be efficient metal accumulators (Yanshin et al. 2021). Given its near-cosmopolitan distribution, considerable biomass, and ability to grow within a broad range of temperatures and salinity, V. fucoides deserves special attention from the perspective of its global use as a bioindicator, as well as for removal of heavy metals and radionuclides from contaminated wastewaters.