Placoderm Desmids Research Articles

Diversity of Placoderm Desmids from the Sacred Rivers of Southern Western Ghats, India

Diversity of Placoderm Desmids from the Sacred Rivers of Southern Western Ghats, India

A diverse group of underappreciated zygnematophytes deserves in-depth exploration

ABSTRACT The conjugating green algae (Zygnematophyceae) are the closest relatives of land plants and hence are of great evolutionary interest. Besides the popular placoderm desmids and the filamentous species, there is an underappreciated diversity of unicellular zygnematophytes with a much “simpler” morphology and smooth cell walls – traditionally referred to as “saccoderm desmids”. These saccoderm desmids have a broad geographic distribution and are ecologically diverse. Many species inhabit terrestrial habitats such as dead wood, rock surfaces and glacial ice. Furthermore, several of the saccoderm genera have turned out to be highly polyphyletic and are typically poorly captured by environmental sequencing approaches. One of these genera is Mesotaenium Nägeli, with ~70 described species and infraspecific taxa united only by a relatively simple (plate- or ribbon-like) chloroplast structure. Here, we shed some light on these inconspicuous yet important members of the algal flora and present an updated rbcL gene phylogeny of the conjugating green algae, including several new lineages of Mesotaenium-like zygnematophytes. We depict the subtle morphological differences among these lineages and discuss our updated phylogeny in the light of ecology and cell biology. In addition, we review published knowledge on photoprotective strategies of zygnematophytes, the latest insights into their evolutionary innovations, and address some technical challenges in exploring this elusive group of microalgae. Some new observations of saccoderm desmids in undersampled habitats and of their microbial associates (e.g., parasites) point to interesting avenues for future research.

Cosmarium bahianum, sp. nov. (Desmidiaceae), a new desmid species from a phytotelm habitat in the Brazilian restinga

Cosmarium bahianum sp. nov. is described from samples collected from a bromeliad tank (phytotelmata) in a “restinga” area located at Salvador, Bahia State, northeast Brazil. Cosmarium bahianum belongs to the group of placoderm desmids, and is mainly characterized by rather small, smooth-walled, but remarkably punctate cells that are elliptic to subtriangular in apical view, as well as by a globose to elliptical, smooth-walled zygospore. The new species was collected from Hohenbergia littoralis J.B.Smith, an endangered endemic bromeliad species from the Brazilian Restinga with distribution restricted to the north coast of Bahia. The relationship of C. bahianum with morphologically similar species is discussed.

The extracellular polymeric substances of desmids (Conjugatophyceae, Streptophyta): chemistry, structural analyses and implications in wetland biofilms

S.N. Kiemle, D.S. Domozych and M.R. Gretz. 2007. The extracellular polymeric substances of desmids (Conjugatophyceae, Streptophyta): chemistry, structural analyses and implications in wetland biofilms. Phycologia 46: 617–627. DOI: 10.2216/06–97.1Desmids represent a group of advanced green algae that are commonly found in biofilm communities of freshwater wetlands. Desmids secrete significant amounts of extracellular polymeric substances (EPS) that form an extensive mucilaginous sheath external to the cell wall and function in adhesion, gliding-based movements and ultimate ensheathment within the biofilm complex. We have initiated biochemical and structural analyses of the EPS of desmids isolated from biofilms from the southeastern Adirondack region of New York including Penium cylindris, Penium spirostriolatum, Cosmarium sp. 1, Cosmarium sp. 2, Pleurotaenium trabecula, Tetmemorus brebissonii, Netrium digitus, Netrium oblongum, Netrium interruptum and Netrium interruptum 2509. Cosmarium sp. 1 EPS appeared as a homogeneous sheath that encapsulated the cells, whereas P. trabecula EPS occurred as dispersed patches, and N. oblongum EPS was reticulated and striated. Polysaccharides were the major component of the EPS (52–76%), and lectin labeling revealed differences in polymer organization between saccoderm (labeled with Con-A, WGA and HPA) and placoderm (labeled with BS-I, PSA, UEA and HPA) desmids. Desmid EPS had significant uronic acid (3–29%) and protein (2–10%) content, and the polysaccharides were sulfated to varying degrees. Xylose and Fucose were the predominant monosaccharides with the major glycosyl linkages t-Xylp, 3,4-Fuc and t-Fuc. The unique EPS from N. oblongum was rich in galactose and uronic acid (29.3% w/w), extracellular polymers of P. spirostriolatum were composed predominantly of arabinose, and T. brebissonii EPS was enriched in glucose and galactose. EPS of P. trabecula and T. brebissonii was highly sulfated (10.2% and 14.7%, respectively). The EPS from New York and UTEX strains of N. interruptum were unique from each other. Overall, desmid EPS exhibits a conserved motif with the predominant component an anionic polysaccharide. The presence of deoxy-sugar subunits indicates the potential for hydrophobic interaction where anionic components may play important role in ionic cross-linking. The establishment of this baseline biochemistry provides the foundation for future dissection of core EPS molecules and associated functional groups and studies of EPS involvement in specific stages of biofilm development (e.g. adhesion).

Phylogeny of the Conjugating Green Algae (Zygnematales and Desmidiales) Based on Plastid and Mitochondrial Gene Sequences

AbstractThe conjugating green algae (Order Zygnematales and Desmidiales) are charophyceans, that is, they are among the green algae most closely related to land plants. Compared to the five other groups of charophyceans (Mesostigma, Klebsormidiales, Chlorokybales, Coleochaetales, Charales), the conjugating greens are by far the most diverse taxonomically (6 families, about 55 genera, 3–4000 species) and very widespread in distribution (including common species such as Spirogyra, Zygnema, Cosmarium, and Staurastrum). Thallus shape ranges from simple, oblong unicells (saccoderm desmids, or Mesotaeniaceae), to unbranched filaments of cells with smooth walls (Zygnemataceae), to elaborately ornate and deeply incised unicells or filaments made of such cells (placoderm desmids, in four families). Prior analyses of sequences of the chloroplast‐encoded gene rbcL (1354 bp, or 95% of the gene) sampled from all six families showed that two of the traditional families (saccoderms and filamentous forms) are paraphyletic, i.e. they do not form two independent clades, although the component genera may together constitute a monophyletic group. In contrast, three families of placoderm desmids formed three monophyletic groups within a larger clade. New analyses using partial sequences from the mitochondrial gene coxIII (608 bp, or 76% of the gene) for 21 species of conjugating green algae from all six families supported the conclusions of the rbcL analysis, although bootstrap support for each gene along was weak for many branches. The coxIII sequences are more variable than rbcL and have proportionately more parsimony‐informative sites (43% vs. 34%, respectively). Combining the two genes improved the resolution of the resulting tree: (1) bootstrap support increased for all branches, particularly in regards relationships of placoderm desmids; and (2) monophyly of a clade containing filamentous and saccoderm desmids was strongly supported. We conclude that phylogeny of conjugating green algae will be much improved by sampling from additional genes.

A Checklist Of Desmids In Nigeria

The paper presents a checklist of diversity and distribution of desmid flora in Nigeria. Samples of desmids were collected by means of 55-micrometer mesh plankton net from different locations. There was a high diversity of desmids. A total of 478 taxa are listed in this report. These are categorized into 27 genera, 273 species, 176 varieties and 29 forma. Five of the genera belong to saccoderm desmids while 22 are placoderm desmids. Of all the genera, Cosmarium was dominant with 91 taxa, followed by Closterium with 84 taxa and Staurastrum with 83 taxa. The cosmopolitan taxa were Closterium lunula, Cl. setaceum, Cosmarium monodii, Desmidium swartzii and Pleurotaenium trabecula. Keywords: Taxonomy, desmids, phytogeography, tropics. (Global Journal of Pure and Applied Sciences: 2002 8(2): 223-238)

MONOPHYLY OF THE GENUS CLOSTERIUM AND THE ORDER DESMIDIALES (CHAROPHYCEAE, CHLOROPHYTA) INFERRED FROM NUCLEAR SMALL SUBUNIT rDNA DATA

We newly sequenced the nuclear‐encoded small subunit (SSU) rDNA coding region for 21 taxa of the genus Closterium. The new sequences were integrated into an alignment with 13 known sequences of conjugating green algae representing six traditional families (i.e. Zygnemataceae, Mesotaeniaceae, Gonatozygaceae, Peniaceae, Closteriaceae, and Desmidiaceae) and five known charophycean sequences as outgroups. Both maximum likelihood and maximum parsimony analyses supported with high bootstrap values one large clade containing all placoderm desmids (Desmidiales). All the Closterium taxa formed one clade with 100% bootstrap support, indicating their monophyly, but not paraphyly, as suggested earlier. As to the taxa within the genus Closterium, we found two clades of morphologically closely related taxa in both maximum likelihood and maximum parsimony trees. They corresponded to the C. calosporum species complex and the C. moniliferum‐ehrenbergii species complex. It is of particular interest that the homothallic entity of C. moniliferum v. moniliferum was distinguished from and ancestral to all other entities of the C. moniliferum‐ehrenbergii species complex. Superimposing all 50 charophycean sequences on the higher order SSU rRNA structure model of Closterium, we investigated degrees of nucleotide conservation at a given position in the nucleotide sequence. A characteristic “signature” structure to the genus Closterium was found as an additional helix at the tip of V1 region. In addition, eight base deletions at the tip of helix 10 were found to be characteristic of the C. calosporum species complex, C. gracile, C. incurvum, C. pleurodermatum, and C. pusillum v. maius. These taxa formed one clade with an 82% bootstrap value in maximum parsimony analysis.

SPECIES PHYLOGENY OF COSMARIUM AND STAURASTRUM (DESMIDIACEAE) BASED ON RBC L SEQUENCES

Cosmarium and Staurastrum are the two most diverse genera of placoderm desmids (Family Desmidiaceae), with approximately 1100 and 800 species, respectively. Phylogenetic analysis of relationships of species has been extremely difficult. In a monograph of North American placoderm desmids, Prescott et al. described early phylogenetic work that concluded Staurastrum to be polyphyletic and certainly polymorphic. Likewise, Cosmarium has also been viewed as polyphyletic, and a number of workers have proposed splitting these genera. The classical view of West and West grouped species within each genus into two divisions and 6–8 sections based on wall features and semicell shape. We sequenced rbcL from 18 species of Cosmarium (2 divisions, 7 sections) and 12 species of Staurastrum (2 divisions and 7 sections) and performed a phylogenetic analysis (parsimony, maximum likelihood, bootstrap) using other placoderm desmids and Zygnematales as outgroups. The results exhibit little support for the monophyly of sections or divisions of the two genera. Furthermore, although there is support for the monophyly of clades within each genus, there is also support for a separate clade containing species from both genera.

PHYLOGENY OF ZYGNEMOPHYCEAE BASED ON COXIII GENE SEQUENCE DATA

Molecular phylogenetic analysis of the conjugating green algae (Class Zygnemophyceae) using nuclear (SSU rDNA) and chloroplast (rbcL) gene sequences has resolved hypotheses of relationship at the class, order, and family levels, but several key questions will require data from additional genes. Based on SSU and rbcL sequences, the Zygnemophyceae and Desmidiales are monophyletic, and families of placoderm desmids are distinct clades (Desmidiaceae, Peniaceae, Closteriaceae, and Gonatozygaceae). In contrast, the Zygnemataceae and Mesotaeniaceae are paraphyletic, although whether these two traditional families constitute a clade is uncertain. In addition, relationships of genera within families have proven resistant to resolution with these two oft‐used genes. We have sequenced the coxIII gene from the mitochondrial genome to address some of these ambiguous portions of the phylogeny of conjugating green algae. The coxIII gene is more variable than rbcL or SSU rDNA and offers greater resolving power for relationships of genera. We present preliminary analyses of coxIII sequences from each of the traditional families of Zygnemophyceae and contrast the resulting topologies with those derived from nuclear and chloroplast genes.

PHYLOGENY OF THE CONJUGATING GREEN ALGAE (ZYGNEMOPHYCEAE) BASED ON rbc L SEQUENCES

Sequences of the gene encoding the large subunit of RUBISCO (rbcL) for 30 genera in the six currently recognized families of conjugating green algae (Desmidiaceae, Gonatozygaceae, Mesotaeniaceae, Peniaceae, and Zygnemataceae) were analyzed using maximum parsimony and maximum likelihood; bootstrap replications were performed as a measure of support for clades. Other Charophyceae sensu Mattox and Stewart and representative land plants were used as outgroups. All analyses supported the monophyly of the conjugating green algae. The Desmidiales, or placoderm desmids, constitute a monophyletic group, with moderate to strong support for the four component families of this assemblage (Closteriaceae, Desmidiaceae, Gonatozygaceae, and Peniaceae). The analyses showed that the two families of Zygnematales (Mesotaeniaceae, Zygnemataceae), which have plesiomorphic, unornamented and unsegmented cell walls, are not monophyletic. However, combined taxa of these two traditional families may constitute a monophyletic group. Partitioning the data by codon position revealed no significant differences across all positions or between partitions of positions one and two versus position three. The trees resulting from parsimony analyses using first plus second positions versus third position differed only in topology of branches with poor bootstrap support. The tree derived from third positions only was more resolved than the tree derived from first and second positions. The rbcL-based phylogeny is largely congruent with published analyses of small subunit rDNA sequences for the Zygnematales. The molecular data do not support hypotheses of monophyly for groups of extant unicellular and filamentous or colonial desmid genera exhibiting a common cell shape. A trend is evident from simple omniradiate cell shapes to taxa with lobed cell and plastid shapes, which supports the hypothesis that chloroplast shape evolved generally from simple to complex. The data imply that multicellular placoderm desmids are monophyletic. Several anomalous placements of genera were found, including the saccoderm desmid Roya in the Gonatozygaceae and the zygnematacean Entransia in the Coleochaetales. The former is strongly supported, although the latter is not, and Entransia's phylogenetic position warrants further study.

Spirotaenia alpina(Zygnemaphyceae: Mesotaeniaceae), a saccoderm desmid new to the British Isles: Revised description and observations on cell division

A more complete description than that hitherto available is provided for the rare saccoderm desmid Spirotaenia alpina Schmidle, recently found in quantity in a pool in SW Scotland, and a first record for the British Isles. Smaller numbers have also been found in a pool in mid-Wales. Details are provided concerning its cell division. The migration of the daughter nuclei during division is an event which in some respects is similar to that seen in the Closteriinae. This would seem to place this saccoderm apart from others, and suggests that it may even have affinities with certain placoderm desmids.

Flora of New Zealand: Desmids, Volume I

Abstract This is the first volume of three on the taxonomy of the desmids found in New Zealand's freshwaters. Amongst the Saccoderm and Placoderm desmids covered, it includes the important genera of Closterium, Euastrum, Micrasterias and Cosmarium.

The process and morphology of conjugation in desmids, especially the genusPleurotaenium

The process of conjugation is described for the desmid taxa Pleurotaenium ehrenbergii (Breb.) de Bary and P. trabecula var. mediolaeve (Playf.) Krieger. Anomalous conjugations, resulting in parthenospores or triploid spores, are also described and the process of triple conjugation is illustrated. The exact nature of the conjugation vesicle, and its analogy but not homology with the conjugation tube of Zygnemaceae and some desmids, is discussed. It is suggested that there are two methods of conjugation in placoderm desmids and that the sequence of events described here for Pleurotaenium may be widespread. Two unique features of the Pleurotaenium zygospore are described, a morphologically-differented operculum and a mammillate mesospore.

THE PRODUCTION AND INHERITANCE OF THE TRIRADIATE FORM IN COSMARIUM TURPINII

AtLTHOUGH THE placoderm desmids have been grown in culture for almost 50 years, taxonomists of the desmids have ignored the possible use of cultures in their studies, instead relying on natural populations as a source of material. An overwhelming number of species, varieties, and forms of desmids have been described but many are highly suspect as environmental variants which have been recognized as taxonomic entities. One of the more common variants among desmid genera is an increase in the amount of radiation shown by a cell. Teiling (1950), in his discussion of desmid form, defines radiation in desmids as element of structure which is decisive in the shape of the desmids according to their vertical symmetry-planes. Thus a Cosmarium cell is said to be biradiate in end view; a simple Staurastruim, triradiate in end view. Although the genus Cosmarium is defined as bein,g biradiate, forms which are triradiate on end view have been described both from natural collections and from cultures. Fritsch (1953) pointed out the confusion among taxonomists as to the proper disposition of such triangular forms. In some cases they have been described as species of the triradiate genus Staurastrum; other specimens have been considered as forma triquetra of biradiate species of Cosmarium. Fritsch's suggestion to group such problematical forms under a separate genus to be called Cosmostaurastrum indicates the highly subjective foundations on which the taxonomy of desmids rests. Heterothallic strains of Cosmarium turpinii Br6b.2 were isolated in 1953 by the writer and since that time have been used in studies of the sexual process, germination of the zygospore, and the inheritance of mating type and a zygotic lethal factor (Starr, 1954a, b, 1955). The appearance of triradiate cells in the biradiate populations of Cosmarium turpinii provided material for the present investigation of the production of the triradiate form and its inheritance in sexual as well as asexual reproduction. The heterothallic strains of C. turpinii (no. 3, 4) are identical morphologically. A typical semicell (fig. 1, 11) in front view is angular, being broad at

ZYGOSPORE GERMINATION IN COSMARIUM BOTRYTIS VAR. SUBTUMIDUM

IN MOST haplobiontic green algae, the zygospores enter a dormant state which, often lasting for months, is broken only after a series of environmental changes that are fatal, in many instances, to the species in its vegetative state. The placoderm desmids are no exception. The rarity of observed sexual union in the desmids and the interval between zygo^pore formation and zygospore germination have resulted in a scarcity of published accounts of the latter. Inasmuch as the taxonomy of the desmids is based almost entirely on characteristics of the vegetative cells, few workers have concerned themselves with attempts to investigate sexual reproduction in the desmids beyond the chance observation of sexual union in natural collections and the even more rare observation of zygospore germination. The most complete accounts of zygospore germination in the placoderm desmids are those of DeBary (1858) who observed the entire life cycle of several species of Cosmarium, Closterium, and Staurastrum which he maintained in crude mixed culture in his laboratory. Periodic checks of these cultures during the period following zygospore formation resulted eventually in the observation of the germination process. Other workers have contributed to our knowledge of this phenomenon in these and other genera, such as Cosmariuam (Hofmeister, 1857; Klebahn, 1891), Closterium (Millardet, 1870; Klebahn, 1891; Morren, 1836), Staurastrum (Millardet, 1870; Turner, 1922), and Hyalotheca (Potthoff, 1927, 1928). Without exception, these reports have been based on chance collections or crude cultures. In an earlier paper, the writer (Starr, 1954a) reported the isolation of heterothallic strains of Cosmarium botrytis var. subtumidum Wittr. These strains are maintained in unialgal clonal culture and sexual union can be effected at will by mixing the two strains under appropriate environmental conditions. Inasmuch as these strains are being used in continuing cytological and genetic research in the writer's laboratory, it seems desirable to record for future reference the details of zygospore germination. The observations of DeBary (1858) on this process in Cosmarium botrytis are confirmed and expanded. The heterothallic strains of Cosmarium botrytis

Isolation of Sexual Strains of Placoderm Desmids

Isolation of Sexual Strains of Placoderm Desmids