Higher plant origins and the phylogeny of green algae

Summary 5S rRNA sequences from six additional green algae lend strong molecular support for the major outlines of higher plant and green algae phylogeny that have been proposed under varying naming conventions by several authors. In particular, the molecular evidence now available unequivocally supports the existence of at least two well-separated divisions of the Chlorobionta: the Chlorophyta and the Streptophyta (i.e., charophytes) (according to the nomenclature of Bremer). The chlamydomonad 5S rRNAs are, however, sufficiently distinct from both clusters that it may ultimately prove preferable to establish a third taxon for them. In support of these conclusions 5S rRNA sequence data now exist for members of four diverse classes of chlorophytes. These sequences all exhibit considerably more phylogenetic affinity to one another than any of them show toward members of the other cluster, the Streptophyta, or the twoChlamydomonas strains. Among the Charophyceae, new 5S rRNA sequences are provided herein for three genera,Spirogyra, Klebsormidium, andColeochate. All of these sequences and the previously publishedNitella sequence show greater resemblance among themselves and to the higher plants than they do to any of the other green algae examined to date. These results demonstrate that an appropriately named taxon that includes these green algae and the higher plants is strongly justified. The 5S rRNA data lack the resolution needed, however, to unequivocally determine which of several subdivisions of the charophytes is the sister group of the land plants. The evolutionary diversity ofChlamydomonas relative to the other green algae was recognized in earlier 5S rRNA studies but was unanticipated by ultrastructural work. These new data provide further evidence for the relative uniqueness of the chlamydomonads and are discussed further.     

Fatty acids filamentous green algae

Fatty acid analyses of several filamentous green algae were conducted using gas-liquid chromatography. Two bryophytes were also examined. Qualitatively, the genera of algae studied were divided into two groups: (A) algae that have significant amounts of polyunsaturated C₂₀ fatty acids and (B) algae that lack or only have very small amounts of the C₂₀ acids. On the basis of fatty acid content, the algae of Group A more closely resemble the bryophytes than do the algae of Group B. Culture age was shown to cause quantitative but not qualitative variations in fatty acid content. It is evident from this study that extrapolation to land plants, from studies on the fatty acid content of the green algae, should include the filamentous forms.     

AN UNRECOGNIZED ANCIENT LINEAGE OF GREEN PLANTS PERSISTS IN DEEP MARINE WATERS1

We provide molecular phylogenetic evidence that the obscure genera Palmophyllum Kütz. and Verdigellas D. L. Ballant. et J. N. Norris form a distinct and early diverging lineage of green algae. These palmelloid seaweeds generally persist in deep waters, where grazing pressure and competition for space are reduced. Their distinctness warrants recognition as a new order, the Palmophyllales. Although phylogenetic analyses of both the 18S rRNA gene and two chloroplast genes (atpB and rbcL) are in agreement with a deep‐branching Palmophyllales, the genes are in conflict about its exact phylogenetic placement. Analysis of the nuclear ribosomal DNA allies the Palmophyllales with the prasinophyte genera Prasinococcus and Prasinoderma (Prasinococcales), while the plastid gene phylogeny placed Palmophyllum and Verdigellas as sister clade to all other Chlorophyta.     

Hidden levels of phylodiversity in Antarctic green algae: further evidence for the existence of glacial refugia

Recent data revealed that metazoans such as mites and springtails have persisted in Antarctica throughout several glacial–interglacial cycles, which contradicts the existing paradigm that terrestrial life was wiped out by successive glacial events and that the current inhabitants are recent colonizers. We used molecular phylogenetic techniques to study Antarctic microchlorophyte strains isolated from lacustrine habitats from maritime and continental Antarctica. The 14 distinct chlorophycean and trebouxiophycean lineages observed point to a wide phylogenetic diversity of apparently endemic Antarctic lineages at different taxonomic levels. This supports the hypothesis that long-term survival took place in glacial refugia, resulting in a specific Antarctic flora. The majority of the lineages have estimated ages between 17 and 84 Ma and probably diverged from their closest relatives around the time of the opening of Drake Passage (30–45 Ma), while some lineages with longer branch lengths have estimated ages that precede the break-up of Gondwana. The variation in branch length and estimated age points to several independent but rare colonization events.     

球状绿藻的隐性生物多样性及其分类学进展

球状绿藻主要指绿藻门中多为单细胞, 形状为球形、近球形, 或由球形衍生出来的其他形状的藻类。球状绿藻分布广泛, 遍布全球, 生活于淡水、海水和亚气生等生境中, 其相似的简单形态下隐藏着复杂的物种多样性。球状绿藻分类学上主要位于绿藻门的两纲四目及其一些独立支系的类群。球状绿藻分类学正由传统的基于形态特征向基于分子信息的复合分类方法转变。球状绿藻隐性的物种多样性涉及约40属, 其中15属是依据新标本材料而建立的新属, 12个属是依据新证据建立的新组合。本文重点介绍了1998年以后在共球藻纲和绿藻纲中新发现和命名的单细胞球状绿藻, 介绍了它们的形态特征、分类学迁移及理由, 特别对常见的小球藻属和栅藻属的分类学概念变迁作了详细介绍, 对一些尚没有中文名的拉丁学名给出了中文命名。另外, 本文讨论了基于DNA的分类学方法在球状绿藻分类学中的应用, 目前系统发育位置的不同已经成为球状绿藻分类的主要依据。目前球状绿藻分类面临的问题是大多分类位置未用分子系统发育方法的验证。 未来, 球状绿藻分类学家应用系统发育研究结合形态学研究探索单细胞球状绿藻的生物多样性。     

The Charophycean green algae as model systems to study plant cell walls and other evolutionary adaptations that gave rise to land plants

The Charophycean green algae (CGA) occupy a key phylogenetic position as the evolutionary grade that includes the sister group of the land plants (embryophytes), and so provide potentially valuable experimental systems to study the development and evolution of traits that were necessary for terrestrial colonization. The nature and molecular bases of such traits are still being determined, but one critical adaptation is thought to have been the evolution of a complex cell wall. Very little is known about the identity, origins and diversity of the biosynthetic machinery producing the major suites of structural polymers (i. e., cell wall polysaccharides and associated molecules) that must have been in place for land colonization. However, it has been suggested that the success of the earliest land plants was partly based on the frequency of gene duplication, and possibly whole genome duplications, during times of radical habitat changes. Orders of the CGA span early diverging taxa retaining more ancestral characters, through complex multicellular organisms with morphological characteristics resembling those of land plants. Examination of gene diversity and evolution within the CGA could help reveal when and how the molecular pathways required for synthesis of key structural polymers in land plants arose.     

Chloroplast genome evolution and phylogeny of the early-diverging charophycean green algae with a focus on the Klebsormidiophyceae and Streptofilum

The Klebsormidiophyceae are a class of green microalgae observed globally in both freshwater and terrestrial habitats. Morphology-based classification schemes of this class have been shown to be inadequate due to the simple morphology of these algae, the tendency of morphology to vary in culture versus field conditions, and rampant morphological homoplasy. Molecular studies revealing cryptic diversity have renewed interest in this group. We sequenced the complete chloroplast genomes of a broad series of taxa spanning the known taxonomic breadth of this class. We also sequenced the chloroplast genomes of three strains of Streptofilum, a recently discovered green algal lineage with close affinity to the Klebsormidiophyceae. Our results affirm the previously hypothesized polyphyly of the genus Klebsormidium as well as the polyphyly of the nominal species in this genus, K. flaccidum. Furthermore, plastome sequences strongly support the status of Streptofilum as a distinct, early-diverging lineage of charophytic algae sister to a clade comprising Klebsormidiophyceae plus Phragmoplastophyta. We also uncovered major structural alterations in the chloroplast genomes of species in Klebsormidium that have broad implications regarding the underlying mechanisms of chloroplast genome evolution.     

Expressed sequence tag (EST) survey of the highly adapted green algal parasite, Helicosporidium

Helicosporidia are obligate invertebrate pathogens with a unique and highly adapted mode of infection. The evolutionary history of Helicosporidia has been uncertain, but several recent molecular phylogenetic studies have shown an unexpectedly close relationship to green algae, and specifically to the opportunistic pathogen Prototheca. To date, molecular sequences from Helicosporidia are restricted to those genes used for phylogenetic reconstruction and genes related to the existence and function of its cryptic plastid. We have therefore conducted a small expressed sequence tag (EST) project on Helicosporidium sp., yielding about 700 unique sequences. We have examined the functional distribution of known genes, the distribution of EST abundance, and the prevalence of previously unknown gene sequences. To demonstrate the potential utility of large amounts of data, we have used ribosomal proteins to test whether the phylogenetic position of Helicosporidium inferred from a small number of genes is broadly supported by a large number of genes. We conducted phylogenetic analyses on 69 ribosomal proteins and found that 98% supported the green algal origin of Helicosporidia and 80% support a specific relationship with Prototheca. Overall, these data multiply the available molecular information from Helicosporidium 100-fold, which should provide the basis for new insights into these unusual but interesting parasites.     

Lunachloris lukesovae gen. et sp. nov. (Trebouxiophyceae,Chlorophyta), a novel coccoid green alga isolated from soil in South Bohemia,Czech Republic

Culture collections of microorganisms can still hold undiscovered biodiversity; with molecular techniques, considerable progress has been made in characterizing microalgae which were isolated in the past and misidentified due to a lack of morphological features. However, many strains are still awaiting taxonomic reassessment. Here we analysed the phylogenetic position, morphology and ultrastructure of the strain CCALA 307 previously identified as Coccomyxa cf. gloeobotrydiformis Reysigl isolated in 1987 from field soil in South Bohemia, Czech Republic. Molecular phylogenetic analyses based on SSU rDNA and the plastid rbcL gene revealed that the strain CCALA 307 formed a distinct sister lineage to Neocystis and Prasiola clades within the Trebouxiophyceae. We describe this strain as a new genus and species, Lunachloris lukesovae. Multiple conserved nucleotide positions identified in the secondary structures of the highly variable ITS2 rDNA barcoding marker provide further evidence of the phylogenetic position of Lunachloris. Minute vegetative cells of this newly recognized species are spherical or ellipsoid, with a single parietal chloroplast without a pyrenoid. Asexually, it reproduces by the formation of 2–6 autospores. Since the majority of recent attention has been paid to algae from the tropics or extreme habitats, the biodiversity of terrestrial microalgae in temperate regions is still notably unexplored and even a ‘common’ habitat like agricultural soil can contain new, as yet unknown species. Moreover, this study emphasizes the importance of culture collections of microorganisms even in the era of culture-independent biodiversity research, because they may harbour novel and undescribed organisms as well as preserving strains for future studies.     

A multi-locus time-calibrated phylogeny of the siphonous green algae

The siphonous green algae are an assemblage of seaweeds that consist of a single giant cell. They comprise two sister orders, the Bryopsidales and Dasycladales. We infer the phylogenetic relationships among the siphonous green algae based on a five-locus data matrix and analyze temporal aspects of their diversification using relaxed molecular clock methods calibrated with the fossil record. The multi-locus approach resolves much of the previous phylogenetic uncertainty, but the radiation of families belonging to the core Halimedineae remains unresolved. In the Bryopsidales, three main clades were inferred, two of which correspond to previously described suborders (Bryopsidineae and Halimedineae) and a third lineage that contains only the limestone-boring genus Ostreobium. Relaxed molecular clock models indicate a Neoproterozoic origin of the siphonous green algae and a Paleozoic diversification of the orders into their families. The inferred node ages are used to resolve conflicting hypotheses about species ages in the tropical marine alga Halimeda.     

SPERMATOGENESIS IN COLEOCHAETE PULVINATA (CHAROPHYCEAE): SPERM MATURATION1

Ultrastructural study showed that the sequence of developmental events occurring during spermatozoid maturation in Coleochaete pulvinata Braun was similar in a number of respects to sperm development in the Charales and lower land plants. Elaboration of cytoskeletal components and associated flagellar basal bodies occurs early, and is followed by an extensive decrease in cytoplasmic volume and increases in densities of cytoplasm and nucleoplasm. Volume decrease and density increases appear to result from exocytosis involving dictyosome vesicles and perhaps endoplasmic reticulum. Elongation of flagella is accompanied by deposition of flagellar and body scales as in the Charales. During final stages of sperm maturation, the MLS undergoes changes in organization of the lamellar strip, which may also occur in archegoniates. In mature sperm of C. pulvinata the MLS and basal bodies are so occluded by dense material that determination of absolute orientation (configuration) is difficult. Thus, absolute orientation of the flagellar apparatus was determined by study of mid-stage spermatids, and found to be the same as previously described by Sluiman for zoospores of C. pulvinata. Finally, it is proposed that the large complex, striated fiber which connects basal bodies in C. pulvinata has been evolutionarily reduced in the Charales and Phaeoceros sp., ultimately disappearing from most embryophytic lines of descent.     

FINE STRUCTURE OF MITOSIS AND CLEAVAGE IN FRIEDMANNIA ISRAELENSIS (CHLOROPHYCEAE: CHLOROSARCINACEAE)1

Observations on the ultrastructure of Friedmannia israelensis Chantanachat & Bold revealed the presence of a phycoplast and zoospores with cruciate rootlets. During mitosis, the nuclear envelope partially disintegrates and the basal bodies remain at the cell surface on either side of the developing cleavage furrow. The events during mitosis and cleavage in Friedmannia resemble those reported in the other green algae, Platymonas and Pleurastrum.     

The new phylogeny of eukaryotes

Molecular phylogeny has been regarded as the ultimate tool for the reconstruction of relationships among eukaryotes-especially the different protist groups-given the difficulty in interpreting morphological data from an evolutionary point of view. In fact, the use of ribosomal RNA as a marker has provided the first well resolved eukaryotic phylogenies, leading to several important evolutionary hypotheses. The most significant is that several early-emerging, amitochondriate lineages, are living relics from the early times of eukaryotic evolution. The use of alternative protein markers and the recognition of several molecular phylogeny reconstruction artefacts, however, have strongly challenged these ideas. The putative early emerging lineages have been demonstrated as late-emerging ones, artefactually misplaced to the base of the tree. The present state of eukaryotic evolution is best described by a multifurcation, in agreement with the 'big bang' hypothesis that assumes a rapid diversification of the major eukaryotic phyla. For further resolution, the analysis of genomic data through improved phylogenetic methods will be required.     

The charophycean green algae provide insights into the early origins of plant cell walls

Numerous evolutionary innovations were required to enable freshwater green algae to colonize terrestrial habitats and thereby initiate the evolution of land plants (embryophytes). These adaptations probably included changes in cell-wall composition and architecture that were to become essential for embryophyte development and radiation. However, it is not known to what extent the polymers that are characteristic of embryophyte cell walls, including pectins, hemicelluloses, glycoproteins and lignin, evolved in response to the demands of the terrestrial environment or whether they pre-existed in their algal ancestors. Here we show that members of the advanced charophycean green algae (CGA), including the Charales, Coleochaetales and Zygnematales, but not basal CGA (Klebsormidiales and Chlorokybales), have cell walls that are comparable in several respects to the primary walls of embryophytes. Moreover, we provide both chemical and immunocytochemical evidence that selected Coleochaete species have cell walls that contain small amounts of lignin or lignin-like polymers derived from radical coupling of hydroxycinnamyl alcohols. Thus, the ability to synthesize many of the components that characterize extant embryophyte walls evolved during divergence within CGA. Our study provides new insight into the evolutionary window during which the structurally complex walls of embryophytes originated, and the significance of the advanced CGA during these events.     

绿藻光合生物制氢技术进展

氢能作为可再生、环境友好的能源,已成为营造可持续发展的经济节约型社会的理想能源。绿藻因能利用光能分解水产氢,被称为最有应用前景的方法之一。本文将综述绿藻光合产氢的原理,介绍该生物制氢技术的研究现状和最新进展,并对其发展趋势做以展望。     

Comparison of the long-wave chlorophyll fluorescence in various green and blue-green algae and diatoms

Two types of long-wave fluorescence bands with similar band shape occur at room temperature in various algae: F_II700 and F_I715. F_II700 occurs in a limited number of algae, follows PS II transients, increases with culture age and is moderately increased by cooling to 83 K. F_I715 occurs in most algae, especially Anabaena, but much less in most diatoms and Tribonema. It does not follow PS II transients, does not increase with culture age and is much increased by cooling to 83 K.An interpretation for the characteristics of F_II700 and F_I715 is given.