New insights into the nature and phylogeny of prasinophyte antenna proteins: Ostreococcus tauri, a case study

The basal position of the Mamiellales (Prasinophyceae) within the green lineage makes these unicellular organisms key to elucidating early stages in the evolution of chlorophyll a/b-binding light-harvesting complexes (LHCs). Here, we unveil the complete and unexpected diversity of Lhc proteins in Ostreococcus tauri, a member of the Mamiellales order, based on results from complete genome sequencing. Like Mantoniella squamata, O. tauri possesses a number of genes encoding an unusual prasinophyte-specific Lhc protein type herein designated "Lhcp". Biochemical characterization of the complexes revealed that these polypeptides, which bind chlorophylls a, b, and a chlorophyll c-like pigment (Mg-2,4-divinyl-phaeoporphyrin a5 monomethyl ester) as well as a number of unusual carotenoids, are likely predominant. They are retrieved to some extent in both reaction center I (RCI)- and RCII-enriched fractions, suggesting a possible association to both photosystems. However, in sharp contrast to previous reports on LHCs of M. squamata, O. tauri also possesses other LHC subpopulations, including LHCI proteins (encoded by five distinct Lhca genes) and the minor LHCII polypeptides, CP26 and CP29. Using an antibody against plant Lhca2, we unambiguously show that LHCI proteins are present not only in O. tauri, in which they are likely associated to RCI, but also in other Mamiellales, including M. squamata. With the exception of Lhcp genes, all the identified Lhc genes are present in single copy only. Overall, the discovery of LHCI proteins in these prasinophytes, combined with the lack of the major LHCII polypeptides found in higher plants or other green algae, supports the hypothesis that the latter proteins appeared subsequent to LHCI proteins. The major LHC of prasinophytes might have arisen prior to the LHCII of other chlorophyll a/b-containing organisms, possibly by divergence of a LHCI gene precursor. However, the discovery in O. tauri of CP26-like proteins, phylogenetically placed at the base of the major LHCII protein clades, yields new insight to the origin of these antenna proteins, which have evolved separately in higher plants and green algae. Its diverse but numerically limited suite of Lhc genes renders O. tauri an exceptional model system for future research on the evolution and function of LHC components.     

PRASINOPHYTES FORM INDEPENDENT LINEAGES WITHIN THE CHLOROPHYTA: EVIDENCE FROM RIBOSOMAL RNA SEQUENCE COMPARISONS1

Complete nuclear-encoded small-subunit ribosomal RNA (rRNA) sequences were determined from Nephroselmis olivacea Stein, Pseudoscourfieldia marina (Throndsen) Manton, Scherffelia dubia (Perty) Pascher, and Tetraselmis striata Butcher (Chlorophyta) to investigate the evolutionary position of these scaly green flagellates. Results of neighbor-joining and maximum parsimony phylogenetic analyses demonstrate at least two independent prasinophyte lineages defined by N. olivacea/P. marina and S. dubia/T. striata, which together with the Chlorophyceae, Pleurastrophyceae, and Ulvophyceae form a monophyletic group. Within this assemblage, N. olivacea and P. marina represent an early-diverging lineage that is evolutionarily distinct from the later-diverging S. dubia/T. striata clade. The branch point of the S. dubia/T. striata clade precedes the near-simultaneous radiation of the Chlorophyceae, Ulvophyceae, and Microthamniales. Though interrelationships between these three latter groups of algae are not resolved, the phylogenetic analyses demonstrate that the Prasinophyceae (sensu Moestrup and Throndsen) and the Pleurastrophyceae (sensu Mattox and Stewart) are not monophyletic classes.     

PHYLOGENY OF THE GENUS PYRAMIMONAS (PRASINOPHYCEAE,CHLOROPHYTA) INFERRED FROM THE rbcL GENE1

A 1089-basepair fragment (approx. 75%) of the large subunit of the chloroplast-encoded gene, ribulose-1,5-bis-phosphate carboxylase/oxygenase (rbcL), was sequenced from 16 species of the genus Pyramimonas Schmarda. Electron microscopic and biochemical studies of Pyramimonas, one of the most morphologically diverse genera within the potential sister groups to the chlorophyll a- and b-containing plants, suggest that this genus consists of at least four separate subgenera. Using the homologous sequence of rbcL from Cymbomonas tetramitiformis Schiller (Halosphaeraceae) as an outgroup and applying the maximum likelihood method, we show that the inferred topology is congruent with traditional delimitations of the taxa based on observations of periplast, internal ultrastructure, and biochemical features. A bootstrap analysis also supports division at the subgeneric level; however, the low bootstrap support associated with the deep nodes precludes resolution of these branches. A maximum likelihood relative rate test revealed that the rbcL gene in these single-celled green flagellates has a heterogeneous rate of substitution. The rbcL gene in species of the subgenus Pyramimonas has evolved at an accelerated rate relative to that of congenerics.     

Isolation,purification, and characterization of flagellar scales from the green flagellateTetraselmis striata (Prasinophyceae)

Summary Flagellar scales from the green flagellateTetraselmis striata (Prasinophyceae) were isolated, purified by isopycnic cesium chloride-gradient and zonal sucrose gradient centrifugation and their structure and biochemical composition investigated. Three types of flagellar scales were purified to more than 90% purity, a fourth type up to 75% purity. In addition to the previously known types of flagellar scales (pentagonal scales, rod-shaped scales, hair-scales), a novel scale type (i.e., the knotted scales) was discovered. New information about the asymmetric structure of the rod-shaped scales is presented and consequently they are renamed man scales. Flagellar scales consist mainly of carbohydrate (50–70%), significant amounts of protein (11% of dry weight) were found only in pentagonal scales. The main sugars (90%) of the pentagonal and man scales are the unusual 2-keto-sugar acids 3-deoxy-5-O-methyl-2-octulosonic acid (5 OMeKDO), 3-deoxy-2-heptulosaric acid (DHA), and 3-deoxy-2-octulosonic acid (KDO), the knotted scales contain as major sugars galactose and arabinose in addition to KDO and 5 OMeKDO but lack DHA. 13 major polypeptides were identified in flagellar scales by one-dimensional SDS-PAGE, 11 of these are of high molecular mass (>116 kDa). While the majority of polypeptides was found associated with pentagonal scales, at least 4 polypeptides were tentatively assigned to the hair-scales and knotted scales.Abbreviations CSF crude scale fraction - PS pentagonal scales - MS man scales - HS hair-scales - KS knotted scales - SDS-PAGE sodium dodecyl sulfate polyacrylamide gel electrophoresis - DHA 3-deoxylyxo-2-heptulosaric acid - 5 OMeKDO 3-deoxy-5-O-methyl-manno-2-octulosonic acid - KDO 3-deoxy-manno-2-octulosonic acid - GA Golgi apparatus     

Le paléoplancton des Grès de Landévennec (Gedinnien de la rade de Brest — Finistère) étude biostratigraphique

In the Gedinnian Landévennec Sandstones Formation(Rade de Brest, Finistère, Massif Armoricain), rich and well-preserved assemblages of organic-walled microplancton were observed. In the Lanvéoc section, worked out bed after bed, fifty-five species of this organic-walled microphytoplancton of uncertain algal affinity and Prasinophyceae are described. Four genera and fifteen species are new in this total. The assemblage shows many relationship to others gedinnian microplancton from Europa. Africa and United States of America.     

EVIDENCE FOR SEXUAL REPRODUCTION IN THE PRIMITIVE GREEN ALGA NEPHROSELMIS OLIVACEA (PRASINOPHYCEAE)1

Evidence for a specialized sexual process in Nephroselmis olivacea Stein is presented. This alga is a member of the Prasinophyceae, which is regarded by some as the most primitive Class of green plants. N. olivacea has a heterothallic type of mating system. Plus and minus gametes were morphologically similar but showed different behaviors during the mating process. The minus gamete settled to the substratum, attaching by its ventral side. The plus gamete attached to the dorsal side of the minus gamete by the region near the flagellar bases of the plus gamete. The mature zygotes were spherical and strongly adhered to the substratum. After zygote germination, two biflagellate daughter cells, each with two pyrenoids were liberated. These cells divided, resulting in four vegetative cells, each with a single pyrenoid.     

Prasinoxanthin—a chemosystematic marker for algae

A new coccoid marine alga (clone Ω 48-23) contained chlorophylls a and b and carotenoids consisting of β,β-carotene (3% of total), β,ε-carotene (1%), zeaxanthin (2%), neoxanthin (21%), two minor unknowns (2 + 2%) and prasinoxanthin (69%). Prasinoxanthin is identical with xanthophyll K, previously considered characteristic of prasinophytes. From spectroscopic and chemical evidence prasinoxanthin is assigned the structure (3′R,-6′R)-3,6,3′ trihydroxy-7,8-dihydro-γ,ε-caroten-8-one, with tentative 3R,6R chirality from biogenetic considerations, thus representing the first algal carotenoid with a γ-end group. The structural relationship between prasinoxanthin and siphonaxanthin (ex Prasinophyceae and Siphonales) is discussed in chemosystematic terms.     

EFFECT OF NUTRIENT LIMITATION ON FATTY ACID AND LIPID CONTENT OF MARINE MICROALGAE1

Phaeodactylum tricornutum and Chaetoceros sp. (Badllariophyceae), Isochrysis galbana (clone T-Iso) and Pavlova lutheri (Prymnesiophyceae), Nannochloris atomus (Chlorophyceae), Tetraselmis sp. (Prasinophyceae), and Gymnodinum sp. (Dinophyceae) were cultured at different extents of nutrient-limited growth: 50 and 5% of μ_max. The lipid content of the algae was in the range 8.3–29.5% of dry matter and was generally higher in the Prymnesiophyceae than in the Prasinophyceae and the Chlorophyceae. Increasing extent of phosphorus limitation resulted in increased lipid content in the Bacillariophyceae and Prymnesiophyceae and decreased lipid content in the green flagellates N. atomus and Tetraselmis sp. The fatty acid composition of the algae showed taxonomic conformity, especially for the Bacillariophyceae, where the major fatty adds were 14:0, 16:0, 16:1, and 20:5n-3. These fatty acids were dominant also in the Prymnesiophyceae together with 22:6n-3. An exception was I. galbana, in which 18:1 was the major monounsaturated fatty add and 20:5n-3 was absent. The fatty acids of N. atomus and Tetraselmis sp. varied somewhat, but 16:0, 16:1, 18:1, 18:3n-3, and 20:5n-3 were most abundant. Gymnodinum sp. contained mainly 16:0, 18:4n-3, 20: 5n-3, and 22:6n-3. An increased level of nutrient limitation (probably phosphorus) resulted in a higher relative content of 16:0 and 18:1 and a lower relative content of 18:4n-3, 20:5n-3, and 22:6n-3. The nutrient limitation probably reduced the synthesis of n-3 polyunsaturated fatty acids.     

Phylogeny and Molecular Evolution of the Green Algae

The green lineage (Viridiplantae) comprises the green algae and their descendants the land plants, and is one of the major groups of oxygenic photosynthetic eukaryotes. Current hypotheses posit the early divergence of two discrete clades from an ancestral green flagellate. One clade, the Chlorophyta, comprises the early diverging prasinophytes, which gave rise to the core chlorophytes. The other clade, the Streptophyta, includes the charophyte green algae from which the land plants evolved. Multi-marker and genome scale phylogenetic studies have greatly improved our understanding of broad-scale relationships of the green lineage, yet many questions persist, including the branching orders of the prasinophyte lineages, the relationships among core chlorophyte clades (Chlorodendrophyceae, Ulvophyceae, Trebouxiophyceae and Chlorophyceae), and the relationships among the streptophytes. Current phylogenetic hypotheses provide an evolutionary framework for molecular evolutionary studies and comparative genomics. This review summarizes our current understanding of organelle genome evolution in the green algae, genomic insights into the ecology of oceanic picoplanktonic prasinophytes, molecular mechanisms underlying the evolution of complexity in volvocine green algae, and the evolution of genetic codes and the translational apparatus in green seaweeds. Finally, we discuss molecular evolution in the streptophyte lineage, emphasizing the genetic facilitation of land plant origins.     

Starch-type polysaccharide and mannitol in Platymonas

d-Mannitol and a starch-type glucan were isolated from a Platymonas species.