Mary-Hélène Noël

Taxonomic revision of Chattonella antiqua, C. marina and C. ovata (Raphidophyceae) based on their morphological characteristics and genetic diversity

Demura M., Noël M.-H., Kasai F., Watanabe M.M. and Kawachi M. 2009. Taxonomic revision of Chattonella antiqua, C. marina and C. ovata (Raphidophyceae) based on their morphological characteristics and genetic diversity. Phycologia 48: 518–535. DOI: 10.2216/08-98.1. Three raphidophyte species, Chattonella antiqua, C. marina and C. ovata, are red tide–forming harmful phytoplankton species, causing mass mortality at fish farms. These species were originally distinguished from each other solely based on their morphological characters. Recent genetic diversity analyses showed that the three species were extremely similar. It was therefore necessary to re-examine these three species to determine if they were independent single species. We compared 104 strains of the three species using morphological characters, nuclear ITS rDNA regions, the chloroplast rbcL gene, the mitochondrion COI gene and selected microsatellite regions. The morphological characters for the three species formed a continuum of variation instead of clearly defined limits. Compared with other selected heterokontophytes, their genetic divergence in the analyzed three regions was found to be at the intraspecies level. Microsatellite markers distinguished each of the 104 strains, but the strains apparently formed a single geographical population. Based on the morphological characters and the phylogenetic analyses, three groups, corresponding to each of the three Chattonella species, were roughly recognized. Nevertheless, this three-group distribution was insufficient to justify their distinction at the species rank. We therefore proposed a taxonomic revision with C. antiqua and C. ovata reduced to varieties of C. marina, this latter having name priority. We proposed a new status: C. marina var. antiqua (Hada) Demura & Kawachi and C. marina var. ovata (Y. Hara & Chihara) Demura & Kawachi.

Diversity and oceanic distribution of the Parmales (Bolidophyceae), a picoplanktonic group closely related to diatoms

Bolidomonas is a genus of picoplanktonic flagellated algae that is closely related to diatoms. Triparma laevis, a species belonging to the Parmales, which are small cells with a siliceous covering, has been shown to form a monophyletic group with Bolidomonas. We isolated several novel strains of Bolidophyceae that have permitted further exploration of the diversity of this group using nuclear, plastidial and mitochondrial genes. The resulting phylogenetic data led us to formally emend the taxonomy of this group to include the Parmales within the Bolidophyceae, to combine Bolidomonas within Triparma and to define a novel species, Triparma eleuthera sp. nov. The global distribution of Bolidophyceae was then assessed using environmental sequences available in public databases, as well as a large 18S rRNA V9 metabarcode data set from the Tara Oceans expedition. Bolidophyceans appear ubiquitous throughout the sampled oceans but always constitute a minor component of the phytoplankton community, corresponding to at most ~4% of the metabarcodes from photosynthetic groups (excluding dinoflagellates). They are ~10 times more abundant in the small size fraction (0.8–5 μm) than in larger size fractions. T. eleuthera sp. nov. constitutes the most abundant and most widespread operational taxonomic unit (OTU) followed by T. pacifica, T. mediterranea and the T. laevis clade. The T. mediterranea OTU is characteristic of Mediterranean Sea surface waters and the T. laevis clade OTU is most prevalent in colder waters, in particular off Antarctica.

Diversity and oceanic distribution of prasinophytes clade VII, the dominant group of green algae in oceanic waters

Prasinophytes clade VII is a group of pico/nano-planktonic green algae (division Chlorophyta) for which numerous ribosomal RNA (rRNA) sequences have been retrieved from the marine environment in the last 15 years. A large number of strains have also been isolated but have not yet received a formal taxonomic description. A phylogenetic analysis of available strains using both the nuclear 18S and plastidial 16S rRNA genes demonstrates that this group composes at least 10 different clades: A1–A7 and B1–B3. Analysis of sequences from the variable V9 region of the 18S rRNA gene collected during the Tara Oceans expedition and in the frame of the Ocean Sampling Day consortium reveal that clade VII is the dominant Chlorophyta group in oceanic waters, replacing Mamiellophyceae, which have this role in coastal waters. At some location, prasinophytes clade VII can even be the dominant photosynthetic eukaryote representing up to 80% of photosynthetic metabarcodes overall. B1 and A4 are the overall dominant clades and different clades seem to occupy distinct niches, for example, A6 is dominant in surface Mediterranean Sea waters, whereas A4 extend to high temperate latitudes. Our work demonstrates that prasinophytes clade VII constitute a highly diversified group, which is a key component of phytoplankton in open oceanic waters but has been neglected in the conceptualization of marine microbial diversity and carbon cycle.

Ultrastructure and phylogenetic position of Chrysoculter rhomboideus gen. et sp. nov. (Prymnesiophyceae), a new flagellate haptophyte from Japanese coastal waters

T. Nakayama, M. Yoshida, M.-H. Noël, M. Kawachi and I. Inouye. 2005. Ultrastructure and phylogenetic position of Chrysoculter rhomboideus gen. et sp. nov. (Prymnesiophyceae), a new flagellate haptophyte from Japanese coastal waters. Phycologia 44: 369–383. We isolated a novel haptophyte alga from the coastal waters of the northern part of Japan. The cell is asymmetrical spindle-to knife-shaped and possesses two nearly equal flagella and a haptonema from the anterior tip. Two yellowish chloroplasts including immersed pyrenoids are situated asymmetrically. Two types of organic scales, small elliptical and large rhomboidal scales, cover the cell. The rhomboidal scale has two tubular projections at the longitudinal poles and closely touched to each other. The transition region of the flagellum includes only one transitional plate, which is probably homologous with the distal plate in other prymnesiophyceans. The flagellar apparatus of this alga has basic components found in other prymnesiophyceans. Developed root 1 (R1) is divided into two components that join again. The R1 extends toward the posterior tip of the cell with associated endoplasmic reticulum (ER) and forms a cytoplasmic tongue. A fibrous root, which has only been reported in some coccolithophorids, connects R1 and basal body 1. The proximal end of R1 is associated with the electron-dense plate and teeth-like structure. Four microtubules of the root 2 are arranged in an arc shape and have appendages. The free part of the haptonema includes five microtubules. In the traditional taxonomy, this alga is apparently a member of the genus Chrysochromulina because the cell possesses a long haptonema and no coccoliths. However, recent molecular phylogenetic studies have shown the polyphyly of the genus Chrysochromulina. Diagnostic characters of Chrysochromulina would be plesiomorphies of the Prymnesiophyceae. Phylogenetic analysis using 18S rDNA and rbcL sequences indicated that the alga reported here is not closely related to any other prymnesiophyceans, including Chrysochromulina, but suggested the basal position of the coccolithophorids. Based on these results, we propose Chrysoculter rhomboideus gen. et sp. nov. and Chrysoculteraceae fam. nov. for this unique haptophyte.

Life cycle of Chattonella marina (Raphidophyceae) inferred from analysis of microsatellite marker genotypes

Red tides of Chattonella spp. have caused continuous damage to Japanese aquaculture, however, the life cycle of this organism remains incompletely understood. To further investigate this matter, we assessed genotypes at 14 microsatellite markers in three varieties of Chattonella marina, viz., C. marina var. antiqua, C. marina var. marina, and C. marina var. ovata, to establish whether Chattonella undergoes asexual diploidization or sexual reproduction. After genotyping 287 strains of C. marina, all but one of these strains was shown to be heterozygous for at least some loci, and thus, in the diploid state, suggesting that Chattonella strains undergo sexual reproduction. In addition, we performed single‐cell amplification on ‘small cells’ that are derived from vegetative cells under dark and low‐nutrient conditions. The results indicated the existence of two types of small cells. The ‘Small cell Type 1’ was found to be heterozygous, genotypically equivalent to the vegetative cells, and is therefore diploid. These small cells may change to resting cells (cysts) directly. The ‘Small cell Type 2’ was homozygous at all analyzed loci, suggesting that these small cells are haploid and may be derived by meiosis. As fusion between small cells has previously been observed, the ‘Small cell Type 2’ may be the gamete of Chattonella. We present a construct of the full life cycle of Chattonella marina based on our own and previous results.

Application of cryopreservation to genetic analyses of a photosynthetic picoeukaryote community.

Cryopreservation is useful for long-term maintenance of living strains in microbial culture collections. We applied this technique to environmental specimens from two monitoring sites at Sendai Bay, Japan and compared the microbial diversity of photosynthetic picoeukaryotes in samples before and after cryopreservation. Flow cytometry (FCM) showed no considerable differences between specimens. We used 2500 cells sorted with FCM for next-generation sequencing of 18S rRNA gene amplicons and after removing low-quality sequences obtained 10,088-37,454 reads. Cluster analysis and comparative correlation analysis of observed high-level operational taxonomic units indicated similarity between specimens before and after cryopreservation. The effects of cryopreservation on cells were assessed with representative culture strains, including fragile cryptophyte cells. We confirmed the usefulness of cryopreservation for genetic studies on environmental specimens, and found that small changes in FCM cytograms after cryopreservation may affect biodiversity estimation.

Chloropicophyceae, a new class of picophytoplanktonic prasinophytes

Prasinophytes are a paraphyletic group of nine lineages of green microalgae that are currently classified either at the class or order level or as clades without formal taxonomic description. Prasinophyte clade VII comprises picoplanktonic algae that are important components of marine phytoplankton communities, particularly in moderately oligotrophic waters. Despite first being cultured in the 1960s, this clade has yet to be formally described. Previous phylogenetic analyses using the 18S rRNA gene divided prasinophyte clade VII into three lineages, termed A, B and C, the latter formed by a single species, Picocystis salinarum, that to date has only been found in saline lakes. Strains from lineages A and B cannot be distinguished by light microscopy and have very similar photosynthetic pigment profiles corresponding to the prasino-2A pigment group. We obtained phenotypic and genetic data on a large set of prasinophyte clade VII culture strains that allowed us to clarify the taxonomy of this important marine group. We describe two novel classes, the Picocystophyceae and the Chloropicophyceae, the latter containing two novel genera, Chloropicon and Chloroparvula, and eight new species of marine picoplanktonic green algae.

Tyge Christensen Prize 2006

The Tyge Christensen Prize, announced in 1997 (Phycologia 36:1), is awarded annually for the best paper published in Phycologia. The Prize consists of a cash award of US

Optical characteristics of E. huxleyi in an axenic tank culture

1000 and commemorative certificates/plaques for each author. Judging of the awards is based on scientific significance, originality in subject matter and/or techniques, comprehensiveness, and clarity of presentation. The International Phycological Society is pleased to announce that the Tyge Christensen Prize for the 2006 publication year has been won by the following paper:

Origins of plastids and glyceraldehyde-3-phosphate dehydrogenase genes in the green-colored dinoflagellate Lepidodinium chlorophorum.

The aim of this study is to find the algorithms for estimating the biomass of the Emiliania huxleyi from the irradiance data of the sea water surface. For this purpose, the relationships between the biomass of the Emiliania huxleyi in an axenic tank culture and the water irradiance data measured by a spectroradiameter were found. During the growing phase and stationary phase of the Emiliania huxleyi, the total attenuation, absorption and back scattering coefficients of the water in the tank were calculated from the irradiance data. The ratio of the water irradiance data at 670nm and 814nm were well correlated with the total cell concentration of the Emiliania huxleyi. It is expected to help for the biomass estimation of natural blooms in the ocean.