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

Abstract

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.

Comparison of coastal phytoplankton composition estimated from the V4 and V9 regions of the 18S rRNA gene with a focus on photosynthetic groups and especially Chlorophyta

“…S9A). BLAST analysis and alignment of Chlorophyta OTUs (data not shown) revealed that the V9 region of some Chlamydomonas is very similar to that of prasinophytes clade VII A5 (Lopes dos Santos et al ., ). Interestingly, the number of reads recovered in V4 and V9 for these 2 assignations (i.e., Chlamydomonas sp.…”

“…For Chlorophyta, average similarity is in general lower in V9 than V4 (Tragin et al ., ), which suggests that V9 will be more discriminating than V4 and will be the best choice. This is the case for example for prasinophytes clade VII, an important oceanic group, for which the use of 99% threshold for V9 OTUs allows to discriminate all sub‐clades defined to date (Lopes dos Santos et al ., ), while in V4, several clades collapse together, having identical sequences in that region. The V9 region of some Chlamydomonas is very similar to that of prasinophytes clade VII A5, which could lead to misinterpret the distribution of this specific sub‐clade when using the V9 region.…”

Quantitative biogeography of picoprasinophytes establishes ecotype distributions and significant contributions to marine phytoplankton

“…Recent 18S V9 amplicon‐based studies (de Vargas et al ., ; Lopes Dos Santos et al ., ) cannot discriminate between Ostreococcus OI and OII (Monier et al ., ), or the two Bathycoccus , and have yet to be evaluated in the context of abundance data generated by quantitative methods. These amplicon data have been used to infer which green algal taxa dominate in the open ocean, specifically observing that Mamiellophyceae contribute only 1.8% of total photosynthetic sequences and that other prasinophytes, that have fourfold higher relative amplicon percentages, are therefore the dominant group (Lopes Dos Santos et al ., ). Here, we performed a quantitative enumeration of BI and BII and analysed these in the context of OI and OII abundances and eukaryotic phytoplankton cell counts.…”

Improvement of phytoplankton culture isolation using single cell sorting by flow cytometry

“…We obtained strains of several taxa that, to our knowledge, have not previously been cultured. Within prasinophyte clade VII, we obtained for the first time a representative of clade VIIB3 (Lopes dos Santos et al., ). In addition, three Pelagophyceae cultures corresponded to undescribed taxa.…”