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.

Figure 1

ML tree inferred from nuclear 18S rRNA sequences belonging to prasinophytes clade VII. Nodes supported by NJ, ML and Bayesian methods are shown by a solid dot (▪). Nodes supported by ML and Bayes only are indicated by a gray dot (). Empty dot (○) represents a node supported by only one method, either ML or Bayes.

Figure 2

ML tree inferred from nuclear 16S rRNA sequences belonging to prasinophytes clade VII. Nodes supported by NJ, ML and Bayesian methods are shown by a solid dot (▪).

Figure 3

(a) Average contribution of the different groups of Chlorophyta in Tara Oceans (average of the three size fractions 0.8–5, 5–20 and 20–180 μm) and in OSD (0.8–200 μm fraction). (b) Average contribution of the different groups of Chlorophyta in the Tara Oceans data set as a function of size fraction in surface (left) and at the DCM (right).

Figure 4

Map of the contribution of Chlorophyta to photosynthetic sequences (size of circles) and dominant Chlorophyta group (color of circles) for Tara Oceans (0.8–5 μm) and OSD samples in surface samples. Circles with black border correspond to OSD stations.

Figure 5

Top graph: contribution of the different clades of prasinophytes clade VII to the smallest size fraction (0.8–5 μm) at each Tara stations. Bottom graph: relative contribution of clade VII sequences to the total photosynthetic sequences. (a) Surface. (b) DCM.

Figure 6

Abundance of the prasinophytes clade VII clades as a function of depth (a) and temperature (b). The radius of the circles is proportional to the contribution of clade to the total number of photosynthetic sequences.