Abstract
Mamiellophyceae (unicellular green algae) are a key phytoplankton group in coastal waters. Although extensively studied over the last 20 years, the overall oceanic distribution of the major species/clades is still poorly known. To address this problem, we analyzed the 2014 Ocean Sampling Day (OSD) metabarcoding dataset providing sequences from the V4 hypervariable region of the 18S rRNA gene for 157 samples collected at 143 mostly coastal stations. Mamiellophyceae were found at nearly all OSD stations and represented 55% of the green microalgae (Chlorophyta) reads. We performed phylogenetic analyses of unique OSD metabarcodes (amplicon single variants, ASVs) and GenBank reference sequences from cultures and from the environment, focusing on the four most represented genera: Ostreococcus (45% of the Mamiellophyceae reads), Micromonas (34%), Bathycoccus (10%) and Mantoniella (8.7%). These analyses uncovered novel diversity within each genus except Bathycoccus. In Ostreococcus, a new clade (E) was the second most represented clade after Ostreococcus “lucimarinus”. Micromonas could be separated into nine clades, exceeding the six species and candidate species already described. Finally, we found two new environmental clades within Mantoniella. Each Mamiellophyceae clade had a specific distribution in the OSD dataset suggesting that they are adapted to different ecological niches.
Highlights
Mamiellophyceae consists of three orders: Mamiellales, Dolichomastigales and Monomastigales[7]
We focused on the LGC dataset which encompasses a much larger number of samples than the Life Watch (LW) dataset which corresponds to a subset of the Ocean Sampling Day (OSD) samples that were processed in a completely independent manner compared to the LGC dataset
Mamiellophyceae are clearly the dominant group of green algae in coastal waters[25,26], previous analysis could not relate their relative abundance to any environmental variable[25]
Summary
Mamiellophyceae consists of three orders: Mamiellales, Dolichomastigales and Monomastigales[7]. The European Ocean Sampling Day (OSD) project sampled global coastal waters in 2014 at approximately the summer solstice (21 June) with the aim of analyzing the diversity and distribution of marine microorganisms[23] especially in coastal waters using 18S rRNA metabarcodes (V4 and V9 hypervariable regions)[24]. This dataset allowed the distribution of fourteen classes of Chlorophyta to be determined[25]. Our analyses reveal the existence of novel clades within Ostreococcus, Micromonas and Mantoniella and that most species/clades have specific oceanic distributions
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