Quantifying long-term recurrence in planktonic microbial eukaryotes.

How much temporal recurrence is present in microbial assemblages is still an unanswered ecological question. Even though marked seasonal changes have been reported for whole microbial communities, less is known on the dynamics and seasonality of individual taxa. Here, we aim at understanding microbial recurrence at three different levels: community, taxonomic group and operational taxonomic units (OTUs). For that, we focused on a model microbial eukaryotic community populating a long-term marine microbial observatory using 18S rRNA gene data from two organismal size fractions: the picoplankton (0.2-3 µm) and the nanoplankton (3-20 µm). We have developed an index to quantify recurrence in particular taxa. We found that community structure oscillated systematically between two main configurations corresponding to winter and summer over the 10 years studied. A few taxonomic groups such as Mamiellophyceae or MALV-III presented clear recurrence (i.e., seasonality), whereas 13%-19% of the OTUs in both size fractions, accounting for ~40% of the relative abundance, featured recurrent dynamics. Altogether, our work links long-term whole community dynamics with that of individual OTUs and taxonomic groups, indicating that recurrent and non-recurrent changes characterize the dynamics of microbial assemblages.