No evidence of Phago-mixotropy in Micromonas polaris (Mamiellophyceae), the Dominant Picophytoplankton Species in the Arctic

In the Arctic Ocean, the small green alga Micromonas polaris dominates picophytoplankton during the summer months but is also present in winter. It has been previously hypothesized to be phago-mixotrophic (capable of bacteria ingestion) based on laboratory and field experiments. Prey uptake was analyzed in several M. polaris strains isolated from different regions and depths of the Arctic Ocean and in Ochromonas triangulata, a known phago-mixotroph used as a control. Measuring ingestion of either fluorescent beads or fluorescently labeled bacteria by flow cytometry, we found no evidence of phago-mixotrophy in any M. polaris strain while O. triangulata was ingesting both beads and bacteria. In addition, in silico predictions revealed that members of the genus Micromonas lack a genetic signature of phagocytotic capacity.