TEP Production and Aggregate Formation in Eukaryotic Picophytoplankton

Picophytoplankton (phytoplankton ca. ≤2 µm) dominate phytoplankton biomass in the oligotrophic subtropical gyres, but due to their small size, have traditionally been viewed as contributing little to export production. Recent studies have shown that picocyanobacteria can produce transparent exopolymeric particles (TEP), the matrix of sinking particles, and form aggregates, increasing their effective size; however, whether eukaryotic picophytoplankton are also able to produce TEP and aggregate is unknown. Here, we test the hypothesis that eukaryotic picophytoplankton form aggregates via the production of TEP and attachment to ballasting minerals. We investigated the production of TEP and formation of suspended (ca. ≤60 µm) aggregates in batch cultures of the prasinophyte Micromonas pusilla and the diatom Minidiscus trioculatus, species that had previously been identified in sinking particles collected in the Sargasso Sea. We also used roller tanks to investigate the formation of visible (ca. ≥0.1 mm) sinking aggregates. M. pusilla produced little TEP (45 ± 5 × 10^–9 µg XG eq. µm^–3) and did not form aggregates, even with the addition of ballasting minerals. M. trioculatus produced more TEP (371 ± 35 × 10^–9 µg XG eq. µm^–3) and suspended aggregates in batch cultures than M. pusilla, and formed visible aggregates in roller tanks. We will also discuss the role of heterotrophic bacteria in TEP production and aggregation of these picoeukaryotes. Our study contributes to the mechanistic understanding of the role of eukaryotic picophytoplankton in the biological carbon pump.