Despite their high abundance in oligotrophic ocean regions, small-celled phytoplankton (<5 µm) have traditionally been viewed as contributing little to export production due to their small size. However, recent studies have shown that the picocyanobacterium Synechococcus forms abundant microaggregates (5-60 µm) and produces transparent exopolymer particles (TEP), the sticky matrix of marine aggregates. TEP and aggregation were also shown to be enhanced under nutrient limited growth conditions. In this study we carried out experiments to test whether other small phytoplankton species exude TEP and form microaggregates, and if these are enhanced under growth-limiting conditions. The picocyanobacterium Prochlorococcus marinus (0.8 µm), the nanodiatom Minutocellus polymorphus (4 µm), and the picoprasinophyte Ostreococcus lucimarinus (0.6 µm) were grown in axenic batch culture experiments under nutrient replete and limited conditions. We found that nutrient limitation increased TEP concentration in all three species, and formation of microaggregates was significantly enhanced in Minutocellus and Ostreococcus in distinct size ranges. Surprisingly, Ostreococcus, the smallest eukaryote known, produced more TEP than Prochlorococcus and Minutocellus per unit cell volume. These findings show how nutrient limited conditions enhance TEP production and microaggregation of phytoplankton with some of the smallest cells, providing a mechanism for their incorporation into larger, sinking particles and contribution to export production in oligotrophic oceans.

Primary Presenter: Catrina Shurtleff, Arizona State University (cshurtle@asu.edu)

Authors:

Catrina Shurtleff, Arizona State University  (cshurtle@asu.edu)

Britni Livar, Arizona State University  ()

Susanne Neuer, Arizona State University  ()