Bacteria-mediated aggregation of the marine phytoplanktonThalassiosira weissflogiiandNannochloropsis oceanica

The ecological relationships between heterotrophic bacteria and marine phytoplankton are complex and multifaceted, and in some instances include the bacteria-mediated aggregation of phytoplankton cells. It is not known to what extent bacteria stimulate aggregation of marine phytoplankton, the variability in aggregation capacity across different bacterial taxa or the potential role of algogenic exopolymers in this process. Here we screened twenty bacterial isolates, spanning nine orders, for their capacity to stimulate aggregation of two marine phytoplankters,Thalassiosira weissflogiiandNannochloropsis oceanica. In addition to phytoplankton aggregation efficiency, the production of exopolymers was measured using Alcian Blue. Bacterial isolates from theRhodobacterales,FlavobacterialesandSphingomonadalesorders stimulated the highest levels of cell aggregation in phytoplankton cultures. When co-cultured with bacteria, exopolymer concentration accounted for 34.1% of the aggregation observed inT. weissflogiiand 27.7% of the aggregation observed inN. oceanica. Bacteria-mediated aggregation of phytoplankton has potentially important implications for mediating vertical carbon flux in the ocean and in extracting phytoplankton cells from suspension for biotechnological applications.