Presenter: Bianca Cruz (Arizona State University)

Description:
The aggregation of phytoplankton is an important process in marine biogeochemical cycles, as it leads to the settling of particulate organic carbon in the form of marine snow. Diatoms > 20 µm in size are considered to contribute appreciably to sinking particle fluxes as a result of aggregation and the production of transparent exopolymer particles (TEP), the matrix for marine snow particles; however, it is not known whether nano-sized (2-20 µm) diatoms are also able to aggregate and produce TEP. DNA metabarcoding and microscopy observations of particle trap material suggest a significant contribution of nano-sized diatoms to particle fluxes, but the mechanisms leading to this contribution have not been explored. Recent studies suggest the significance of interactions with heterotrophic bacteria in TEP production and aggregation, which if widespread, have significant impacts on ocean biogeochemistry by mediating particle fluxes. Here, we tested the aggregation and production of TEP by the nano-diatom Minutocellus polymorphus and investigated if interactions with bacteria play a role in this aggregation and production of TEP by comparing axenic M. polymorphus cultures with co-cultures of M. polymorphus and bacterial taxa known to colonize marine snow particles. We found that M. polymorphus forms sinking aggregates and produces TEP comparably to other phytoplankton groups. Additionally, we found that aggregation was significantly enhanced when in the presence of Marinobacter adhaerens HP15, but not in the presence of Pseudoalteromonas carrageenovora or Vibrio thalassae. We will further discuss the role of each bacterial species in the aggregation of M. polymorphus by investigating their production of TEP.