Role of Particle-Associated Bacteria in Aggregate Formation in the Current and Future Ocean

The formation of marine snow is an important part of the biological carbon pump as carbon is exported to depth. These particles are composed of cells, minerals, and other sources of organic carbon that can be held together by transparent exopolymeric particles (TEP) acting as a glue-like matrix. Due to their small size, heterotrophic bacteria were previously expected to not contribute appreciably to aggregation. However, heterotrophic bacteria have been found to influence the aggregation of phytoplankton and in some cases result in an increase in TEP production, but it is unclear exactly how they contribute to aggregation. We determined the aggregation potential of Pseudoalteromonas carrageenovora, Vibrio thalassae, and Marinobacter adhaerens HP15 through growth experiments with axenic cultures to determine growth, TEP production, and aggregation. The bacteria were also inoculated into roller tanks to determine their ability to form macroaggregates. M. adhaerens HP15 had the lowest cell-normalized TEP production at all growth stages and the lowest microaggregate formation. The cell-normalized TEP production and microaggregate formation was not significantly different between P. carrageenovora and V. thalassae. All bacteria formed visible macroaggregates in roller tanks with clay addition and exhibited high sinking velocities (150-1200 m d-1) that are comparable to those of aggregates formed by large mineral ballasted phytoplankton. Experiments involving the simulation of ambient and future ocean conditions using natural seawater in mesocosms have revealed a stark difference in macroaggregate formation between treatments, potentially indicating a difference in microbial community and TEP composition. These findings expand our understanding of how heterotrophic bacteria can contribute to export flux in the ocean.