Chemotaxis toward phytoplankton drives organic matter partitioning among marine bacteria

Significance Microscale interactions between bacteria and phytoplankton underpin ocean biogeochemistry and frequently involve bacterial chemotaxis to phytoplankton dissolved organic matter (DOM). Yet, it remains unclear how the effects of this interaction propagate to ecosystem scales. We address this gap through a hybrid approach where high-resolution observations of chemotaxis toward a diatom are directly used in a resource utilization model. We find that chemotactic bacteria consume most diatom DOM under resource-rich or bacteria-rich conditions, that DOM is partitioned among distinct populations based on diffusivity, and that consumption is skewed toward very few cells. Nonmotile oligotrophic bacteria dominate when productivity is low. Motile copiotrophs dominate during blooms. Ocean chemotaxis thus partitions resources spatially, by molecular size, and temporally through seasonal and episodic blooms.