Bacterial capture and lysis on diatom spines reveal a suspension-feeding strategy
Phytoplankton contribute nearly half of global primary production and play a central role in ocean biogeochemistry. Although traditionally viewed as phototrophs, a growing body of evidence indicates that many phytoplankton are mixotrophic, supplementing photosynthesis with heterotrophic acquisition of nutrients and organic matter. Yet, the mechanisms enabling heterotrophic feeding in diatoms remain poorly understood. Here, we show that the chitinous spines of the diatom Conticribra weissflogii can serve as attachment sites for the bacterium Marinobacter adhaerens. Cryo-electron tomography and viability staining revealed that a substantial fraction of spine-associated M. adhaerens undergoes lysis. Consistent with bacterial lysis on the spines, Raman spectroscopy showed that diatom cells bearing spine-associated bacteria acquired more bacteria-derived nitrogen than uncolonized cells. Although microfluidic assays revealed a low bacterial affinity for the spines, our results suggest that attachment can be promoted by fucoidan deposits identified on the spines - a compound for which bacteria showed significantly higher binding affinity than chitin - as well as by their geometry, which encounter rate calculations show to be particularly efficient at intercepting bacteria. Together, these findings reveal a previously undescribed form of suspension-feeding in phytoplankton, reminiscent of the role of zooplankton pseudopods in prey capture. Diatom spines intercept bacteria from the surrounding water, retain them near the cell surface, and facilitate subsequent nutrient acquisition, probably through bacterial lysis. Our results expand the functional repertoire of diatom spines beyond defense and buoyancy, identifying spine-mediated bacterial capture as a previously overlooked pathway contributing to phytoplankton mixotrophy.