Surface properties of SAR11 bacteria facilitate grazing avoidance

Ayelet Dadon-Pilosof, Keats R. Conley, Yuval Jacobi, Markus Haber, Fabien Lombard, Kelly R. Sutherland, Laura Steindler, Yaron Tikochinski, Michael Richter, Frank Oliver Glöckner, Marcelino T. Suzuki, Nyree J. West, Amatzia Genin, Gitai Yahel

Oceanic ecosystems are dominated by minute microorganisms that play a major role in food webs and biogeochemical cycles (1) . Many microorganisms thrive in the dilute environment due to their capacity to locate, attach to, and use patches of nutrients and organic matter (2,3) . We propose that some free-living planktonic bacteria have traded their ability to stick to nutrient-rich organic particles for a non-stick cell surface that helps them evade predation by mucous filter feeders. We used a combination of in situ sampling techniques and next-generation sequencing to study the biological filtration of microorganisms at the phylotype level. Our data indicate that some marine bacteria, most notably the highly abundant Pelagibacter ubique and most other members of the SAR 11 clade of the Alphaproteobacteria, can evade filtration by slipping through the mucous nets of both pelagic and benthic tunicates. While 0.3 µm polystyrene beads and other similarly-sized bacteria were efficiently filtered, SAR11 members were not captured. Reversed-phase chromatography revealed that most SAR11 bacteria have a much less hydrophobic cell surface than that of other planktonic bacteria. Our data call for a reconsideration of the role of surface properties in biological filtration and predator-prey interactions in aquatic systems.In situ sampling reveals that members of the SAR11 clade show significantly lower retention by mucous filter feeders, and that this is probably due to their reduced hydrophobic cell surface, suggesting that cell surface properties are important factors in predator-prey interactions.