Oceanic ecosystems are dominated by minute microorganisms that play a major role in food webs and biogeochemical cycles . Many microorganisms thrive in the dilute environment due to their capacity to locate, attach to, and use patches of nutrients and organic matter . 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.

Suspended food particles in oligotrophic waters are scarce and the planktonic community is dominated by minute picoplanktonic (< 2 lm) cells. Consequently, suspension feeders inhabiting such environments are faced with the dual challenge of capturing extremely small particles and efficiently processing large volumes of water. We used direct in situ techniques to characterize the size-dependent capture efficiency curve of four solitary ascidian species (Chordata, Tunicata) from the ultra-oligotrophic East-Mediterranean, and two species from the oligotrophic Gulf of Aqaba (Northern Red-Sea). To control for confounding factors such as particle surface chemistry, we used a suspension of artificial polystyrene microspheres with a wide range of sizes (from 0.3 lm to 10 lm) but identical surface chemistry. This suspension was introduced into the inhalant siphon while the water inhaled and exhaled by the otherwise undisturbed ascidian was cleanly sampled. All the investigated ascidian species captured ! 1 lm microspheres at 95% 6 8% efficiency (mean 6 95% Confidence Interval) with no significant difference between species. Moreover, and in contrast to reports from eutrophic waters, both in situ and laboratory experiments revealed an efficient capture of submicron 0.3 lm microspheres at close to 50% efficiency. For most of the species tested, the capture efficiency of microspheres dropped below optimum between 1 lm and 0.5 lm providing an estimation of their mucus mesh pore size. We suggest that ascidians from oligotrophic waters are adapted for capturing submicron particles picoplankton and other. Such adaptation enables "oligotrophic" ascidians to access a much larger food source and facilitates the transfer of planktonic C, N, and P from the water column to the benthos.

Direct measurements of the capture efficiency of planktonic cells by seven solitary ascidians were made in situ and in the laboratory and compared with the capture efficiency of polystyrene microspheres. The capture efficiency of the microspheres was significantly higher than that of planktonic cells over the entire tested size range (0.3–15 μm). Submicron polystyrene spheres with a surface modification consisting of an adsorbed layer of a nonionic, long‐chain surfactant were removed at lower efficiencies than uncoated particles whereas for larger microspheres (1–3 μm), the coating had no effect. Our findings strengthen the concept that some planktonic cells evade capture by mucus‐based suspension feeders, and that evasion happens throughout the pico‐ and nanoplankton size range. Thus, the common assumption that particles larger than ~ 1 μm are always captured at a 100% efficiency by ascidians should be reconsidered. Some large microalgae cells (> 3–12 μm) were captured at a lower efficiency than the largest microspheres used (3 μm) suggesting that other factors, such as surface interactions and particle shape, play an important role in capture throughout the tested size range. Furthermore, given the lack of a known active selection mechanism in ascidians, we propose that some plankton possess traits that allow them to evade predation by mucus‐based suspension feeders.