Bacteria and other microorganisms play a major part in the food webs of the open ocean. Benthic ascidians and planktonic appendicularians are small, free-floating marine creatures that take in nutrients by filtering seawater through sieve-like structures made of mucous. Microorganisms that become trapped in the mucosal structures are consumed by the filter feeders and later expelled in fecal pellets that can sink to the bottom of the ocean. Little research concerns the precise interactions between these organisms and their prey. Our study explores mucosal filter feeder action in greater depth to improve understanding of marine microbial relationships and the nutrient cycles of open ocean ecosystems.

 

We investigated the filtering capabilities of benthic ascidians and planktonic appendicularians in situ (in their natural habitats). Using flow cytometry (a technology that uses laser light to measure the number, size and other properties of microscopic particles in a fluid), we measured the microbial content of the seawater before and after it was filtered by the organisms. We then used DNA sequencing to determine which species of microorganisms were efficiently trapped by the mucous nets and which were able to pass through unharmed. Finally, we analyzed the various microbes using reversed-phase chromatography (a process that measures cell stickiness).

 

Key Findings:

• SAR11 marine bacteria were not captured in the salp mucous mesh. 

• Particles smaller in size than SAR11 bacteria were trapped and processed by the filter feeders, suggesting that factors other than size affect filtration. 

• SAR11 bacteria and other marine bacteria that were able to pass through the mucous mesh had lower HIC indexes, meaning that they had non-sticky cell surfaces.

• Some marine bacteria may have traded their ability to stick to nutrient-rich organic particles for a non-stick cell surface that helps them avoid being consumed by mucous filter feeders. 

 

Our findings demonstrate that size and shape are not the sole factors involved in mucous filter  feeding. We hypothesize that cell surface characteristics play a key role in this process and that certain microorganisms—particularly, SAR11 bacteria—have adapted non-stick cell surfaces that allow them to slip through the mucous mesh. Since non-sticky cell surfaces reduce bacteria’s ability to adhere to (and consume) nutrient-rich organic particles, we conclude that the SAR11 bacteria likely have made an evolutionary trade-off in order to avoid predation. Overall, these results suggest that the predator–prey interactions of oceanic microorganisms are much more complex than previously thought.

Dadon-Pilosof, Ayelet, Keats R. Conley, Yuval Jacobi, Markus Haber, Fabien Lombard, Kelly R. Sutherland, Laura Steindler, Yaron Tikochinski, Michael Richter, Frank Oliver Glӧckner, Marcelino T. Suzuki, Nyree J. West, Amatzia Genin, and Gitai Yahel. "Surface Properties of SAR11 Bacteria Facilitate Grazing Avoidance." Nature Microbiology 2, no. 12 (October 2017): 1608-1615. https://doi.org/10.1038/s41564-017-0030-5

 

.Sampling of water inhaled and exhaled by a filter feeder.

Image courtesy of T. Shlesinger, taken in the Red Sea.