Differing Escape Responses of the Marine Bacterium Marinobacter adhaerens in the Presence of Planktonic vs. Surface-Associated Protist Grazers

Protist grazing pressure plays a major role in controlling aquatic bacterial populations, affecting energy flow through the microbial loop and biogeochemical cycles. Predator-escape mechanisms might play a crucial role in energy flow through the microbial loop, but are yet understudied. For example, some bacteria can use planktonic as well as surface-associated habitats, providing a potential escape mechanism to habitat-specific grazers. We investigated the escape response of the marine bacterium <i>Marinobacter adhaerens</i> in the presence of either planktonic (nanoflagellate: <i>Cafeteria roenbergensis</i>) or surface-associated (amoeba: <i>Vannella anglica</i>) protist predators, following population dynamics over time. In the presence of <i>V. anglica</i>, <i>M. adhaerens</i> cell density increased in the water, but decreased on solid surfaces, indicating an escape response towards the planktonic habitat. In contrast, the planktonic predator <i>C. roenbergensis</i> induced bacterial escape to the surface habitat. While <i>C. roenbergensis</i> cell numbers dropped substantially after a sharp initial increase, <i>V. anglica</i> exhibited a slow, but constant growth throughout the entire experiment. In the presence of <i>C. roenbergensis</i>, <i>M. adhaerens</i> rapidly formed cell clumps in the water habitat, which likely prevented consumption of the planktonic <i>M. adhaerens</i> by the flagellate, resulting in a strong decline in the predator population. Our results indicate an active escape of <i>M. adhaerens</i> via phenotypic plasticity (i.e., behavioral and morphological changes) against predator ingestion. This study highlights the potentially important role of behavioral escape mechanisms for community composition and energy flow in pelagic environments, especially with globally rising particle loads in aquatic systems through human activities and extreme weather events.