<i>Vibrio cholerae</i> interaction with predatory bacteria on chitin suggests an alternative mode of biofilm formation in marine snow conditions

<i>Vibrio cholerae</i> is a ubiquitous marine microbe that solubilizes and consumes chitin in the marine water column. In both the marine environment and the intestinal track, <i>V. cholerae</i> forms biofilms; a key question regarding the lifestyle of <i>V. cholerae</i> is how do the diverse substrates that it encounters influence its biofilm formation and, in turn, shape its ecological interactions. Here, we use the predator-prey interaction between <i>Bdellovibrio bacteriovorus</i> and <i>V. cholerae</i> as a model to explore how the environmental chitin substrate alters <i>V. cholerae</i> biofilm formation and predator-prey interactions. We find that glass-bound biofilms provide strong protection for <i>V. cholerae</i> against predation while also allowing a population of predatory <i>B. bacteriovorus</i> to remain in place. In contrast, chitin-bound biofilms offer less protection against <i>B. bacteriovorus</i> predation and do not maintain a stable population of <i>B. bacteriovorus</i>. Using percolation and population dynamics models, we predict that these changes in predator-prey dynamics can be mostly explained by alterations in biofilm architecture between the two conditions, which changes the fraction of prey available to <i>B. bacteriovorus</i>. Performing targeted biofilm matrix deletions, we confirm this prediction by recapitulating key features of the chitin predator-prey interactions on glass surfaces. Following on this observation, we show that <i>V. cholerae</i> biofilms grown on chitin produce much less of the canonical biofilm matrix components and instead rely on other extracellular structures. Overall, our experiments detail how growth substrate can alter biofilm matrix composition and how these changes in biofilm architecture and cellular arrangement can impact higher-order ecological interactions.