Chemotaxis, growth, and inter-species interactions shape early bacterial community assembly.

From the perspective of a marine copiotrophic bacterium, the surface ocean is a mosaic of exploitable hotspots of organic matter released from living and senescing phytoplankton. Bacterial success in exploiting this patchy microscale environment relies on sensing and swimming towards the hotspots, and upon arrival, growing on available substrates. However, the combined effect of chemotaxis and growth rate on bacterial community assembly has never been quantified. Here, we characterized chemotaxis and growth rate responses of seven representative copiotrophic marine bacteria to phytoplankton exometabolites, both for single species and for pairs of species. We compared these results to prediction of a mathematical null model of hotspot community assembly. Our results revealed that the bacterial strains exhibit diverse responses to phytoplankton metabolites, which can act as either signals, substrates, or both. Interactions between bacterial pairs resulted in chemotactic responses or growth rates different from model predictions in all of the 12 pairs tested (83% differed in chemotaxis, 33% in growth). These community dynamics indicate that inter-species interaction is another factor shaping early colonization of metabolite hotspots, with beneficial, detrimental, and neutral associations observed between bacterial species. Such complex ecological interactions impact chemotactic behaviors and growth rates of marine bacteria on resource hotspots, affecting their community compositions and associated functions relevant to the cycling of key elements including carbon.