Microbial community assembly on micro-scale marine particles
At ecosystem scales, particulate organic matter represents one of the major reservoirs of carbon in the planet, transporting fixed CO2 from surface waters to the ocean floor. At micrometer scales, particles represent both nutrient patches and habitat on which heterotrophic bacteria colonize and assemble in dense communities. Within these communities local interactions via secondary metabolites or degradation by-products can control community dynamics and productivity, thereby establishing a direct link between micrometer scale ecology and global ecosystem processes. To study these micro-scale ecological processes we developed an experimental system based on artificial biopolymer particles of defined composition and size. These particles are introduced in coastal waters and act as “baits” on which we can harvest microbial communities as scales of ~50µm to study the role of local ecological interactions in shaping community structure and dynamics.
By tracking community composition during particle degradation we found that populations dynamics is ordered in a succession of ‘species’ that attach, grow and then migrate out of particles, all in a highly reproducible manner despite the high diversity of the community. Further analysis of the traits associated with this dynamics, both with cultured isolates as well as with meta-genomics, showed that early community members were able to chemotax towards particles and produce the hydrolytic enzymes that initiate the successional cascade. On the other hand, late community members were dominated by organisms that were limited by dispersal and depended on local neighbors for their growth on particles. Analysis of pairwise interactions between these bacteria suggests that strong facilitation, and not antagonism, dominates the interaction network of the community and that these positive interactions could increase the total productivity of the system. These results thus show that important ecosystem processes can be controlled by local interactions networks at micron scales and that synergistic interactions between microbes are common and can have major effects on the performance of multi-species systems.