Bacteria-protozoan interactions in a mixed-species biofilm community

Abstract

Environmental biofilms typically consist of mixed-species communities exhibiting complex inter- and intra-species synergistic/antagonistic interactions. Intense competition in aquatic systems is driven by nutrient and space limitation. Bacterial biofilm members cooperate under such conditions to resist environmental perturbations, processes that subsequently determine community composition. This thesis reports on environmental bacterial screens for amensal/antagonistic metabolites against protozoa and nematodes. Further, defined mixed-species model communities isolated from the surface of the green alga Ulva australis were tested for their protozoan predation response under varying nutrient conditions. Biofilms exhibited higher levels of antiprotozoal and anthelmintic activity than their planktonic counterparts. Biofilm inhibitory activities were highly diverse, commonly targeting either nematodes or protozoa, but rarely targeting both with broad-spectrum activity. Predation resistance was explored using a model marine four-species community. Protozoan grazers of different feeding types were used on different-stage biofilms; Rhynchomonas nasuta (early-stage), Tetrahymena pyriformis and Acanthamoeba castellanii (intermediate- and late-stage). As single-species biofilms, all four species displayed varying levels of grazing. All were susceptible to R. nasuta whereas only Dokdonia donghaensis and Acinetobacter lwoffii were vulnerable to T. pyriformis. A. castellanii selectively preferred Shewanella japonica and A. lwoffii. Mixed-species biofilms facilitated synergistic and antagonistic interactions between consortia members. R. nasuta selectively preferred A. lwoffii, resulting in a ‘reverse grazer’ effect, allowing other members to benefit. T. pyriformis preferred grazing on D. donghaensis. The mixed-species community was resistant to A. castellanii grazing and thus commensal to D. donghaensis and A. lwoffii. Under high nutrient conditions, both three- and four-species biofilms resisted predation. By comparison, low nutrient conditions increased both mixed- and single-species biofilms susceptibility to A. castellanii grazing. Under low nutrient conditions, the removal of Microbacterium phyllosphaerae from the four-species consortium destabilized the community. Under low carbon conditions, S. japonica ii demonstrated increased grazing susceptibility, possibly due to resource reallocation from defence to growth/maintenance. Bacterial grazing resistance depends on survival strategies, e.g. the ability to exploit nutrients and the trade-off between growth and defence. The differential levels of grazing resistance of both mixed- and single-species consortia, resulting from ‘top-down’ and ‘bottom-up’ factors, affected community structure and composition. Successful application of the carbon:nutrient balance hypothesis to S. japonica's defence strategy, highlights the potential for eukaryotic theories as predictive models for microbial systems.