Theory of island biogeography on a microscopic scale: organic aggregates as islands for aquatic pathogens

Four predictions of the MacArthur-Wilson theory of island biogeography were evaluated to assess the degree to which detrital-based organic aggregates (e.g. marine snow, organic detritus, and bioflocs) may provide a favorable microhabitat (i.e. an 'island') for bacteria in general, and specifically aquatic pathogens. We demonstrate the theory's relevance for microbial communities in aquatic environments by describing the community metabolic response and functional diversity of individual organic aggregates while documenting the persistence of potential pathogens and fecal indicator bacteria. Our results support the 4 predictions, including a significant species-area relationship, consistency of species richness at equilibrium, non-zero level of species turnover at equilibrium, and variance to mean ratios of less than 0.5 at equilibrium. The aggregate-associated microbial communities demonstrated significantly higher rates of metabolic response and functional diversity, and contained higher concentrations of culturable vibrios and fecal indicator bacteria compared to aggregate-free water, supporting the idea that organic aggregates are sites of favorable habitat surrounded by a less favorable matrix. These results substantiate that organic aggregates may be represented as microscopic islands. Using island biogeography theory to understand the microbial ecology of aquatic pathogens associated with organic aggregates is important with respect to environmental sampling of recreational waters and mathematical modeling of the transmission of waterborne diseases from aquatic reservoirs to humans.