Tiny islands: Colonization and extinction of microbial species on marine aggregates

Background/Question/Methods

Island biogeography theory argues that the balance of colonization and extinction determines species richness, and this theory has been invoked hundreds of times on systems from oceanic islands to lakes. The relevance of this theory to microorganisms has been more rarely tested. Here we examine how organic aggregates (also referred to as “marine snow” or bioflocs) may act as microscopic islands for bacteria in aquatic environments. Importantly, we are able to build upon empirically parameterized models to examine the colonization and extinction process mechanistically. We developed a multi-species, stochastic extension of a deterministic whole-community model of bacterial colonization and trophic dynamics on aggregates. The influence of aggregate size and bacterial concentration, i.e. the island size and distance from mainland, could then be considered. Specifically, we developed a birth-death-colonization model that was solved with by simulation using an approximation of Gillespie’s direct method.

Results/Conclusions

We found that while colonization rates increase linearly with aggregate size and density of potential colonists, extinction rates are non-linear on small aggregates in a low background density of colonists. Under these conditions the species have a low probability of presence on the aggregates and a low abundance when present. The observed extinction rates are a result of bacterial traits, including the propensity to permanently attach to aggregates, and of the mortality imposed by flagellate predators. By providing mechanistic understanding of how island size interacts with biotic factors to control species presence, the model provides testable predictions for bacterial dynamics on aggregates. Further, the nonlinear extinction rates allow us to predict what direction the species equilibrium will shift and what the magnitude will be with respect to a change in aggregate size, providing insight into species persistence and community dynamics in these microbial communities. Predicting these dynamics is also applicable to determining the potential role of aggregates as vectors that may enhance transmission and persistence of waterborne pathogens.