Marine snow microbial communities: scaling of abundances with aggregate size

Marine aggregates are inhabited by diverse microbial communities, and the concentration of attached microbes typically exceeds concentrations in the ambient water by orders of magnitude. An extension of the classical Lotka-Volterra model, which includes 3 trophic levels (bacteria, flagellates, ciliates) and considers colonization, detachment, growth and predator-prey interactions on the surface of the particle, was used to examine the processes that govern abundances of attached micro-organisms. Effects of sinking on colonization rates as well as the fractal nature of natural aggregates were also taken into account. As input for the model, I used experimentally determined encounter and detachment rates, and density-dependent growth and grazing rates, as well as information on relevant properties of natural aggregates, all taken from the literature. The model reproduces the temporal development of attached populations of bacteria, flagellates, and ciliates, as observed in experimental systems, and also predicts steady-state abundances of attached micro-organisms that are close to those observed on field-collected aggregates. The model suggests that attached bacterial populations are controlled by flagellate grazing, while flagellate and ciliate populations are governed by colonization and detachment. The model also suggests that microbial populations are turned over rapidly (1 to 20 times d-1) due to continued colonization and detachment. The model overpredicts somewhat the scaling of microbial abundances with aggregate size observed in field-collected aggregates. This may be because it disregards the aggregation/disaggregation dynamics of aggregates, as well as interspecific interactions between bacteria.