Advantages to microbes of growth in permeable aggregates in marine systems1

Highly porous microbial aggregates occur in a variety of environments ranging from marine snow in oceans to flocs in biological reactors. For permeable aggregates undergoing gravitational settling or subjected to fluid shear, predicted velocities of intra‐aggregate flow range from 1 to 100 µ m s −1 . Estimation of intra‐aggregate velocities requires specification of the aggregate size, porosity, and permeability, as well as a mean fluid shear rate characterizing the fluid turbulence. We have examined substrate removal by microorganisms in permeable aggregates with an analysis of mass transfer. The overall uptake by bacteria in aggregates can be up to 60% greater than uptake by dispersed bacteria. In general, a model of substrate uptake based on advective transport is more appropriate than a model based on diffusive transport for cells in large, permeable aggregates that utilize substrates with diffusivities <10 −6 cm 2 s −1 .