Advantages to microbes of growth in permeable aggregates in marine systems l

Highly porous microbial aggregates occur in a variety of environments ranging from marine snow in oceans to floes 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 pm s I. Estimation of intra-aggregate velocities requires spccihcation 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 dispcrscd 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 < 1 Op6 cm2 s-l. Microbial attachment to surfaces, particles, and other organisms is a ubiquitous phenomenon that has been studied in various environments. In natural environments bacteria inhabit large, amorphous aggregates that can contain at various times almost every species present in the water column (Silver et al. 1978; Alldredge 1979). Large, amorphous aggregates can be composed of bacteria attached to zooplankton fecal pellets, bacteria attached to each other via long polymeric fibers, and bacteria, phytoplankton, and other suspended material ’ attached to the cast houses of mucous net feeders (Prezelin and Alldredge 1983; Pomeroy 1984). In engineered systems, such as wastewater treatment reactors, suspended aggregates (floes) occur as a complex mass of inert particles, bacteria, and protozoa. Microbial aggregation also occurs in pure cultures. In the fermentation industry, routine use is made of yeasts that flocculate to achieve inexpensive separation of biomass from the culture fluid. The reasons why microorganisms attach I This research was supported by U.S. Environmental Protection Agency (USEPA) grant R-808382 to the University of California at Berkeley. This article has not been subjected to USEPA peer review. is an active area of research. ZoBell (1943) showed that surfaces readily concentrate bacteria and Jannasch and Pritchard (1972) found that particles introduced into batch cultures could stimulate growth rates. Researchers continue to investigate the association of microorganisms with suspended matter (Melchiorri-Santolini and Hopton 1972), surfaces (Characklis 198 I), and other microorganisms (Calleja 1984). The difficultics in understanding microbial attachment stem from both the multiplicity of attachment mechanisms and a failure to define the nutritional and environmental advantages of attachment. Large, amorphous aggregates arise from a dynamic balance among coagulation of small particles by fluid shear or differential sedimentation, breakup of larger aggregates, and removal by settling (Hunt 1980; McCave 1984), although distinguishing mechanisms from field data is difficult. Rising bubbles can also concentrate particles in the water column (Weber et al. 1983) and may aid in aggregate formation (Baylor and Sutcliffe 1963; Barber 1966). Particle collisions will result in the formation of aggregates only if the particles remain in contact. In marine systems, high salt concentrations destabilize particles and allow attachment via compression of the electrical double lay-