Microbial Loop

Mounting evidence from studies of lakes and marine systems suggests that carbon flux through microbial food webs may be much greater than previously recognized (Pomeroy and Wiebe 1988). In these ecosystems, uptake of dissolved organic carbon (DOC) by bacteria is thought to be a significant energy pathway. The microbial biomass that results from DOC uptake is consumed by protozoans and micro-metazoans, and perhaps subsequently by macro-metazoans. There is much uncertainty about the amount of energy that reaches the "usual" metazoan food web by this route, for several reasons. Our knowledge of bacterial production is scant. It is suspected that several trophic transfers take place within the microbial web, with energy loss at each step. The extent of consumption of protozoans and micrometazoans by larger metazoans is unknown. Thus, it is possible that little of the biomass produced by bacterial growth ever reaches larger consumers. Instead, energy may be dissipated and minerals recycled within a largely distinct and separate "microbial loop." Even if no energy is transferred to larger consumers, however, the loop is still critically important in re-mineralizing organic matter (Edwards and others 1990).

In all likelihood, the microbial loop is of considerable significance in streams and rivers. The amount of carbon present as DOC and in FPOM in running water is substantial. Some portion of this energy source is processed by microbial populations, constituting a carbon flux about which little is known.