Do microbial numbers count?: Quantifying the regulation of biogeochemical fluxes by population size and cellular activity

In order to enhance understanding of the interrelationships among community members and between them and their environment, the concept of regulation analysis is extended from biochemistry into microbial ecology. Ecological regulation analysis quantifies how biogeochemical fluxes are regulated by the microorganisms performing the process; the degree to which changes in fluxes are due to changes in population size and to changes in activity cell(-1) (cellular activity). Regulation analysis requires data on biogeochemical fluxes and the numbers of cells through which these fluxes run. Its application to five biogeochemical processes (aerobic methane oxidation, aerobic nitrite oxidation, methanogenesis, sulfate reduction and reductive dehalogenation) revealed that in general, but not always, flux was primarily regulated by cellular activity, i.e. by changes in the size and properties of the enzyme pool and in the concentrations of substrates and metabolites. Thus, it is often not sufficient to count the numbers of cells performing a particular step in a biogeochemical process in order to estimate its flux. Ecological regulation analysis can be extended to address which aspects of cellular activity require quantification in order to describe biogeochemical fluxes better. Its application is discussed in the context of the complexity of microbial communities (e.g. functional redundancy) and their functioning.