ABSTRACT
Net flux of sulfur from the inorganic mineral nutrient form into biomass, primarily protein, occurs only through the reductive sulfate assimilation pathway of microorganisms and plants. The sulfur content of bulk protein is essentially constant among all organisms, and the predominance of protein synthesis in sulfur metabolism makes it a good tracer of this crucial growth-related process. Incubations for sulfate assimilation experiments follow procedures common for almost any rate process measurement involving mixed microbial communities. After inoculation with S-sulfate, any of a variety of in situ, simulated in situ, light gradient, or dark incubation techniques at ambient temperature may be used. The high activities encountered in sulfate assimilation tracer studies require special attention to adsorption of unassimilated sulfate onto filters. The subcellular fractionation of labeled components of filtered cells yields information pertaining not only to protein synthesis rates but also to the autotrophic vs. heterotrophic contributions to total community protein synthesis.
