On the use of tetrazolium salts for the measurement of microbial activity in sediments

In an attempt to understand the significance of microbiol activity measurements in anaerobic sediments by the tetrazolium reduction assay I examined the effect of addition of tetrazolium salts on a variety of anaerobic bacteria of different metabolic types. Triphenyletrazolium chloride was found to be reduced by a variety of obligately anaerobic bacteria that lack cytochromes, and evolve hydrogen during fermentation; thus the assay not only measures respiratory electron transport as was often assumed. Hydrogen evolution is inhibited in hydrogen-producing fermentative bacteria upon addition of tetrazolium salts, thereby depriving, e.g., the surfate-reducing bacteria of an important electron donor. Moreover, a dissimilatory sulfate-reducing bacterium tested did not reduce tetrazolium salts at significant rates. It is thus suggested that the tetrazolium reduction assay applied to anaerobic ecosystems gives a measure for activity of hydrogen-evolving fermentative bacteria, rather than for overall bacterial metabolism or of anaerobic respiration.