5-Cyano-2,3-ditolyl tetrazolium chloride (CTC) reduction in a mesophilic anaerobic digester: measuring redox behavior, differentiating abiotic reduction, and comparing FISH response as an activity indicator.

The tetrazolium salt 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) has been widely applied to assess microbiological activity in environmental samples. CTC reduction has previously been quantified in a variety of anaerobic systems (i.e., fermentative, nitrate reducing, sulfate reducing) using direct microscopy, solvent extraction, and flow cytometry. In this work, extracellular CTC reduction was observed and distinguished from its intercellular counterparts by the amorphous character and near uniform fluorescence of the resulting formazan precipitates (CTF). Fluorescence yielded by non-cellular-associated formazan precipitates bleached much more rapidly than CTF formed within cells under identical UV exposure (<2 min). Dehydrogenase activity assays and fluorescent in situ hybridization (FISH) were simultaneously carried out in microcosms containing active anaerobic digester biomass, propylene glycol, and settled sewage centrate for direct comparison. In substrate limited microcosms, quantitative FISH measurements remained well above their detection limit indicating sustained intercellular ribosomal RNA concentrations over a 5-day period, while dehydrogenase assays (CTC) decreased to background levels within 14 h of substrate limitation. Results from this work suggest that CTC reduction in cell-free samples may impede accurate enzyme activity measurements, particularly when quantification involves solvent extraction, flow cytometry, or software-aided counting. In addition, activity assessment in anaerobic digesters using FISH and CTC reduction assays may be comparable until substrate becomes limited.