Bacterial dissimilatory reduction of arsenate and sulfate in meromictic Mono Lake, California

The stratified (meromictic) water column of alkaline and hypersaline Mono L ake, California, contains high concentrations of dissolved inorganic arseni c (similar to 200 mu mol/L). Arsenic speciation changes from arsenate [As ( V)] to arsenite [As (III)] with the transition from oxic surface waters (mi xolimnion) to anoxic bottom waters (monimolimnion). A radioassay was devise d to measure the reduction of As-73 (V) to As-73 (III) and tested using cel l suspensions of the As (V)-respiring Bacillus selenitireducens, which comp letely reduced the As-73 (V). I, field experiments, no significant activity was noted in the aerobic mixolimnion waters, but reduction of As-73 (V) to As-73 (III) was observed in all the monimolimnion samples. Rate constants ranged from 0.02 to 0.3/day, with the highest values in the samples from th e deepest depths (24 and 28 m). The highest activities occurred between 18 and 21 m, where As (V) was abundant (rate, similar to 5.9 mu mol/L per day) . In contrast, sulfate reduction occurred at depths below 21 m, with the hi ghest rates attained at 28 m (rate, similar to 2.3 mu mol/L per day). These results indicate that As (V) ranks second in importance, after sulfate, as an electron acceptor for anaerobic bacterial respiration in the water colu mn. Annual arsenate respiration may mineralize as much as 14.2% of the pela gic photosynthetic carbon fixed during meromixis. When combined with sulfat e-reduction data, anaerobic respiration in the water column can mineralize 32-55% of this primary production. As lakes of this type approach salt satu ration, As (V) can become the most important electron acceptor for the biog eochemical cycling of carbon. Copyright (C) 2000 Elsevier Science Ltd.