Low-molecular-weight sulfonates, a major substrate for sulfate reducers inmarine microbial mats

Several low-molecular-weight sulfonates were added to microbial mat slurrie s to investigate their effects on sulfate reduction. Instantaneous producti on of sulfide occurred after taurine and cysteate mere added to all of the microbial mats tested. The rates of production in the presence of taurine a nd cysteate were 35 and 24 mu M HS- h(-1) in a stromatolite mat, 38 and 36 mu M HS- h(-1) in a salt pond mat, and 27 and 18 mu M HS- h(-1) in a salt m arsh mat, respectively. The traditionally used substrates lactate and aceta te stimulated the rate of sulfide production 3 to 10 times more than taurin e and cysteate stimulated the rate of sulfide production in all mats, but w hen ethanol, glycolate, and glutamate were added to stromatolite mat slurri es, the resulting increases were similar to the increases observed with tau rine and cysteate. Isethionate, sulfosuccinate, and sulfobenzoate mere test ed only with the stromatolite mat slurry, and these compounds had much smal ler effects on sulfide production. Addition of molybdate resulted in a grea ter inhibitory effect on acetate and lactate utilization than on sulfonate use, suggesting that different metabolic pathways were involved. In all of the mats tested taurine and cysteate were present in the pore water at nano molar to micromolar concentrations. An enrichment culture from the stromato lite mat was obtained on cysteate in a medium lacking sulfate and incubated anaerobically. The rate of cysteate consumption by this enrichment culture was 1.6 pmol cell(-1) h(-1). Compared to the results of slurry studies, th is rate suggests that organisms with properties similar to the properties o f this enrichment culture are a major constituent of the sulfidogenic popul ation. In addition, taurine was consumed at some of highest dilutions obtai ned from most-probable-number enrichment cultures obtained from stromatolit e samples. Based on our comparison of the sulfide production rates found in various mats, low-molecular-weight sulfonates are important sources of C a nd S in these ecosystems.