Low-molecular-weight sulfonates, a major substrate for sulfate reducers in marine microbial mats:

Several low-molecular-weight sulfonates were added to microbial mat slurries to investigate their effects on sulfate reduction. Instantaneous production of sulfide occurred after taurine and cysteate were added to all of the microbial mats tested. The rates of production in the presence of taurine and cysteate were 35 and 24 M HS ؊ h ؊1 in a stromatolite mat, 38 and 36 M HS ؊ h ؊1 in a salt pond mat, and 27 and 18 M HS ؊ h ؊1 in a salt marsh mat, respectively. The traditionally used substrates lactate and acetate stimulated the rate of sulfide production 3 to 10 times more than taurine and cysteate stimulated the rate of sulfide production in all mats, but when ethanol, glycolate, and glutamate were added to stromatolite mat slurries, the resulting increases were similar to the increases observed with taurine and cysteate. Isethionate, sulfosuccinate, and sulfobenzoate were tested only with the stromatolite mat slurry, and these compounds had much smaller effects on sulfide production. Addition of molybdate resulted in a greater 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 nanomolar to micromolar concentrations. An enrichment culture from the stromatolite 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, this rate suggests that organisms with properties similar to the properties of this enrichment culture are a major constituent of the sulfidogenic population. In addition, taurine was consumed at some of highest dilutions obtained from most-probable-number enrichment cultures obtained from stromatolite samples. Based on our comparison of the sulfide production rates found in various mats, low-molecular-weight sulfonates are important sources of C and S in these ecosystems. The sulfur cycle plays an important role in the geomicrobiology of intertidal and coastal sediments The sulfonates are a group of organosulfur compounds that are commonly found in household and industrial wastewater. Linear alkyl sulfonates are major constituents of detergents (58), and the catabolism of these compounds has been well documented (6, 18) and includes carbon-sulfur bond breakage pathways (39, 44). Recently, it has been shown that a range of low-molecular-weight sulfonates are present in the marine environment. A variety of different marine sediments contain significant amounts of sulfonates, which comprise 20 to 40% of the organosulfur pool (65). Unfortunately, the composition of the sulfonate pool was not characterized further in the study of Vairavamurthy et al. (65). However, several sources of sulfonates have been identified in the marine environment. Sulfonates can be the sole source of sulfur or nitrogen for marine phytoplankton (7), and sulfolipids can be major biomembrane constituents in microscopic algae (56, 57), cyanobacteria (2, 26), diatoms (1), and bacteria (24). Sulfonate-containing exopolymers may be present in benthic diatoms and support the gliding motility of these organisms, like suggestions made for bacteria (25). Taurine (2-aminoethanesulfonate) is present in marine diatoms (34) and zooplankton (9), presumably as a major osmolyte. The intracellular concentration of these compounds typically exceeds 0.2 M, which could easily explain the occurrence of sulfonates in the estuarine environment. Similarly, cysteate (alanine 3-sulfonate) is one of the oxidation products of cysteine residues in proteins (55, 60), and isethionate (2-hydroxyethanesulfonate) has been found in marine algae (31). Clearly, many different sources contribute to the pool of low-molecular-weight sulfonates in the marine environment. Taurine is one of the several amino acids that are readily bioavailable in marine sediments (48). In addition to being used for anabolic purposes, amino acids are catabolized by microbes. Under anoxic conditions, this occurs through fermentation and, more importantly, sulfate reduction (8, 51, 61). The nitrogen-containing sulfonates taurine and cysteate can also be utilized by sulfate-reducing bacteria (SRB) Microbial mats, including microbial mats associated with sal