Autotrophic DIC uptake rates and their association to microbial community structure in microbial mats from a sulfidic, saline, warm spring, Utah, USA

Stinking Springs is a warm (28-40 C), saline spring which drains into the northeastern side of Great Salt Lake, Utah. Sulfide concentrations range from 250 μM to negligible in the span of 50 meters of spring flow. Microbial mats are developed throughout the discharge region, with floating and streamer-style communities near the inflow region and associated with higher flow rates, and layered mats where the flow is dispersed to thin (1 cm) sheet flow. Bicarbonate uptake rates of the mat layers were determined with 24-hour light and dark incubations of 13HCO3-spiked spring water. Uptake in the light ranged from 0-2.1 %/day, with maximum rates obtained in mats floating on the water surface and in lower pink-colored layers. Uptake in the dark ranged from 0-0.3 %/day and was also highest in the lower layers. 16S rRNA analyses as well as pigment extractions showed no correlation between high autotrophic rates and cyanobacteria or chlorophyll A, nor did lipid analyses indicate a correlation between uptake and diatom abundance. Photoheterotrophs, purple sulfur bacteria and anaerobic heterotrophs were most abundant in layers with high 13HCO3 uptake in the light. Organisms filling the same metabolic niches, but from different phyla, were most closely correlated with uptake in the dark. These results suggest that the C cycling in this mat system is dominated by heterotrophs and anaerobic phototrophs despite abundant cyanobacteria. Calcium carbonate crystals were found embedded in both green and pink mat layers but carbonate abundance was not correlated with autotrophic rates. Large depletion of sulfate (16 mM to near 0), small concentrations of measured sulfide, and a large presence precipitated elemental sulfur suggest that sulfate reduction coupled to oxidation of sulfide to elemental sulfur crystals is a major sink of S in this system. There was also a notable absence of barite in any mat layer or associated with diatoms, as had been previously described (Bonny & Jones 2007).