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Project Progress

We refined the model of hypersaline mats Microbial BioGeoChemistry (MBGC). Working towards publication of the first version of the model (submitted May), we adopted many improvements into the model, or researched them as future adaptations. One assumption that was questioned was that oxygen supersaturation inhibits oxygenic photosynthesis. Whereas this simplifying assumption was effective in earlier models, the midday reduction in photosynthesis that occurs in the mats is more likely due to photorespiration. This has consequences for the carbon cycle and we have researched the inclusion of photorespiration into the model.

We ran sensitivity tests to answer outstanding questions about the relative diel changes in the rates of sulfate respiration and dissimilatory sulfate reduction (dsr); (Table 1; Figure 1). The relative frequency of these processes has consequences to differential interference contrast (DIC) emissions (Figure 1). We found that cyanobacterial fermentation is critical in supplying H₂ for dsr and that nighttime sulfur oxidation in colorless sulfur bacteria is critical in supplying S^o for cyanobacterial fermentation.

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These findings have consequences for further development of the carbon cycle that will include organic carbon and methanogenesis. We have begun the development of these new carbon components by gathering literature on photorespiration, cyanobacterial fermentation, and methanogenesis in microbial mats. We focused on several sources of carbon, specifically exudation during oxygen supersaturation leading to photorespiration, fermentation, and decomposition. This can be described as a ‘top-down’ approach because it examines sources of organic C. We are concurrently taking a ‘bottom-up’ approach looking at one critical sink of organic C: methanogens. In hypersaline, sulfate-rich environments, these microbes use methylamines because they are noncompetitive substrates. The precursors of methylamines are osmoregulants released during the decomposition of hypersaline bacteria, such as glycine betaine. The current challenge is to find or model pool sizes of these critical osmoregulants.