Salinity-dependent limitation of photosynthesis and oxygen exchange in microbial mats

We used benthic flux chambers and microsensor profiling under standardized incubation conditions to compare the short-term (hours) and long-term (days) functional responses to salinity in eight different hypersaline microbial mats. The short-term response of productivity to changes in salinity was specific for each community and in accordance with optimal performance at the respective salinity of origin. This pattern was lost after long-term exposure to varying salinities when responses to salinity were found to approach a general pattern of decreasing photosynthesis and oxygen exchange capacity with increasing salinity. Exhaustive measurements of oxygen export in the light, oxygen consumption in the dark and gross photosynthesis indicated that a salinity-dependent limitation of all three parameters occurred. Maximal values for all three parameters decreased exponentially with increasing salinity; exponential decay rates (base 10) were around 4-5 mL.g^-1. The values of mats in steady state with respect to salinity tended to approach this salinity-dependent limit. On the basis of environmental and ecophysiological data, we argue that this limitation was not caused directly by salinity effects on the microorganisms. Rather, the decreasing diffusive supply of O₂ in the dark and the increasing diffusion barriers to O₂ escape in the fight, which intensify with increasing salinity, were likely responsible for the salinity-dependent limitations observed.