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 s pecific for each community and in accordance with optimal performance at th e respective salinity of origin, This pattern was lost after long-term expo sure to varying salinities when responses to salinity were found to approac h a general pattern of decreasing photosynthesis and oxygen exchange capaci ty with increasing salinity, Exhaustive measurements of oxygen export in th e light, oxygen consumption in the dark and press photosynthesis indicated that a salinity-dependent limitation of all three parameters occurred, Maxi mal 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 ecophysiol ogical data, we argue that this limitation was not caused directly by salin ity effects on the microorganisms, Rather, the decreasing diffusive supply of O-2 in the dark and the increasing diffusion barriers to O-2 escape in t he light, which intensify with increasing salinity, were likely responsible for the salinity-dependent limitations observed.