CARBON-ISOTOPE VARIATIONS IN A SOLAR POND MICROBIAL MAT - ROLE OF ENVIRONMENTAL GRADIENTS AS STEERING VARIABLES

A biogeochemical traverse is presented for a juvenile benthic mat cove ring the depth profile of an artificially stratified and eutrophicated hypersaline heliothermal pond with known gradients of temperature, sa linity, pH, and light transmission. It can be shown that visual mat de velopment depends primarily on temperature and salinity as main enviro nmental steering variables whose increase with depth goes along with t he attenuation and final disappearance of a visible microbial film in the pond's hypolimnic compartment. Recorded biogeochemical parameters (C(org) content, cell numbers, chlorophyll-a content) evidently reflec t, as either biomass- or productivity-related index functions, the vis ually perceptible growth gradient of the microbial ecosystem along the pond slope. The observed coincidence of maxima in these index functio ns with maxima in deltaC-13org clearly identifies high rates of primar y productivity as the agent ultimately responsible for the generation of isotopically heavy (C-13-enriched) biomass in these and related env ironments. Extreme demands placed on the local feeder pool of dissolve d inorganic carbon by high rates of primary productivity entertained b y the mat-forming microbenthos obviously give rise to severe CO2 limit ation, enforcing the operation of a diffusion-(supply-)limited assimil atory pathway with an isotopically indiscriminate metabolization of th e available CO2 resources.