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 covering the depth profile of an artificially stratified and eutrophicated hypersaline heliothermal pond with known gradients of temperature, salinity, pH, and light transmission. It can be shown that visual mat development depends primarily on temperature and salinity as main environmental steering variables whose increase with depth goes along with the 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 reflect, as either biomass- or productivity-related index functions, the visually perceptible growth gradient of the microbial ecosystem along the pond slope. The observed coincidence of maxima in these index functions with maxima in deltaC-13org clearly identifies high rates of primary productivity as the agent ultimately responsible for the generation of isotopically heavy (C-13-enriched) biomass in these and related environments. Extreme demands placed on the local feeder pool of dissolved inorganic carbon by high rates of primary productivity entertained by the mat-forming microbenthos obviously give rise to severe CO2 limitation, enforcing the operation of a diffusion-(supply-)limited assimilatory pathway with an isotopically indiscriminate metabolization of the available CO2 resources.