SUPERHEAVY ORGANIC-CARBON FROM HYPERSALINE MICROBIAL MATS - ASSIMILATORY PATHWAY AND GEOCHEMICAL IMPLICATIONS

Microbial mats [primarily consisting of Synechococcus sp., Synechocystis sp., Gloeocapsa sp., Gloeothece sp., Lyngbya sp., Phormidium sp., Anabaena sp., Microcoleus sp., Oscillatoria sp., Schizothrix sp., and Spirulina sp.] from coastal brine pools of the Sinai Peninsula are consistently enriched in the heavy C isotope (13C) as compared to average organic matter; those from the Solar Lake locality representing the heaviest organic C ever encountered in the biosphere (.delta.13C = -5.7 .+-. 1.4.permill. vs. PDB). Ineffective discrimination against 13C in the C-fixing pathways of the mat-forming microbenthos seems due to substantially reduced CO2 solubilities in heliothermal brines pushing C into the exceptional role of a limiting nutrient, and the consequent establishment of a diffusion-limited assimilatory pathway in which the large isotope fractionations of the ribulose-1,5-bisphosphate carboxylase reaction are basically suppressed.