The isotopic composition of organic carbon from extant stromatolite-type microbial ecosystems is commonly slanted toward heavy delta¹³carbon values with respect to the average composition of organic matter (including that from Precambrian stromatolites). This seems the more enigmatic as the bulk of primary producers from benthic microbial communities are known to fix carbon via the carbon3 pathway normally entailing the sizable fractionations of the ribulose diphosphate carboxylase reaction. There is reason to believe that the small fractionations displayed by aquatic microorganisms result from the limitations of a diffusion-controlled assimilatory pathway in which the isotope effect of the enzymic reaction is largely suppressed. Apart from the diffusion-control exercised by the aquatic environment, transport of carbon dioxide to the photosynthetically active sites will be further impeded by the protective slime (polysaccharide) coatings commonly covering microbial mats in which gas diffusivities are extremely low. Ineffective discrimination against ¹³carbon becomes, however, most pronounced in hypersaline environments where substantially reduced carbon dioxide solubilities tend to push carbon into the role of a limiting nutrient (brine habitats constitute preferential sanctuaries of mat-forming microbenthos since the emergence of Metazoan grazers). As the same microbial communities had been free to colonize normal marine environments during the Precambrian, the carbon dioxide concentration effect was irrelevant to the carbon-fixing pathway of these ancient forms. Therefore, it is not surprising that organic matter from Precambrian stromatolites displays the large fractionations commonly associated with carbon3 photosynthesis. Increased mixing ratios of carbon dioxide in the Precambrian atmosphere may have additionally contributed to the elimination of the diffusion barrier in the carbon-fixing pathways of ancient mat-forming microbiota. -- AATA