Geochemical implications of induced changes in C13 fractionation by blue-green algae

The photosynthetic fractionation of carbon isotopes by blue-green algae in laboratory culture is dependent in a non-linear fashion on the CO₂ concentration in the feed gas. For the three species tested, the minimum fractionation occurred at a CO₂ concentration of 0.2% in air and was approximately zero for the two marine species tested. Enrichment of C¹² in the reduced carbon is not an inevitable result of photosynthetic carbon fixation. Temperature and pH had no detectable effect on fractionation. The maximum fractionation observed in the laboratory cultures or in recent blue-green algal mats was 18‰. Differences in the isotope ratio of coexisting oxidized and reduced carbon in Precambrian stromatolites are as great as 31‰. Present carbon isotopic evidence is not consistent with the idea that blue-green algae were major contributors to the organic matter in Precambrian sediments.