| Carbon isotopic fractionation has been studied in a diversity of microorganisms, especially of rather recently discovered "extremophiles" (including a species capable of growth at temperatures as high as 113°C). The 21 species studied include 6 for which the entire genome has been or is currently being sequenced, providing opportunity to correlate carbon isotopic signatures with differing biochemical pathways. The results indicate that microorganisms using the reductive citric acid cycle, the 3-hydroxypropionate cycle, and the Archaeoglobales (using the acetyl-CoA pathway) do not significantly fractionate carbon isotopes, whereas methanogens using the acetyl-CoA pathway can be substantially enriched in 12C.; The results reported indicate that the magnitude of carbon isotopic fractionation observed for biomass-producing carbon fixation in species of Methanococcus and Methanopyrus were related to the particular growth-status that had been attained by the various cultures when the experiments were stopped with low fractionation initially and the largest fractionations observed as the cells were beginning stationary growth.; A phylogenetic "Tree of Life" has been constructed based on the observed presence or absence of families of protein-encoding genes in eleven complete genomes of free-living prokaryotic microorganisms. Despite apparently widespread lateral gene transfer among prokaryotes, use of this new method appears to establish a single robust underlying evolutionary history for these microorganisms. This novel and robust evolutionary tree is used as a template from which the magnitude of carbon isotopic fractionation expected to have been exhibited by various groups of ancient (including extinct) microorganisms is inferred.; Lastly, ion microprobe measurements with an accuracy and precision of ∼1 to 2‰ have been made of carbon isotopes in 30 specimens representing six fossil genera of microorganisms petrified in stromatolitic cherts of the ∼850-Ma-old Bitter Springs Formation, Australia and the ∼2,100-Ma-old Gunflint Formation, Canada. The results generally yield delta13CPDB values consistent with carbon fixation via the Calvin cycle and, thus, morphology-based assignment of these microbes to the cyanobacteria. |
