Carbon and hydrogen isotopic compositions of algae and bacteria from hydrothermal environments,Yellowstone National Park

Stromatolites forming today on a small scale in hydrothermal environments are chemical and biological analogues of much larger Precambrian formations. Carbon isotopic composition varied as a function of CO₂ concentration, pH, and species composition. Stratiform, layered stromatolites grew in silica-depositing springs at 55° to 70°C; they consisted mainly of a unicellular alga, Synechococcus, and a filamentous, photosynthetic bacterium, Chloroflexus. These thermophiles become enriched in ¹²C as the concentration of carbon dioxide in the effluent waters increases. At a concentration of 40 ppm total inorganic C, and δ¹³C of organic carbon was ～ ?12%., whereas at 900 ppm total inorganic C, the δ¹³C of similar species was ～ ?25%.. Conical stromatolites or conophytons (principally a filamentous, blue-green alga Phormidium and Chloroflexus) grew at 40°-55°C. In older, broader conophytons, Chloroflexus was the dominant organism. Their δ¹³C values were ～ ?18%. in a variety of hot springs. In carbonate-depositing springs, i.e., carbon dioxide saturated, conophytons and stromatolites consisting of a variety of blue-green algae and photosynthetic bacteria had the most negative δ¹³C values (to ?30%.). These carbon isotope ratios are directly comparable to carbon isotope ratios of kerogen from Precambrian stromatolites. The presence and activity of methanogenic bacteria or heterotrophic, aerobic and anaerobic bacteria did not alter significantly the δ¹³C of the original organic matter.The hydrogen isotopic fractionation between thermophilic organisms and water is 0 to ?74 for temperatures of 85° to 46°C, respectively. Acidophilic algae fractionated hydrogen isotopes to a lesser extent than did the photosynthetic organisms inhabiting neutral pH springs. Because organic matter retains some of its original isotopic signature, relationships of CO₂ levels, pH, temperature, and species composition between modern stromatolites and their environment and those of the Precambrian can be inferred.