CYANOBACTERIAL EMERGENCE AT 2.8 GYA AND GREENHOUSE FEEDBACKS

The earliest robust evidence for cyanobacteria comes from molecular biomarkers preserved in 2.75 Gya sediments and their earliest fossil record, an emergence time consistent with molecular phylogeny (Hedges et al., 2001). Dismukes et al. (2001) argued that bicarbonate preceded water as the preferred reductant for oxygenic photosynthesis, postulating a transition stage in the Archean when a bacterial photosynthetic precursor to cyanobacteria utilized bicarbonate, then with the drop of carbon dioxide level cyanobacteria emerged using water instead of bicarbonate. Thermodynamic calculations give a 5 to 10 fold drop in bicarbonate levels with the onset of a methane-dominated greenhouse assuming surface temperatures of about 60 deg C and a drop in the level of atmospheric carbon dioxide from about 1 to < 0.03 bars. Cyanobacterial emergence at 2.8 Gya coincides with the first evidence of atmospheric methane, the negative excursion in the organic carbon isotope record. We propose that the long-term buildup of atmospheric methane in the early Archean culminated with a methane-dominated greenhouse at about 2.8 Gya, with a concomitant reduction in atmospheric carbon dioxide levels. Surface temperatures were declining from about 80-85 to 60-70 deg C from 3.8 to 2.8 Gya, with temperature the stronger driver of silicate weathering than atmospheric carbon dioxide level (Schwartzman, 1999). Far less methane is needed to maintain climatic temperatures than carbon dioxide. This scenario is also consistent with lower ambient temperatures for this transition. Thus, we propose the onset of the methane-dominated greenhouse triggered the emergence of cyanobacteria. The earlier emergence of phototrophy is consistent with its upper temperature limit of about 75 deg C. The proposed drop in atmospheric carbon dioxide level had two consequences, the emergence of cyanobacterial oxygenic photosynthesis and its carbon dioxide-concentrating mechanism, an adaptation to declining carbon dioxide to oxygen ratios in the external environment since the Archean greenhouse transition.

References

Dismukes et et al., 2001. PNAS USA 98, No. 5: 2170-2175. Hedges et al., 2001. BMC Evolutionary Biology 1: 4. Schwartzman, D., 1999. Life, Temperature, and the Earth: The Self-Organizing Biosphere, Columbia Univ. Press.