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Author Kirschvink, Joseph L.
Source CiteSeerX
Content type Text
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Subject Domain (in DDC) Computer science, information & general works ♦ Data processing & computer science
Subject Keyword Oxygen Mediating Enzyme ♦ Late Origin ♦ Paleoproterozic Icehouse ♦ Oxygenic Photosynthesis ♦ Second Problem ♦ Recent Work ♦ Subsequent Incorporation ♦ Ultraviolet Radiation ♦ Lethal Level ♦ Photochemically-produced H2o2 ♦ Major Geological Problem ♦ Late Archaean Atmosphere ♦ Anoxygenic Phototrophs ♦ Glacier Base ♦ Oxygen Tolerance ♦ Oxygen-dependent One ♦ Lipid Biomarkers ♦ Late End ♦ Main Piece ♦ Sterane Biomarkers ♦ Biological Oxygen Production ♦ Anaerobic Sterol Synthesis Pathway ♦ Anaerobic Enzyme ♦ Common Evolutionary Pattern ♦ Significant Quantity ♦ Skeptical View ♦ Geological Evidence ♦ Unique Biomarkers ♦ First Problem
Abstract Two major geological problems regarding the origin of oxygenic photosynthesis are: (1) identifying a source of oxygen predating biological oxygen production and capable of driving the evolution of oxygen tolerance, and (2) determining when oxygenic photosynthesis evolved. One solution to the first problem is the accumulation of photochemically-produced H2O2 at the surface of glaciers and its subsequent incorporation into ice. Melting at the glacier base would release H2O2, which interacts with seawater to produce O2 in an environment shielded from the lethal levels of ultraviolet radiation needed to produce H2O2. Answers to the second problem are controversial and range from 3.8 to 2.2 Ga. A skeptical view, based on metals that have redox potentials close to oxygen, argues for the late end of the range. The preponderance of geological evidence suggests little or no oxygen in the late Archaean atmosphere (< 1 ppm). The main piece of evidence for an earlier evolution of oxygenic photosynthesis comes from lipid biomarkers. Recent work, however, has shown that 2-methylhopanes, once thought to be unique biomarkers for cyanobacteria, are also produced anaerobically in significant quantities by at least two strains of anoxygenic phototrophs. Sterane biomarkers provide the strongest evidence for a date ≥2.7 Ga but could also be explained by the common evolutionary pattern of replacing anaerobic enzymes with oxygen-dependent ones. Although no anaerobic sterol synthesis pathway has been identified in the modern
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