As Earth’s primordial environment was anoxic, the molecular oxygen generated by the earliest oxygenic photosynthesizes would have been rapidly consumed, removed from the atmosphere by its reaction with previously Kinesin inhibitor unoxidized substrates (e.g., volcanic gases, unoxided minerals, and huge amounts of ferrous iron dissolved in the world’s oceans) to be buried in rock-forming minerals. Only after all such substrates had been completely oxidized could the content of atmospheric oxygen have permanently increased, a time lag from the origin of O2-producing photosynthesis that lasted several and perhaps many hundreds of millions of years. Taken
as a whole, the evidence available indicates that O2-producing photosynthetic microorganisms originated earlier than 2,450 Ma ago; that such microbes were likely in place by 2,700 Ma ago; and that the origin of oxygenic photosynthesis may date from as early as, or even earlier than, 3,500 Ma ago. Paleobiological evidence of photosynthesis Three principal lines of evidence are available to address the question of the time of origin of oxygenic photosynthesis—stromatolites, cellular
microfossils, and the chemistry of ancient organic matter—each of which is discussed, in turn, below. Stromatolites this website As preserved in the geological record, stromatolites are finely layered rock structures, typically composed of carbonate minerals (e.g., calcite, CaCO3), that are formed by the microbially mediated accretion of laminae, layer upon layer, from the surface of an ancient seafloor or lake bottom. Their layered structure reflects the photosynthetic metabolism of the mat-building microorganisms. Thin (mm-thick) mats composed of such microbes formed as the microorganisms multiplied and spread across surfaces that were intermittently veneered by detrital or precipitated mineral grains that blocked sunlight. To maintain photosynthesis, mobile members of such communities, such as gliding oscillatoriacean cyanobacteria, moved upward through the accumulated mineral
matter to establish a new, overlying, microbial mat. The repeated accretion and subsequent lithification of such mats, ADAMTS5 commonly augmented by an influx of non-mobile microbes (such as colonial chroococcacean, entophysalidacean, and Tozasertib purchase pleurocapsacean cyanobacteria), can result in the formation of stromatolitic structures that range from small millimetric columns and pustular mounds to large, decimetric bioherms. During diagenesis, the series of changes that lead to the lithification and preservation of such structures, silica (quartz, SiO2), can replace the initially precipitated carbonate matrix. If replacement occurs early in the history of a deposit, before the mat-building microorganisms decay and disintegrate, cellularly intact microbes can be preserved.