Molecular ‘fossils’ and the emergence of photosynthesis

The most familiar photosynthesis is that associated with green plants, members of the Eucarya, in which organelles known as chloroplasts play a crucial role.  Lyn Margulis’ theory of endosymbiotic incorporation of various bacteria in the origin of the eukaryote cell, sees cyanobacteria as the most likely progenitors of chloroplasts in plants.  Aspects of the genetic material in chloroplasts are sufficiently similar to that of blue-green bacteria to make this a robust view.  Tracking down when that melding of bacterial ancestors took place is a difficult task, both for molecular biologists and palaeontologists, partly because the record of cell material similar to that of cyanobacteria goes cold about 2.5 billion years ago.

Stromatolites, which today grow through the action of cyanobacteria excluding calcium from their cells in hypersaline environments, go back into the Archaean 3.46 billion years ago, but there is no guarantee that stromatolite forms were always confined to oxygenic photosynthesisers.  However, the manner in which photosynthesis by blue-greens fractionates carbon isotopes possibly gives a signal in the d13C record of ancient hydrocarbons.  Sadly, the overlaps between carbon-isotope fractionation oxygenic photosynthesisers, chemoautotrophs and anoxygenic photoautotrophs are too broad for this kind of study to give a definitive answer.  Nonetheless, some researchers have claimed an Archaean origin for the cyanobacteria using this approach.

The advance of molecular biology, which compares gene sequences among living organisms to seek degrees of relatedness (phylogenies), steadily moves towards widely accepted molecular “clocks” that might resolve the timing of emergent life processes.  A joint US-Japan team of molecular biologists have compared the photosynthetic genes of two modern photoautotrophs – green sulphur and green nonsulphur bacteria, neither of which are oxygen producing – with those of other photosynthetic bacteria (Xiong, J. et al., 2000.  Molecular evidence for the early evolution of photosynthesis.  Science, v.  289 8 September 2000, p. 1724-1730).  Their results firmly place oxygenic photosynthesis, as in cyanobacteria, as descendent from earlier anoxygenic photoautotrophy, purple bacteria likely being the first to emerge by developing pigments capable of using solar energy to fuel proton pumping across cell walls.  Jin Xiong and co. do not derive any timing for this phylogeny, but palaeobiologists are suggesting from their evidence that the six major photosynthetic bacterial lineages were around in the mid-Archaean (2.8 to 3.0 billion years ago) and maybe earlier.  This comes nowhere close to the greater antiquity of stromatolites, but tagging purple bacteria as the first photosynthetic organisms, albeit not producing oxygen, gives a helping hand.  Organic molecules originating in them are sufficiently distinct to already have shown up in kerogen from ancient shales, and such precursors to petroleum are present in Archaean sediments.

The interest in the emergence of photosynthesis is understandable, because of the huge increase in opportunities that it presented, by comparison with chemoautotrophic metabolism that seems likely to have been the first life strategy.  The latter depends on chemical tricks with reduced materials, such as S, Fe2+ and methane delivered by sea-floor hydrothermal vents.  Assuming appropriate rates for Archaean magmatism, that could sustain about 1012 moles of carbon fixing in cells per year.  The anoxygenic photosynthetic pathway would have multiplied that by ten times.  However, it is oxygenic photosynthesis that exploded life’s potential for interaction with the inorganic world, and that stemmed from the chemical-physical process at the root of what blue-greens did.  The essence of oxygenic photosynthesis is that the pigments (like chlorophyll in plants) involved in transforming photon energies into electron flows, which are essential in the reduction of CO2 and water to carbohydrates, actually break the very strong bond between hydrogen and oxygen in water; that is why it releases free oxygen as a by-product.  That feat involves a combination of the processes used by green sulphur and purple bacteria, which in itself implies the later emergence of cyanobacteria as confirmed by Xiong et al’s work.  By using water molecules in this way, however, oxygenic photosynthesis opened up the whole near-surface of the hydrosphere, increasing potential bioproductivity by a further two or three orders of magnitude at least.  It can be said that such a development truly brought life onto the front stage from hiding in obscure nooks and crannies.  But we still have little precise idea of when that happened.

See also:  Des Marais, D., 2000.  When did photosynthesis emerge on Earth? Science, v.  289 8 September 2000, p. 1703-1705.

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