Mitochondria, oxygen toxicity and the quahog

One of the many crises through which life passed during its evolution was the widespread appearance of oxygen.  This occurred once the release of soluble iron-2 to the oceans from sea-floor processes fell below a rate that buffered the photosynthetic generation of oxygen through the precipitation of iron-3 oxides in marine sediments.  Oxygen is life-threatening, largely through its encouraging the formation of simple compounds that are more potent oxidizers than oxygen (O2) itself, such as O, H2O2 and HO.  In cells they can lead to genetic degeneration, progressive ageing and eventually cell death.  Free oxygen in the environment was a stealthy threat to all life forms that existed around 2200 Ma.  A possible evolutionary response that may have opened the way for the later rise of the Eucarya, and the huge diversification that permitted, is nicely summarized by Doris Abele in Nature of 7 November 2002 (Abele, D. 2002.  The radical life-giver.  Nature, v. 420, p. 27).  The main strand of her argument is that mitochondria, the energy converters in eukaryote cells, also serve to keep oxygen levels inside cells high enough for metabolism, yet low enough to minimise the formation of threatening oxidants.  Her object of study has been the noble ocean quahog, Arctica islandica (incidentally, a clam often referred to by Herman Melville in Moby Dick) which mysteriously burrows into anoxic muds for a while and drops its metabolism alarmingly.  By this habit, the quahog has achieved what middle-aged Californians yearn for; spectacular life extension to as much as 220 years.  Abele believes that this protective function of mitochondria  is deployed by the quahog, having arisen in the earliest Eucarya, after the oxygenation of the planet.  However, as Lyn Margulis observed in developing her endosymbiotic hypothesis for the emergence of eukaryotes, mitochondrial RNA is very like that of oxygen-respiring purple bacteria.  Anti-oxidant mechanisms may therefore be more ancient.  The other main defence against free radicals takes the form of a range of vitamins and other complex compounds, some of which seem to have their origins in heat-shock proteins; possibly harking back to life’s origins near deep-ocean hydrothermal vents.

In a similar vein, linked to the rise of oxygen concentrations, doubt has been cast on the role of photosynthesising cyanobacteria since the earliest times..  Most geologists hold them responsible for creating stromatolites since 3500 Ma, and also for providing an early source of oxygen that was rapidly scavenged by the precipitation of iron oxides in banded iron formations.  Carrine Blank, a palaeobiologists at Washington University in St Louis, has genetically compared cyanobacteria with a range of other living Bacteria, to asses their relatedness.  Her work suggests that the blue-greens were late additions to early life, perhaps long after the first BIFs appeared (report on the annual meeting of the Geological Society of America, in New Scientist 9 November 2002, p. 25).

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