I would like to have opinions of Biogeochemists about whether most ancient anoxygenic phototrophs, the predecessors of cyanobacteria, could have suffered from oxidative stress (ROS) when they oxidized Fe2+ in an ancient anoxic ocean. I would like to notice that an Archean ocean is supposed was not free of H2O2. I look forward to your points and opinions. Thanks. Igor
To Igor Brown:
Why do you assume that oxidation of Fe(II) in Archean waters was carried out by the types of photosynthetic bacteria that do not release oxygen (i.e. green and purple bacteria), and why do you think this occurred before the evolution of cyanobacteria? My understanding is that cyanobacteria themselves are thought to have oxidised the Fe(II) with the oxygen that they produced during photosynthesis, thereby causing the deposition of the distinctive Precambrian banded iron formations (possibly as a means of preventing toxic effects of free oxygen). This makes sense, because [1] cyanobacteria carry out oxygen-releasing "plant photosynthesis," unlike the green and purple bacteria, which carry out "bacterial photosynthesis," which does not produce oxygen; and [2] there is fossil evidence that cyanobacteria were in existence at least 3.5 billion years ago, along with geochemical evidence (a banded iron formation in Greenland) suggesting that they were in existence at least 3.8 billion years ago (at the very beginning of the Archean aeon).
T.A. Jackson, Ph.D. E-mail address: [email protected]
Dear Dr. Jackson: I am not an author of the hypothesis that most ancient anoxygenic phototrophs started to oxidize Fe2+ and to deposit mineralized iron significantly before the emergence of cyanobacteria. You can find the description of this hypothesis in the articles D. Newman, A. Kappler, K. Konhauser and other biogeochemists. The main reason for my question was that contemporary anoxygenic phototrophs oxidize Fe2+ with the help of other oxidizers than O2 but in its presence, even under anaerobic conditions because all contemporary Earth is contaminated with O2. The latter can be the source of free radicals. In contrast, early Archean hydrosphere, as it seems to me, was completely free of O2 but very rich with Fe2+; the latter could have penetrated in any live cells. So, my concern was whether free radicals could be generated under such conditions or not.
Thank you. IB
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