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On the origins of oxygenic photosynthesis and aerobic respiration in Cyanobacteria

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Science  31 Mar 2017:
Vol. 355, Issue 6332, pp. 1436-1440
DOI: 10.1126/science.aal3794

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  • RE: Cyanobacteria are oxygenic phototrophic prokaryotes
    • Himadri B. Pakrasi, University Distinguished Professor, Washington University, St. Louis, MO 63130, USA
    • Other Contributors:
      • Jonathan P. Zehr, University Distinguished Professor, University of California, Santa Cruz, CA 95064, USA

    Cyanobacteria are photosynthetic microbes that are responsible for the oxic environment in our planet. Several decades ago, cyanobacteria were defined as phototrophic prokaryotes (1). Such an operational definition of this important class of bacteria was based on their oxygenic photosynthetic life style. Soo et al. recently reported a new class of bacteria, Sericytochromatia, that have homologs to a limited set of genes in cyanobacteria (2), but lack genes for components of the photosynthetic machinery, similar to the previously identified Melainabacteria (3). These authors have argued that Melainabacteria and Sericytochromatia should be included into a single group with cyanobacteria, and called this entire group Cyanobacteria (2).

    To date, more than 400 cyanobacterial genomes have been sequenced and the evolutionary relationships between cyanobacteria and chloroplasts in plants and algae have been firmly established. All but a small number of these cyanobacteria are oxygenic. In fact, even the outliers such as UCYN-A (4) that lack photosystem II and RUBISCO and are obligate symbionts of eukaryotic algae have been shown to have originated from genome reduction of free living unicellular nitrogen fixing cyanobacteria such as Cyanothece (5), and evolved within the cyanobacteria lineage, rather than being a distantly related ancestral branch. In their analysis of Melainabacteria, Ley and colleagues were careful to describe them as belonging to ‘a new candidate phylum...

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    Competing Interests: None declared.
  • RE: Horizontal gene transfer of reaction center proteins
    • Tanai Cardona, Scientist (Photosynthesis Research), Department of Life Sciences, Imperial College London

    This is a fantastic paper and will greatly improve our understanding of the evolution of oxygenic photosynthesis, aerobic metabolism, and the early diversification of life.

    The paper shows that the sequenced genomes of Melainabacteria and Sericytochromatia lack genes encoding components of photosynthesis, but it does not indicate that Oxyphotobacteria obtained photochemical reaction centres via horizontal gene transfer nor does it show that oxygenic photosynthesis evolved after the divergence of the Melainabacteria.

    The paper disregards the evolution of the photosynthetic machinery, which rules out the possibility that Oxyphotobacteria obtained reaction centres via horizontal gene transfer from any of the known clades of anoxygenic phototrophic bacteria.

    I challenge the authors to take a close look at the evolution of the reaction centre proteins and to inspect them in detail. Please, take the sequences of the core proteins and compare them among each other using the available crystal structures. Then draw conclusions based on the phylogeny of reaction centre proteins within the context of their structural and photochemical properties.

    Once you do that, it will quickly become apparent that Oxyphotobacteria did not obtain reaction centres from Proteobacteria, Chloroflexi, Chlorobi, Heliobacteria, or Acidobacteria. And it will also become evident that the roots of oxygenic photosynthesis go a lot deeper than can be accounted for with a ‘horizontal...

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    Competing Interests: None declared.