BacterioChlorophyll A in the Open Ocean - Do We Need to Reassess its Role as a Biomarker?

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ABSTRACTOCE-0118045Bacteriochlorophyll A (bChl a) is a photosynthetic pigment which has been formerly considered a unique molecular biomarker for the anoxic photoheterotrophic bacteria. Although anoxygenic photoheterotrophic bacteria which live in oxic waters were discovered some 20 years ago, bChl a was considered to be expressed solely during, and thus diagnostic of, anoxygenic bacterial photosynthesis. These bacteria require anoxic conditions to synthesize their bacteriochlorophylls and, in the presence of light, use molecules other than water (e.g. sulfide, sulfur compounds, hydrogen, Fe etc) as photosynthetic electron donors. This is to be contrasted to the mode of existence of cynanobacteria and eukaryotes whose primary pigment, chlorophyll a (or one of its close derivatives), enables water to serve as the electron donor for photosynthesis, incidentally forming molecular oxygen as a waste product. Recent biophysical methods that detect bChl a have suggested that aerobic bacterial photosynthesis may be much more prevalent in the world's surface ocean, and also not limited to niche (anoxic microzonal) environments. As well as limiting the use of bChl a as a paleoecological biomarker for anoxic environments (a subject of this study), confirmation of the presence of a significant concentration of bChl a in the global surface ocean implies a significant biomass and photosynthetic energy flux due to aerobic anoxygenic phototrophs. This may or may not have important implications to carbon fixation and ocean carbon transport, but certainly is important in explaining the marine ecology of aerobic anoxygenic phototrophs in relation to non-phototrophic bacteria, especially in oligotrophic waters. In order to systematically confirm the importance of the bChl a signal in oxic seawater, this study will, in water samples from several sites: i) rigorously chemically identify the pigment, bChla, and its related forms, ii) investigate associated particle size relationships of its occurrence (indicative of microzonal anoxia) iii) conduct preliminary microbiological studies of its occurrence in likely target microbial (e.g Roseobacter clade) culturesiv) investigate the biogeochemical transformation of bChla in water columns and sedimentary environments.