217 000-year-old DNA sequences of green sulfur bacteria in Mediterranean sapropels and their implications for the reconstruction of the paleoenvironment
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KeywordAncient DNA; Oceanic anoxic events; Chlorobiaceae; Fossil DNA; Green sulfur bacteria; Sapropels
Deep-sea sediments of the eastern Mediterranean harbor a series of dark, organic carbon-rich layers, so-called sapropels. Within these layers, the carotenoid isorenieratene was detected. Since it is specific for the obligately anaerobic phototrophic green sulfur bacteria, the presence of isorenieratene may suggest that extended water column anoxia occurred in the ancient Mediterranean Sea during periods of sapropel formation. Only three carotenoids (isorenieratene, β-isorenieratene and chlorobactene) are typical for green sulfur bacteria and thus do not permit to differentiate between the ~80 known phylotypes. In order to reconstruct the paleoecological conditions in more detail, we searched for fossil 16S rRNA gene sequences of green sulfur bacteria employing ancient DNA methodology. 540 bp-long fossil sequences could indeed be amplified from up to 217,000-year-old sapropels. In addition, such sequences were also recovered from carbon-lean intermediate sediment layers deposited during times of an entirely oxic water column. Unexpectedly, however, all the recovered 16S rRNA gene sequences grouped with freshwater or brackish, rather than truly marine, types of green sulfur bacteria. It is therefore feasible that the molecular remains of green sulfur bacteria originated from populations which thrived in adjacent freshwater or estuarine coastal environments rather than from an indigenous pelagic population.
Author Posting. © The Authors, 2006. This is the author's version of the work. It is posted here by permission of Society for Applied Microbiology and Blackwell for personal use, not for redistribution. The definitive version was published in Environmental Microbiology 9 (2007): 238–249, doi:10.1111/j.1462-2920.2006.01134.x.
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