| Summary: | 14th International Symposium on Microbial Ecology (ISME14), 19-24 August 2012 Copenhagen, Denmark The deep ocean constitutes the largest habitat in the biosphere yet it is still unexplored in its majority. Based on low temperatures and substrate availability, a postulated large decrease in all biological processes with depth pictures a deep oceanic realm with almost negligible activity. Recently, this assumption of a 'dead' deep-sea has become questioned by increasing data that show deep-ocean bacteria and archaea to be growing at rather high rates. Open ocean samples were collected during the worldaround Malaspina Expedition in 2010-2011 from the sunlit epipelagic waters to the dark bathypelagic zone, covering the major oceanic basins of the global ocean including the North and South Atlantic, the Pacific and the Indian Ocean. Preliminary results indicate higher total abundances of heterotrophic prokaryotes (Bacteria and Archaea) in deep North Atlantic waters (4 × 105 cells mL-1) as compared to the other basins (8 × 103 cells mL-1 in the South Atlantic and 3 × 104 cells mL-1 in the Indian and Pacific Oceans). Despite these basin-scale differences, a common >1 order of magnitude decrease in abundance from the photic zone to the dark ocean was observed. We also measured single cell and bulk activities which point to significant activity levels in deep waters of all the major basins. Using archaeal inhibitors of bacterial activity, we show that Archaea contribute to approximately 40% of total heterotrophic prokaryotic production in the deep oceanic waters Peer Reviewed
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