Origin of deep-biosphere bacteria in sediments of the Black Sea and the Namibian Upwelling Area

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The marine deep biosphere represents the largest biotope on Earth. Throughout the last years, we have obtained interesting insights into its community composition. As some of the microbial groups detected in deep marine sediments have close relatives in other environments, the question arises how the populations enter their habitat. The scientific goal of this project is to understand the origin of deep-subsurface bacteria. We would like to clarify whether bacteria that are settling down from the water column survive after being buried in the sediment or whether the deep biosphere is composed of specific types that have evolved or thrive in the sediment, only. To reach these goals, samples from the Black Sea collected during Meteor cruise M72/5 and samples taken from the coast off Namibia during Meteor cruise M76/1 will be analyzed by microbiological and molecular methods. Instead of performing a complete community analysis, we are focusing on four bacterial representatives as model organisms to study their distribution and trace their way from open waters into deeper sediment layers. Three of the bacterial groups to be studied (Rhizobium, Photobacterium, Roseobacter clade) are facultative aerobes and have cultured representatives in marine sediments and in the water column, while one (Chloroflexi) represents the most abundant bacterium in the deep biosphere, but has not yet been cultured from this habitat. A specific quantification by CARD-FISH and qPCR will be performed along regional transects from shallow to deep sediments. Molecular screening of serial dilution cultures will be used to identify and isolate microorganisms with a unique phylotype. New isolates will be characterized with respect to their specific physiological adaptations (e.g. to pressure). Special attention will be given to an Eemian (about 120.000 years old) Black Sea sapropel which allows a comparison with previously studied Mediterranean sapropels of similar age.