Authors: Martin Krüger and Axel Schippers
Integrated Ocean Drilling Program (IODP) Expedition 347 to the Baltic Sea in 2013 was in line with the IODP Science Plan main research theme “Deep biosphere responses to glacial–interglacial cycles,” addressing questions such as deep biosphere evolution, its biogeochemical processes, and how the postglacial diffusive penetration of conservative seawater ions may alter the chemical composition and microbial physiology in the subseafloor biosphere. Consequently, we tried to enrich indigenous microorganisms at in situ conditions using a broad range of electron acceptors (for fermenters; Fe, Mn, and sulfate reducers; and methanogens), simple and complex carbon substrates (in mixtures or as single compounds), and a wide range of culture conditions (temperature and salinity) to cover varying environmental conditions and metabolic requirements. The most successful were enrichment cultures with a mix of polymeric substrates, which proved to be successful for all samples investigated. Also, iron- and manganese-reducing organisms could be enriched from all sites, whereas nitrate as an electron acceptor did not work well. Methanogenic enrichments were only successful for a few of the samples investigated. In these cases, different monomeric as well as complex substrates were converted to methane, indicating a metabolically versatile indigenous microbial community in the sediments.