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Abstract: Filter samples from the oxic zone and suboxic zone of the physically stratified water column and sediment samples of the Gotland Deep, Baltic Sea, were analyzed for bacteriohopanepolyol (BHP) and phospholipid fatty acid (PLFA) concentrations. In total, eight BHPs were identified, with the greatest diversity in the suboxic zone. There, 35-aminobacteriohopane-31,32,33,34-tetrol (aminotetrol) and 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol), whose concentrations decreased concurrently from the lower to the upper suboxic zone, indicated type I methanotrophic bacteria and thus aerobic oxidation of methane. The presence and activity of type I aerobic methanotrophic bacteria was further supported by the presence of 13C-depleted PLFAs, specifically 16:1ω8c and 16:1ω5c (δ13C as low as −41.2‰). However, the relative amount of methanotroph-specific compounds was low (aminopentol, <0.2% of total BHPs; 16:1ω8c, ca. 0.5% of total PLFAs), suggesting a minor contribution of aerobic methanotrophic bacteria to the particulate organic matter. The distinctive BHP pattern in the suboxic zone, including aerobic methanotroph biomarkers and a tentative marker for a pelagic redoxcline [putative 22S isomer of the ubiquitous 22R-bacteriohopanetetrol (BHT)], was mirrored in the sediment samples. Our data indicate that a major portion of the sedimentary hopanoids of the Gotland Deep is sourced from the suboxic part of the water column via an effective but unknown transport mechanism. [Copyright &y& Elsevier]

The Baltic Sea, one of the world's largest brackish-marine basins, established after deglaciation of Scandinavia about 17 000 to 15 000 yr ago. In the changeable history of the Baltic Sea, the initial freshwater system was connected to the North Sea about 8000 yr ago and the modern brackish-marine setting (Littorina Sea) was established. Today, a relatively stable stratification developed in the water column of the deep basins due to salinity differences. Stratification is only occasionally interrupted by mixing events, and controls nutrient availability and growth of specifically adapted microorganisms and algae. We studied bacteriohopanepolyols (BHPs), lipids of specific bacterial groups, in a sediment core from the Central Baltic Sea (Gotland Deep) and found considerable differences between the distinct stages of the Baltic Sea's history. Individual BHP structures indicate contributions from as yet unknown redoxcline-specific bacteria (bacteriohopanetetrol isomer), methanotrophic bacteria (35-aminobacteriohopanetetrol), cyanobacteria (bacteriohopanetetrol cyclitol ether isomer) and, through allochthonous input after the Littorina transgression, from soil bacteria (adenosylhopane), whereas the origin of other BHPs in the core has still to be identified. Notably high BHP abundances were observed in the deposits of the brackish-marine Littorina phase, particularly in laminated sediment layers. Because these sediments record periods of stable water column stratification, bacteria specifically adapted to these conditions may account for the high portions of BHPs. An additional and/or accompanying source may be nitrogen-fixing (cyano)bacteria, which is indicated by a good correlation of BHP abundances with Corg and δ15N. [ABSTRACT FROM AUTHOR]