Bacteriohopanepolyols track past environmental transitions in the Black Sea

Abstract. Bacteriohopanepolyols (BHPs) are structurally diverse compounds produced by a wide range of bacteria making them ideal candidates as chemotaxonomic biomarkers and indicators of bacterially-driven biogeochemical processes in the geological record. In this study, we characterize changes in the BHP distribution in the Black Sea over the past 20 thousand years (ka), as the basin underwent three distinct environmental phases: (i) an oxic lacustrine phase where the Black Sea was disconnected from the global ocean; (ii) a transition period marked by the initial influx of marine water into the basin; and (iii) a marine phase where the basin was permanently euxinic. During the lacustrine phase we observe a high abundance and diversity of nucleoside BHPs (Nu-BHPs) that are likely derived from elevated terrigenous inputs as well as production of Nu-BHPs in the brackish-to-fresh water column. The transition phase is marked by a decrease in the abundance of most Nu-BHPs and an increase in the abundance of methoxylated-BHPs as well as BHPs such as aminobacteriohopanetriol which are ubiquitous across a wide range of environments including soils as well as marine and freshwater settings. The euxinic marine phase (7.2 ka–present) can be divided into two stages based on changes in BHP composition. The early stage is characterised by a high abundance of aminobacteriohopanetetrol and aminobacteriohopanepentol, which were likely produced by methanotrophs at the oxycline. A shallow oxycline likely allowed for increased transport of these BHPs to the sediment. The later marine phase is characterised by a decline in these BHPs, likely due to a deepening of the oxycline and reduced transport of BHPs from the oxycline to the sediment. The changes in BHP distributions throughout the record may either be attributed to shifts in the bacterial communities or physiological adaptations of bacteria to the changing environment. Throughout the record, diagenetic products of BHPs (e.g., anhydro-bacteriohopanetetrol) were detected. These degradation products, however, remain a small proportion of the overall BHP composition, indicating excellent preservation conditions throughout the record. This study offers new insights into changes in microbial communities and biogeochemical processes that occurred in the Black Sea during the Last Deglaciation and Holocene in response to substantial shifts in the hydrology and oxygen conditions of the basin.