Bacteriohopanepolyols track environmental transitions in the Black Sea

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 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, i.e. the bottom water is characterized by sulfidic conditions and a lack of oxygen. During the lacustrine phase we observe a high abundance and diversity of nucleoside BHPs (Nu-BHPs) that are likely associated with high terrigenous inputs. We also find evidence of BHPs likely associated with the nitrogen-cycle. For instance, an increase in oxazinone-aminobacteriohopanetriol could indicate that there was enhanced nitrite-oxidation during the lacustrine phase. The transition phase is generally characterized by an increase in the abundance of methoxylated-BHPs and various BHPs that are generally associated with aquatic environments (e.g., aminobacteriohopanetriol). Methoxylated-BHPs also persist into the marine phase, indicating that these BHPs may be specific to saline environments. The euxinic marine phase (7.2 ka-present) is composed of two stages. The early stage is characterized by a high abundance of aminobacteriohopanetetrol and aminobacteriohopanepentol, as well as ethenolamine-bacteriohopanepentol and -bacteriohopanehexol, that are likely transported to the sediments from the shallow oxycline niche of their methanotrophic sources. The later marine phase is characterized by a decline in these BHPs, likely due to a deepening of the oxycline. The change in distribution of many BHPs throughout the record may either be attributed to microbial adaptations to significant shifts in the oxygenation and/or hydrology of the basin or are associated with specific groups of bacteria that are suited to these environmental conditions. Throughout the record, diagenetic products of BHPs (e.g., anhydrous-bacteriohopanetetrol) were detected. These BHPs, however, remain a small proportion of the overall BHP composition, indicating high 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 significant shifts in the hydrology and oxygenation of the basin.