Bacteriohopanepolyols Across the Black Sea Redoxcline Trace Diverse Bacterial Metabolisms

Redox gradients in oceanic oxygen minimum zones are hotspots of diverse bacterial populations and metabo-lisms. Here, we test whether bacteriohopanepolyols (BHPs), a suite of membrane-regulating pentacyclic tri-terpenoid lipids, are suitable tracers of major bacterial metabolisms and whether their structural diversity (BHP lipidomics) allows tracing the bacterial ecology across the Black Sea chemocline. We analyzed suspended par-ticulate matter (SPM) sampled along depth-profiles across the redox/chemocline at two sites in the eastern and western gyres and underlying core-top sediments at the western gyre site. Our results show that BHPs abun-dances reflect the stratification of the Black Sea and capture ecological niches across the chemocline, being highest in the lower suboxic zone and in upper to deep sulfidic waters. BHP-inferred bacterial metabolisms in the lower suboxic zone are dominated by anammox and aerobic methane oxidizers (primarily by Type I/gammap-roteobacteria). Bacterial biomass production in the upper to deep sulfidic zone is mirrored by a range of BHPs that are normally associated with aerobic metabolisms, most likely explained by either survival in oxygenated micro-niches or production by anaerobic bacteria (which could possibly include sulfate reducers and nitrite -dependent methane oxidizers). Most surprisingly, this ecological niche harbors seemingly unknown anaerobic bacteria which produce abundant C-2 and C-3 methylated BHPs. Core-top sediments reflect the BHP structural diversity found in SPM, but with notable compositional differences. Most strikingly, the export dynamics seem to result in near-absence of C-2 methylated BHPs in surface sediments. Sedimentary BHP distributions therefore underestimate the importance of C-2 methylated BHPs in the water column, which has implications for the geological record.