INTEGRATED BIOMARKER INVENTORY REVEALS RESPONSE OF MICROBIAL COMMUNITY TO GLOBAL WARMING INDUCED WATER CHEMISTRY CHANGES IN THE BLACK SEA

Abstract

The Black Sea is a site of intense scientific study because of its rich sediment archive. A semi-enclosed marginal sea, the Black Sea has undergone significant paleoceanographic changes during the late Quaternary, most notably its reconnection with the Mediterranean Sea in the early Holocene (~9,500–7,500 years ago) following the last glacial maximum (LGM). During the LGM (~26,500–19,000 years ago), global sea levels dropped by ~120 m, isolating the Black Sea as a freshwater lake until rising waters allowed Mediterranean inflow, first as a gradual incursion (~9,500 years BP) and then as a fully marine connection (~7,500 years BP), leading to the modern stratified water column with a brackish surface layer and anoxic deep waters. This transition is marked by a shift from freshwater lacustrine sediments to marine sapropelic muds, supported by microfossil evidence. The reconnection induced strong salinity-driven stratification, causing oxygen depletion in deeper waters and establishing the Black Sea’s permanently anoxic conditions, with a layered water column consisting of an oxic surface layer (0–100 m), a suboxic zone (100–150 m), and a sulfidic anoxic zone (>150 m) dominated by hydrogen sulfide (H₂S). Sedimentary records reveal redox-sensitive geochemical markers such as molybdenum (Mo) and uranium (U) enrichments, pyrite (FeS₂) formation, and high organic carbon preservation due to limited microbial degradation under anoxia. Lipid biomarkers can provide additional information about the Black Sea’s sedimentation, redox and salinity evolution history. These molecular fossils, combined with isotopic and inorganic geochemical data, reconstruct the lake-to-marine transition, euxinia onset, and anthropogenic impacts, making the Black Sea a critical natural laboratory for studying paleoenvironmental changes, redox evolution, and organic carbon burial in marginal seas. The overarching theme of this work is the application of an integrated biomarker inventory to understand the co-evolution of the planktonic microbial community and the Holocene Black Sea’s salinity and redox changes. A biomarker inventory application rather than the use of single, stand-alone biomarkers is important to capture the existing condition and response of source organisms that dwell at different depths of the water column. This approach potentially helps minimize bias that could arise from the use of single biomarker groups. The work is reported in three chapters. Chapter 1 concerns lipid inventory of targeted biomarkers to reconstruct unique responses of microorganisms dwelling at different depths of the water column. The targeted biomarkers include Chlorophyll-a degradation derivatives, (Chl-a-DDs), long-chain alkenones (LCAs), crenarchaeol, overly branched glycerol dialkyl glycerol tetraether (OB-GDGTs) and dialkyl glycerol ethers (DAGEs) which are markers of organisms that dwell at different depths from surface waters to the sea-sediment interface. The inventory reveals differences in microbial dynamics prior to marine incursion, progressive changes during the incursion and dynamics in the Late Holocene. Mediterranean incursion and the resulting stratification impacted chemocline lead to increasing dominance of photosynthesis over chemosynthesis. Euxinia developed gradually in the basin, and it has remained euxinic with the impact of Late Holocene surface freshening stimulating primary productivity but not having much effect on water column stratification. Chapter 2 focuses on characterizing the unique response of algae versus other photosynthetic organisms to salinity and redox changes. A potential proxy that can help with discriminating Isochrysidales algae from other photosynthetic sources is also discussed. Prior to the incursion, primary productivity was low and relative abundance of other phytoplanktons (proxied by chlorophyll-a-degradation derivations) to haptophytes were stable, with Chl-a-DDs/LCAs values ranging between 0.7 and 1.6. The δ¹³Corg and δ¹⁵Norg data show large variability, likely reflecting mixtures of terrestrial and aquatic organic matter. Isochrysis algae were not well adapted to increasing salinity that accompanied the Mediterranean incursion with estimated concentration dropping to lowest values as marine incursion continued. Progressive increase in TOC, positive carbon isotopic excursion and progressive increase of δ¹⁵Norg support the idea of increased primary productivity and enhanced preservation, although the increased productivity was mainly by other phytoplankton groups. There is resurgence in Isochrysis algae with Late Holocene surface water freshening. Chapter 3 details the detection and distribution of long-chain alkenones and haptophytes’ ecological dynamics and response to climate variation in Holocene. Using reversed-phase liquid chromatography electrospray ionization quadrupole time-of-flight (RPLC-ESI-qTOF-MS) LCAs with 37 to 40 carbons, with unsaturation degrees ranging from di- to tetra unsaturated as well as methyl- and ethyl-compounds were detected and quantified. Baseline resolution was achieved for C37 alkenone double bond isomers as well as distinct separation of C38Me and C38Et alkenone peaks. A novel discovery is the decrease in the abundance of tetra-unsaturated alkenones with increasing carbon chain length and the intolerance of tetra-unsaturated alkenone-producing haptophytes to elevated salinity.