Biomarker evidence for the occurrence of anaerobic ammonium oxidation in the eastern Mediterranean Sea during Quaternary and Pliocene sapropel formation

Abstract

Abstract. The eastern Mediterranean Sea sedimentary record is characterised by intervals of organic-rich sapropel sediments, indicating periods of severe anoxia triggered by astronomical forcing. It has been hypothesised that nitrogen fixation was crucial in injecting the Mediterranean Sea with bioavailable nitrogen (N) during sapropel events. However, the evolution of the N biogeochemical cycle of sapropels is poorly understood. For example, the role of the complementary removal reactions like anaerobic ammonium oxidation (anammox) has not been investigated because the traditional lipid biomarkers for anammox, ladderane fatty acids, are not stable over long periods in the sedimentary record. Using an alternative lipid biomarker for anammox, bacteriohopanetetrol stereoisomer (BHT isomer), we present here for the first time N removal throughout the progression, e.g. formation, propagation, and termination, of basin-wide anoxic events. BHT isomer and ladderanes were analysed in sapropel records taken from three eastern Mediterranean sediment cores, spanning S1 to Pliocene sapropels. Ladderanes were rapidly degraded in sediments, as recently as the S5 sapropel. BHT isomer, however, was present in all sapropel sediments, as far back as the Pliocene, and clearly showed the response of anammox bacteria to marine water column redox shifts in high-resolution records. Two different N removal scenarios were observed in Mediterranean sapropels. During S5, anammox experienced Black Sea-type water column conditions, with the peak of BHT isomer coinciding with the core of the sapropel. Under the alternative scenario observed in the Pliocene sapropel, the anammox biomarker peaked at onset and termination of said sapropel, which may indicate sulfide inhibition of anammox during the core of sapropel deposition. This study shows the use of BHT isomer as a biomarker for anammox in the marine sediment record and highlights its potential in reconstructing anammox during past anoxic events that are too old for ladderanes to be applied, e.g. the history of oxygen minimum zone expansion and oceanic anoxic events.

Origin of Sedimentary BHPs Along a Mississippi River–Gulf of Mexico Export Transect: Insights From Spatial and Density Distributions

“…This hypoxia influences the microbial community structures (Gillies et al, 2015;Gillies Campbell et al, 2019) and BHP assemblages have been shown to vary with redox conditions elsewhere (Wakeham et al, 2007;Sáenz et al, 2011b;Kharbush et al, 2013;Ricci et al, 2014;Rush et al, 2014;Höfle et al, 2015;Matys et al, 2017). BHT II accumulates under suboxic and anoxic conditions (Sáenz et al, 2011b;Matys et al, 2017) and, more recently, it has been used as a proxy for anammox bacteria when present in high abundance relative to BHT [BHT II ratios; BHT II/(BHT + BHT II)] (Rush et al, 2014(Rush et al, , 2019Matys et al, 2017). Support for an anammox origin of BHT II comes from its significantly δ 13 C-depleted values (-50.6%) in a Mediterranean Sea sediment (Hemingway et al, 2018).…”

Dark carbon fixation in the Arabian Sea oxygen minimum zone contributes to sedimentary organic carbon (SOM)

“…Here, we investigated the δ 13 C value of sedimentary organic matter of surface sediments deposited within the OMZ of the Arabian Sea and employed a simple isotope mixing model to investigate the extent of input from OMZ carbon fixation into sedimentary organic matter. As the major process in the Arabian Sea OMZ 10.1029/2019GB006282 Global Biogeochemical Cycles known to produce isotopically light biomass is anaerobic oxidation of ammonia (anammox; Ulloa et al, 2012;Villanueva et al, 2014), in order to determine the isotopic signature of this pathway, we developed and applied a method to determine the δ 13 C values of a novel biomarker, bacteriohopanetetrol stereoisomer (BHT'), which has been found to be unique to anammox bacteria in the marine environment in culture and environmental studies (Rush et al, 2014(Rush et al, , 2019Rush & Sinninghe Damsté, 2017). It has been found in the Arabian Sea and other marine anoxic settings (Matys et al, 2017;Sáenz et al, 2011).…”

Analytical and Computational Advances, Opportunities, and Challenges in Marine Organic Biogeochemistry in an Era of “Omics”

“…Core and intact archaeal isoprenoidal GDGTs can be now analyzed in a single run using reversed-phase chromatography (Zhu et al, 2013), while the analysis of core bacterial and archaeal GDGTs has been improved by the use of ethylene bridged hybrid (BEH) HILIC columns (Hopmans et al, 2016). The detection of BHPs has been improved by the analysis of non-derivatized compounds using either atmospheric pressure chemical ionization (APCI) (Talbot et al, 2016) or electrospray ionization (ESI) (Rush et al, 2019).…”