Bacteriohopanetetrol-<i>x</i>: constraining its application as a lipid biomarker for marine anammox using the water column oxygen gradient of the Benguela upwelling system

Abstract

Abstract. Interpreting lipid biomarkers in the sediment archive requires a good understanding of their application and limitations in modern systems. Recently it was discovered that marine bacteria performing anaerobic ammonium oxidation (anammox), belonging to the genus Ca. Scalindua, uniquely synthesize a stereoisomer of bacteriohopanetetrol (“BHT-x”). The ratio of BHT-x over total bacteriohopanetetrol (BHT, ubiquitously synthesized by diverse bacteria) has been suggested as a proxy for water column anoxia. As BHT has been found in sediments over 50 Myr old, BHT-x has the potential to complement and extend the sedimentary biomarker record of marine anammox, conventionally constructed using ladderane lipids. Yet, little is known about the distribution of BHT-x in relation to the distribution of ladderanes and to the genetic evidence of Ca. Scalindua in modern marine systems. Here, we investigate the distribution of BHT-x and the application of the BHT-x ratio in relation to distributions of ladderane intact polar lipids (IPLs), ladderane fatty acids (FAs) and Ca. Scalindua 16S rRNA genes in suspended particulate matter (SPM) from the water column of the Benguela upwelling system (BUS), sampled across a large oxygen gradient. In BUS SPM, high BHT-x abundances were restricted to the oxygen-deficient zone on the continental shelf (at [O2] < 45 µmol L−1, in all but one case). High BHT-x abundances co-occurred with high abundances of the Ca. Scalindua 16S rRNA gene (relative to the total number of bacterial 16S rRNA genes) and ladderane IPLs. At shelf stations with [O2] > 50 µmol L−1, the BHT-x ratio was < 0.04 (in all but one case). In apparent contradiction, ladderane FAs and low abundances of BHT and BHT-x (resulting in BHT-x ratios > 0.04) were also detected in oxygenated offshore waters ([O2] up to 180 µmol L−1), whereas ladderane IPLs were undetected. The index of ladderane lipids with five cyclobutane rings (NL5) correlates with in situ temperature. NL5-derived temperatures suggested that ladderane FAs in the offshore waters were not synthesized in situ but were transported down-slope from warmer shelf waters. Thus, in sedimentary archives of systems with known lateral organic matter transport, such as the BUS, relative BHT and BHT-x abundances should be carefully considered. In such systems, a higher BHT-x ratio may act as a safer threshold for deoxygenation and/or Ca. Scalindua presence: our results and previous studies indicate that a BHT-x ratio of ≥ 0.2 is a robust threshold for oxygen-depleted waters ([O2] < 50 µmol kg−1). In our data, ratios of ≥ 0.2 coincided with Ca. Scalindua 16S rRNA genes in all samples (n=62), except one. Lastly, when investigating in situ anammox, we highlight the importance of using ladderane IPLs over BHT-x and/or ladderane FAs; these latter compounds are more recalcitrant and may derive from transported fossil anammox bacteria remnants.

Characterization of diverse bacteriohopanepolyols in a permanently stratified, hyper-euxinic lake

“…Notably, stereoisomers of BHT were not observed in any of the samples, despite 'late-eluting BHT isomers' being associated with modern marine water column suboxia/anoxia, as well as bacterial anaerobic oxidation of ammonium (anammox) activity (Sáenz et al, 2011;Wakeham et al, 2012;Kharbush et al, 2013;Rush et al, 2014;Matys et al, 2017Matys et al, , 2019Schwartz-Narbonne et al, 2020;van Kemenade et al, 2022). The lack of BHT isomer(s) supports previous studies which suggest that the isomerization of BHT and ratios of BHT to its isomer(s) are more indicative of anammox activity rather than simply suboxia/anoxia.…”

“…Additionally, it has been suggested that the ratio of bacteriohopanetetrol (BHT) to its isomer(s) in sediments can be used as a proxy for suboxicanoxic water column conditions (Sáenz et al, 2011). The presence of specific BHT isomers has more recently been associated with anaerobic ammonium-oxidation (anammox), an important microbial process in the nitrogen cycle (e.g., Rush et al, 2014;Matys et al, 2017;Schwartz-Narbonne et al, 2020;van Kemenade et al, 2022). Many studies now postulate that C-2 methylated hopanoids (originally thought to be indicative of oxygenic photosynthesis; Summons et al, 1999) are produced in response to myriad environmental stressors (e.g., changes in pH, redox, nutrient availability, solar irradiance), combinations of which are still being revealed (see review by Newman et al, 2016 and references therein;Matys et al, 2019a).…”

Loss of nitrogen via anaerobic ammonium oxidation (anammox) in the California Current system during the late Quaternary

“…Babbin et al (2014) showed, using incubations from the ETNP OMZ, that anammox rates increase in response to the addition of OM. Likewise, in the modern Southern Pacific OMZ, N loss by anammox was found to be strongly correlated with the export of OM, via the release of ammonium into the water column through remineralization (Kalvelage et al, 2013). As such, the co-occurrence of ladderane FA and paleoproductivity proxy maxima during MIS 5 could reflect an increase in Ca.…”

“…modern annual aver- age bottom water temperatures at Site 1012 are < 5 °C; Garcia et al, 2019). Additionally, while the transport of ladderane FAs has been shown to occur within oxygen-depleted systems (van Kemenade et al, 2022), long-distance transport of ladderane FAs with the CU (characteristic DO concentration of ∼ 62 µmol L −1 in modern CU water; Sahu et al, 2022) is unlikely, and it would be expected to yield higher relative abundances of SC-ladderane FAs than detected in the record. Transport of ladderanes is also not reflected in present-day ENP ladderane distributions, as an investigation of ladderanes at more northerly (∼ 20°N) and more southerly (∼ 17°N) located sites showed in situ synthesis by pelagic Ca.…”

“…Thus, SC-ladderane FAs in the sediment archive may be used to trace back anammox cell material that has been exposed to oxic conditions, such as sedimentation through the oxic water underlying an OMZ. Furthermore, the index of ladderane FAs with five cyclobutane rings (NL 5 ) has been shown to correlate with the in situ water temperature at which ladderane FAs are synthesized (Rattray et al, 2010), which has been used to determine the provenance of ladderane lipids (Jaeschke et al, 2009;Rush et al, 2012a;van Kemenade et al, 2022).…”