Bacteriohopanepolyols along redox gradients in the Humboldt Current System off northern Chile

Marine oxygen minimum zones (OMZs) are characterized by the presence of subsurface suboxic or anoxic waters where diverse microbial processes are responsible for the removal of fixed nitrogen. OMZs have expanded over past decades and are expected to continue expanding in response to the changing climate. The implications for marine biogeochemistry, particularly nitrogen cycling, are uncertain. Cell membrane lipids (biomarkers), such as bacterial bacteriohopanepolyols (BHPs) and their degradation products (hopanoids), have distinctive structural attributes that convey information about their biological sources. Since the discovery of fossil hopanoids in ancient sediments, the study of BHPs has been of great biogeochemical interest due to their potential to serve as proxies for bacteria in the geological record. A stereoisomer of bacteriohopanetetrol (BHT), BHT II, has been previously identified in OMZ waters and has as been unequivocally identified in culture enrichments of anammox bacteria, a key group contributing to nitrogen loss in marine OMZs. We tested BHT II as a proxy for suboxia/anoxia and anammox bacteria in suspended organic matter across OMZ waters of the Humboldt Current System off northern Chile, as well as in surface and deeply buried sediments (125-150 ky). The BHT II ratio (BHT II/total BHT) increases as oxygen content decreases through the water column, consistent with previous results from Perú, the Cariaco Basin and the Arabian Sea, and in line with microbiological evidence indicating intense anammox activity in the Chilean OMZ. Notably, BHT II is transported from the water column to surface sediments, and preserved in deeply buried sediments, where the BHT II ratio correlates with changes in δ¹⁵ N sediment values during glacial-interglacial transitions. This study suggests that BHT II offers a proxy for past changes in the relative importance of anammox, and fluctuations in nitrogen cycling in response to ocean redox changes through the geological record.