Vertical variations of bacteriohopanepolyols near the Changjiang Estuary waters: Implications for marine hypoxia and bacterial activities

Bacteriohopanepolyols (BHPs) are widespread in bacteria and are used as lipid biomarkers for specific bacterial activities and environmental changes. However, deposition and degradation dynamics of BHPs are still poorly constrained in marine settings, thereby affecting their reliability as biomarkers. Here, we investigated BHPs in the water columns and sediments near the Changjiang Estuary to evaluate their origins, preservation and responses to seawater hypoxia and bacterial activities. Results indicated that bacteriohopanetetrol stereoisomer (BHT-isomer) in this area was predominantly BHT-x produced by Ca. Scalindua, present in hypoxic bottom seawater. 2-Methyl BHT and nitrogen-fixing cyanobacteria showed similar vertical distribution in the water column, as did amnio-BHPs (aminotetrol and aminopentol) and the aerobic methane oxidation (AMO) gene pmoA. 2-Methyl BHT, aminotetrol and aminopentol were thought to primarily derive from cyanobacteria and aerobic methanotrophs in seawater, respectively, despite other potential sources. Depth profiles suggest that insitu production in sediments is not a major contribution to sedimentary BHPs, compared to the export and deposition from the water column. The abundances of unsaturated BHT and amino-BHPs in surface sediments were decreased compared with those in the water column, indicating that unsaturated BHT and amino-BHPs appear to undergo a certain degree of early diagenetic degradation. BHT-isomer and 2-Methyl BHT preserved preferentially over amino-BHPs. Additionally, BHT-isomer significantly correlated with anammox functional gene amx-16S rRNA, and 2-Methyl BHT significantly correlated with nitrogen-fixing functional gene nifH. This indicated that BHT-isomer and 2-Methyl BHT are suitable as biomarkers for anammox and cyanobacterial nitrogen fixation. The BHT-isomer ratio accurately reflects the situation in the water column and appears to be a very appropriate proxy for hypoxia. Conversely, sedimentary aminotetrol and aminopentol underestimate the importance of aerobic methane oxidation in seawater, as these are more rapidly degraded upon deposition to the sea floor.