Redox indication potential of isoprenoid tetraether lipids in marine environments: Insights from the East China Sea

The response of marine ammonia-oxidizing archaea (AOA) to environmental stress is reflected in changes in their membrane lipid composition, particularly the unique isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs). However, the influence of dissolved oxygen (DO) on the composition of iGDGTs in the ocean remains unclear. This study aims to investigate the link between DO levels and the fractional abundances of iGDGTs in the East China Sea (ECS) to establish a redox proxy. Results suggested that the absolute abundances of iGDGTs were influenced by Thaumarchaeota biomass when DO concentrations exceeded 2 mg/L. Vertical distributions of iGDGTs through the water column suggested their transport from bottom waters to sediments. Increasing proportions of iGDGT-0 and the sum of iGDGT-1, iGDGT-2 and iGDGT-3 from suspended particulate matters to surface sediments indicated their preferential preservation. DO concentrations (2–6 mg/L) in the water column showed a significant positive correlation with the relative abundance of crenarchaeol (cren%) but a negative correlation with iGDGT-0%, suggesting insufficient DO levels to promote AOA cyclization. However, bottom DO concentrations exhibited significant negative correlations with both cren% and iGDGT-0% in surface sediments, attributed to enhanced cyclization of iGDGTs and Euryarchaeota abundance within more reducing sediments, respectively. The consistent relationship between iGDGT-0% and DO in both water column and sediments enabled iGDGT-0% to be a potential redox proxy. Temporal variations in iGDGT-0% in the ECS in recent decades aligned well with in-situ DO monitoring data, further validating iGDGT-0% as a promising redox proxy.