3-Hydroxy fatty acids (3-OH-FAs), lipid biomarkers mainly derived from Gram-negative bacteria, have recently been proposed as proxies for environmental parameters such as temperature and pH. However, their applicability in deep-ocean settings remains poorly understood, as previous studies have primarily focused on terrestrial environments and marginal seas. Moreover, the dynamic responses of Gram-negative bacterial communities to environmental factors may influence the production and distribution of 3-OH-FAs.
In this study, we investigated the spatial distribution of 3-OH-FAs in surface sediments from open-ocean and hadal environments, and conducted incubation experiments using surface sediments from the Yangtze Estuary and the western and central Pacific to evaluate microbial responses to temperature and oxygen conditions. The results reveal pronounced regional heterogeneity in 3-OH-FA distributions, with hadal sediments showing unusually high concentrations that cannot be explained solely by sea surface temperature. Incubation experiments further show that although microbial community structure is shaped by temperature, the production and composition of 3-OH-FAs do not vary linearly with temperature. Instead, micro-oxygen conditions and the stochastic nature of microbial succession during incubation may play more important roles in determining 3-OH-FA composition. These findings suggest that 3-OH-FA signals in deep-marine sediments are influenced by multiple environmental and ecological factors, and their application as paleoenvironmental proxies in the deep ocean requires further caution and calibration.