Effect of salinity on the abundance and distributions of Glycerol dialkyl glycerol tetraethers and 3-hydroxy fatty acids in terrestrial aquatic interface

Lipid biomarkers like Glycerol dialkyl glycerol tetraethers (GDGTs) and 3-hydroxy fatty acids (3-OH FAs) are valuable proxies used in paleoenvironmental studies. Although temperature and pH are essential parameters responsible for the variations in abundance and isomeric distribution patterns of GDGTs and 3-OH FAs, the effect of other factors is still not fully investigated, especially in salt-stress environments. While the impact of salinity on GDGTs is not fully understood, the effects on 3-OH FAs have not been previously explored. In this study, we investigated the sensitivity of both proxies and their response to environmental factors (temperature, pH, and salinity) in the Pulicat Lagoon, India, where salinity ranges from 60 ppt (hypersaline) to 32 ppt (brackish). Interestingly, conventional temperature indicators like the Methylation index of brGDGTs (MBT) and the Isomers ratio of brGDGTs (IR) correlated more with water salinity than with lake water temperature. In contrast, the RAN15 index in 3-OH FA showed a stronger temperature response than the RAN17 index. pH-related indices like the Cyclization ratio of branched tetraethers (CBT and CBT’) in GDGTs correlated negatively with sediment pH, while certain 3-OH FA indices such as ratio of all Iso and Ante-isomers to all Normal isomers (RIAN), ratio of all Iso-isomers to all Normal isomers (RIN), Branched index and Branching ratio correlated significantly with salinity. This disparity in response patterns between GDGTs and 3-OH FAs could be due to bacterial diversity and their salinity adaptation mechanisms. Salinity may alter bacterial membrane lipid structure, affecting branching or methylation in fatty acids to maintain cytoplasmic lipid fluidity and permeability. This adaptation is reflected in the temperature indices of GDGTs and pH indices of 3-OH FAs. We propose these proxies as potential salinity indicators, but caution is advised when using them for paleo-temperature and pH reconstruction in salt-stress environments.