Impacts of pH and temperature on soil bacterial 3-hydroxy fatty acids: development of novel terrestrial proxies

Gram-negative bacterial 3-hydroxy fatty acids (3-OH-FAs) biomarkers are widespread in a variety of environments including both marine and terrestrial sediments (including speleothems). In this study we analysed the hydroxylated membrane lipids of 26 soil samples from an altitudinal transect of Shennongjia Mountain (Mt.) in central China to study the environmental factors controlling the relative distribution of 3-OH-FAs. Our results show that both the ratio of the summed iso and anteiso to the total amount of normal 3-OH-FAs (RIAN), and the ratio of summed iso and anteiso to the total amount of all 3-OH-FAs (Branched Index) were primarily related to the pH of soil (R² = 0.70 and 0.70, respectively). Additionally, the anteiso to normal 3-hydroxy fatty acids ratio of the C₁₅ and C₁₇ homologues (RAN₁₅ and RAN₁₇) shows a significant negative correlation with mean annual air temperature (MAAT) (R²=0.51 and 0.48, respectively). When comparing the 3-OH-FA based indices with established glycerol dialkyl glycerol tetraether (GDGT) based indices from the same soil samples, the RIAN and Branched Index show strong linear correlations with the cyclisation ratio of branched tetraethers (CBT) (R² = 0.77 and 0.74, respectively), and the RAN₁₅ and RAN₁₇ show negative correlations with the MBT/CBT-MAAT (MBT, methylation index of branched tetraethers) (R² = 0.61 and 0.36, respectively). Our new field-based correlations demonstrate the physiological response of Gram-negative bacterial cell membranes to the external environment and suggest that 3-hydroxy fatty acids can be applied in palaeoenvironmental studies to estimate past MAAT and soil pH.