3-Hydroxy fatty acids (3-OH-FAs), derived from Gram-negative bacterial outer membranes, have received recent attention for their potential as new terrestrial pH and temperature proxies for palaeoclimate studies. Initial studies from altitudinal transects of contemporary soils – correlating bacterial 3-OH-FA compositions to air temperature and pH – have shown promising results. But the geographical extent of recent calibrations is limited. In this study, we analyse 3-OH-FA lipid distributions in 186 globally distributed soil samples to study the environmental factors controlling the relative distribution of the 3-OH-FA isomers. Our sample-set covers a wide range of temperatures (−0.4 to 27 °C) and pH (3.6–9.2). For the global compilation we find that the ratio of anteiso to normal 3-OH-FAs of the C15 or C17 homologues (RAN15 or RAN17) shows a strong linear relationship with mean annual air temperature (MAAT) (R2 = 0.48, p < 0.001 and R2 = 0.41, p < 0.001, respectively). Additionally, the negative logarithm of the ratio of the summed iso and anteiso to the total amount of normal 3-OH-FAs (RIAN) is also strongly anticorrelated with the soil pH (R2 = 0.66, p < 0.001). However, we find that for our 3-OH-FA based proxies there are significant differences in slope and intercept of the linear corrections at regional scales. Thus local or regional calibrations are likely preferable (at this stage of 3-OH-FA proxy development) for application to specific palaeoclimate archives. We also explore the relationship of 3-OH-FA isomer fractional abundances to environmental parameters using machine learning tool (a Gaussian Process (GP) emulator). This confirms the first order relationships to environmental parameters highlighted by the empirical equations and also derives several alternative GP emulator models for reconstructing MAAT and pH which give higher R2 values (0.66 for MAAT; 0.63 for pH) and lower RSME values (3.5 °C for MAAT; 0.76 for pH) compared to simple linear regressions at the global scale. We compare our 3-OH-FA based indices with bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs) based indices from the same soil samples. At a global scale RAN15 and RAN17 show negative correlations with the MBT′5ME-MAAT (MBT′5ME, methylation index of 5-methyl branched tetraethers) (r = −0.59, p < 0.001 and r = −0.42, p < 0.001, respectively), whilst RIAN shows strong linear correlations with the cyclisation ratio of branched tetraethers (CBT) (r = 0.77, p < 0.001). Similar to 3-OH-FA based temperature proxies, GDGT based temperature proxy MBT′5ME also showed different regional calibrations. Our new field-based correlations demonstrate the broad physiological response of Gram-negative bacterial cell membranes to external environmental changes on a global scale. We suggest that 3-OH-FA based proxies have widespread potential for palaeoenvironmental studies to estimate past MAAT and soil pH, but that regional/local and context specific calibrations may need to be applied.
Bibliographical noteFunding Information:
We thank Stephan Mantel and Gerard Heuvelink at ISRIC World Soil Information (Wageningen, Netherlands), for sampling and provision of archived soil samples. We thank Dr. Linfeng Gong, Mr. Shijin Zhao, Dr. Xinyue Dang and Dr. Jiantao Xue from China University of Geosciences (Wuhan) for their help during the field sampling in China. Huub Zwart, Vinothan Sivapalan, Caroline Bendle, George Bendle and Ryan Bendle are thanked for their participation in the USA field sampling campaign and Jessica Conway (University of Illinois Urbana Champaign), Chad Broyles and colleagues (International Ocean Discovery Programme - Texas A&M University) and Dr Amelia Shevenell (University of Southern Florida) and David Tubbs and Eimear Orgill (University of Birmingham) for help with equipment, logistics, shipping, sample handling and soil licenses. Thanks to Dr. Xing Xiang (Shangrao Normal University) for helping with the global map plotting. This work was supported by funding bodies in China and the UK, namely the National Natural Science Foundation of China (Grant Nos. 41830319, 41807419, 41821001, 91951208, U20A2094, 41773135), the National Key R&D Project of Ministry of Science and Technology (Grant No. 2016YFA0601100), the 111 project (National Bureau of Foreign Experts and the Ministry of Education of China; Grant No. BP0820004), the Fundamental Research Funds for the National Universities, China University of Geosciences, Wuhan (Grant No. CUGL170815), the Leverhulme Trust (RPG-2018-110), the Birmingham-Illinois Partnership Fund, the UoB School of GEES Pump Priming Fund. We thank the China Scholarship Council (CSC) (Grant No. 201306410031) for supporting Canfa Wang's studies at the University of Birmingham and the UK's Natural Environmental Research Council (NERC) and the NERC CENTA-DTP (Central England NERC Training Alliance ? Doctorial Training Partnership) for funding Alice Hardman's PhD studies and research training grant (NE/L002493/1) and for an independent research fellowship to Sarah Greene (NE/L011050/1). Ilya Mandel is a recipient of the Australian Research Council Future Fellowship FT190100574.
© 2021 Elsevier Ltd
- 3-Hydroxy fatty acid
ASJC Scopus subject areas
- Geochemistry and Petrology