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

Abstract

Gram-negative bacterial 3-hydroxy fatty acids (3-OH-FAs) 20 biomarkers are widespread in a variety of environments including both marine 21 and terrestrial sediments (including speleothems). In this study we analysed 22 the hydroxylated membrane lipids of 26 soil samples from an altitudinal 23 transect of Shennongjia Mountain (Mt.) in central China to study the 24 environmental factors controlling the relative distribution of 3-OH-FAs. Our 25 results show that both the ratio of the summed iso and anteiso to the total 26 amount of normal 3-OH-FAs (RIAN), and the ratio of summed iso and anteiso 27 to the total amount of all 3-OH-FAs (Branched Index) were primarily related to 28 the pH of soil (R 2 = 0.70 and 0.70, respectively). Additionally, the anteiso to 29 normal 3-hydroxy fatty acids ratio of the C15 and C17 homologues (RAN15 and 30 RAN17) shows a significant negative correlation with mean annual air 31 temperature (MAAT) (R 2 =0.51 and 0.48, respectively). When comparing the 3-32 OH-FA based indices with established glycerol dialkyl glycerol tetraether 33 (GDGT) based indices from the same soil samples, the RIAN and Branched 34 Index show strong linear correlations with the cyclisation ratio of branched 35 tetraethers (CBT) (R 2 = 0.77 and 0.74, respectively), and the RAN15 and RAN17 36 show negative correlations with the MBT/CBT-MAAT (MBT, methylation index 37 of branched tetraethers) (R 2 = 0.61 and 0.36, respectively). Our new field-based 38 correlations demonstrate the physiological response of Gram-negative bacterial 39 cell membranes to the external environment and suggest that 3-hydroxy fatty 40 3 acids can be applied in palaeoenvironmental studies to estimate past MAAT 41 and soil pH.

Evaluation of 3-hydroxy fatty acids as a pH and temperature proxy in soils from temperate and tropical altitudinal gradients

“…Preparation of samples for 3-OH FA analysis followed the procedure of Wang et al (2016) with slight modifications. Briefly, ca.…”

“…10 g of freeze-dried and ground soil were subjected to acid hydrolysis. Whereas only free 3-OH FAs are obtained upon direct solvent extraction, additional compounds bound to LPS of Gram-negative bacteria can be released through the hydrolysis step (Wang et al, 2012(Wang et al, , 2016. Acid hydrolysis was shown to be more efficient in extracting 3-OH FAs from soil samples than saponification, suggesting that most of these molecules are linked to macromolecules by amide bonds in soils, the latter being difficult to cleave off by saponification (Yang et al, 2016).…”

“…Recently, Wang et al (2016) suggested that other bacterial lipids, so called 3-hydroxy fatty acids (3-OH FAs), could be used as such a proxy. These compounds, containing 10 to 18 C and a hydroxyl group in third position, are characteristic components of the Gram-negative bacterial lipopolysaccharide (LPS).…”

“…Recently, 3-OH FAs were analysed in 26 soils located between 315 and 2840 m elevation along Mt. Shennongjia (China; Wang et al, 2016). Significant correlations were obtained between the relative abundance of these compounds and temperature or pH.…”

“…Majella, Central Italy. The aims were (i) to examine and compare the distributions of 3-OH FAs in soils from different climatic zones, including the Chinese samples previously analysed by Wang et al (2016) and…”