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.

源自革兰氏阴性细菌外膜的3-羟基脂肪酸（3-OH-FAs）由于其作为古气候研究新的陆地pH和温度替代物的潜力而受到了近期的关注。对当代土壤的高空样带进行的初步研究（将细菌的3-OH-FA成分与气温和pH相关联）已显示出令人鼓舞的结果。但是最近校准的地理范围是有限的。在这项研究中，我们分析了186个全球分布的土壤样品中的3-OH-FA脂质分布，以研究控制3-OH-FA异构体的相对分布的环境因素。我们的样品组涵盖了广泛的温度范围（−0.4至27°C）和pH（3.6–9.2）。对于全局编译，我们发现前等值与法线的比率C ₁₅或C ₁₇同源物（RAN ₁₅或RAN ₁₇）的3-OH-FA与年平均气温（MAAT）表现出很强的线性关系（R ²  = 0.48，p  <0.001和R ²  = 0.41，p  <分别为0.001）。此外，该求和的比率的负对数异和反异向的总量正常3-OH-FAS（RIAN）也强烈与土壤pH反相关（R ²  = 0.66，p <0.001）。但是，我们发现，对于基于3-OH-FA的代理，在区域范围内，线性校正的斜率和截距存在显着差异。因此，对于特定的古气候档案，应用本地或区域校准可能是更可取的（在3-OH-FA代理开发的这一阶段）。我们还使用机器学习工具（高斯过程（GP）模拟器）探索了3-OH-FA异构体分数丰度与环境参数的关系。这证实了经验公式突出显示的与环境参数的一阶关系，并且还推导了用于重建MAAT和pH的多个替代GP仿真器模型，这些模型提供了更高的R ²与简单的线性回归在全球范围内相比，该值（MAAT为0.66； pH为0.63）和较低的RSME值（MAAT为3.5°C； pH为0.76）。我们将基于3-OH-FA的指数与来自相同土壤样品的细菌支链甘油二烷基甘油四醚（brGDGTs）的指数进行比较。在全球范围内，RAN ₁₅和RAN ₁₇与_MBT'5ME -MAAT（_MBT'5ME，5-甲基支链四_醚的甲基化指数）呈负相关（r  = -0.59，p  <0.001和r  = -0.42，p  <分别为0.001）和RIAN与支化四醚（CBT）的环化率显示出强烈的线性相关性（r = 0.77，p  <0.001）。与基于3-OH-FA的温度代理相似，基于GDGT的温度代理_MBT'5ME也显示出不同的区域校准。我们基于新领域的相关性在全球范围内证明了革兰氏阴性细菌细胞膜对外部环境变化的广泛生理反应。我们建议基于3-OH-FA的代理在古环境研究中估计过去的MAAT和土壤pH值具有广泛的潜力，但是可能需要应用区域/局部和特定环境的校准方法。