Altitudinal transects reveal large differences in intact lipid composition among soils

Abstract

Fatty acid-based lipids comprise a small but important component of soil organic matter. Lipids are indispensable components of soil microbes due to their function as components of membranes and as stores of energy and C. Hence, lipid composition is likely under strong selection pressure and there ought to be strong associations between lipid composition of microbial communities and environmental conditions. Associations between microbial lipids and environment likely involve an integrated combination of differences in lipid headgroups (classes) and fatty acyl chains. However, past studies examining associations between soil lipid composition and environmental conditions have focussed on fatty acids hydrolysed from polar lipids and less is known about headgroups (classes) of polar lipids. The aim of this study was to examine associations between environmental conditions changing with altitude and the intact polar and non-polar lipids of soil microbial communities. We used two altitudinal transects, both spanning from forest through to above the alpine treeline, but separated from one another by ~700 km. Liquid chromatography-mass spectrometry identified 174 intact lipids to the level of class and sum composition. Approximately half of the pool of fatty acid-based lipids was accounted for by two classes of non-polar lipids (diacylglycerol and triacylglycerols), while the other half was dominated by three classes of polar lipids (phosphatidylethanolamine, phosphatidylcholine and diacylglyceryl-N,N,N-trimethylhomoserine). There were large differences among sites in the relative amounts of lipid classes. For example, diacylglyceryl-N,N,N-trimethylhomoserine varied among sites from 5 to 41% of the polar lipid pool, phosphatidylcholine from 31 to 60% of the polar lipid pool, and diacylglycerols from 9 to 53% of the total non-polar pool. Relationships of lipid composition with altitude were weak or differed between transects, and pH was the variable most strongly associated with lipid composition. Variation among sites in the relative abundance of phosphatidylcholine were positively associated with pH, while relative and absolute abundance of diacylglycerol was negatively related to pH. We suggest that the accumulation of diacylglycerol at low pH represents slowed hydrolysis and/or microbial utilisation. A large fraction of variance among sites in lipid composition remained unexplained, which highlights the need for additional research on processes leading to production and consumption of fatty acid-based lipids.