Soil microbial populations substitute phospholipids with betaine lipids in response to low P availability

Abstract

Soil microbes can be limited by phosphorus availability. A significant proportion of cellular P is accounted for by the phospholipids that comprise membranes. Experiments with aquatic microbial communities and isolated microbes have shown one way microbes can reduce their cellular P requirements is by substituting phospholipids with P-free polar lipids. We do not know if soil microbial communities can also spare P by using P-free lipids instead of phospholipids.The aims of this study were to examine a range of contrasting soils and a) determine the profile of polar fatty-acid-based lipids; and b) examine if P-free polar lipids are more abundant in P-poor soils. To obtain a broad range in P availability we examined soils from 16 sites that encompassed a range of geologies and vegetation types. Intact lipids were identified and quantified using liquid-chromatography-mass spectrometry. An independent estimate of polar lipid classes was obtained by hydrolysing lipids and quantifying headgroups with capillary electrophoresis-mass spectrometry.Among sites soil P status varied from supra-optimal (N:P = 2.4 g g−1) through to severely deficient (N:P = 80 g g−1). The dominant polar lipids among sites were phosphatidylethanolamine (PE), phosphatidylcholine (PC), and the betaine lipid diacylglyceryl-N,N,N-trimethylhomoserine (DGTS). Among sites as P availability declined a larger proportion of polar lipids was accounted for by betaine lipids such that in the most severely P-deficient soils phospholipids were only 61% of total polar lipids. If we assume that phospholipids account for one-third of cellular P, the measured substitution of phospholipids with betaine lipids would reduce the amount of P required for growth by around 10%. The substantial P saving due to lipid substitution could contribute to the selection of taxa under P deficiency.