Lipid Analyses for Viable Microbial Biomass, Community Composition, Metabolic Status, and In Situ Metabolism

Abstract

This chapter discusses how phospholipid fatty acids (PLFA) and other lipids can be used to estimate total viable microbial biomass, community composition, and metabolic status as well as the use of in situ 13C incorporation to determine metabolic pathways in environmental samples. The viable microbial biomass increases with the availability of metabolizable substrates and may decrease after their exhaustion. Gas chromatography-mass spectrometry (GC-MS) of PLFA as their methyl esters provides greater sensitivity and specificity than the LP method and additionally provides detailed information on the microbial community composition and metabolic status. Some PLFA are indicative of particular phylogenic groups, and considered together with the environment from which the sample was retrieved, can be helpful in interpreting results. Biomarkers for the metabolic status of microbial communities include those for starvation and toxicity, unbalanced growth, and aerobic versus anaerobic growth, among others. Ubiquinones (UQ) are biomarkers for oxic respiration that mediate electron transport to oxygen and nitrate, while menaquinones (MK) can carry electrons to any electron acceptor. Besides the major respiratory quinones UQ and MK, Bacteria and Archaea also contain desmethylmenaquinones, methionaquinones, plastoquinones, rhodoquinones, and caldariellaquinones, which can also vary in the length of the side chain and, in the case of MK, in the degree of hydrogenation of the side chain.