Estimation of bacterial biomass in subsurface sediments by quantifying intact membrane phospholipids

In an earlier study of deep subsurface sediments from Nankai Trough (ODP Leg 190, offshore Japan) we employed intact phospholipids (PLs) as molecular indicators of living microorganisms. The current study extends this work by quantifying absolute amounts of sedimentary PLs by liquid chromatography-mass spectrometry (LC-MS) and by converting PL data into cell numbers in order to improve methods to estimate the extent of bacterial life in the subsurface. Investigations were carried out on 90 cm short cores of Lake Baikal sediment. High amounts of identified intact PLs are interpreted as reflecting the constituents of living bacteria due to high organic matter decomposition and oxic mineralisation between the epilimnion and the sediment-water interface. Concentrations of ester-bound PLs reach up to 13,120 ng/g sediment dry weight. Predominance of ethanolamine and glycerol PL head groups confirms the bacterial origin. The most abundant side-chain pairs are combinations including 14:0 and 16:0 fatty acids and to a minor extent 15:0 and 16:1 fatty acids. Depth profiles of PL concentrations converted from conventional PL fatty acid analysis are of the same order of magnitude and show comparable trends as those for intact PLs. An approximate estimation of bacterial cell numbers is inferred from intact PL quantification using LC-MS.