Comparison of extraction methods for quantitative analysis of core and intact polar glycerol dialkyl glycerol tetraethers (GDGTs) in environmental samples

Abstract

Archaeal membranes are composed of core lipids called glycerol dialkyl glycerol tetraethers (GDGTs) that contain polar head groups (intact GDGTs) in living cells. The reliability of Archaeal membrane lipids in studies of paleogeochemistry, population distributions, and physiological state depend on efficient extraction and detection of these lipids. We compare existing methods for extraction of core and intact GDGTs so that they can be used as quantitative indicators of living archaeal biomass. We used an active culture of the marine archaeon, Nitrosopumilus maritimus, to document limitations of methods in current use and as a reference for methodological improvement. We found that all extractable GDGTs in the culture were intact and that acid hydrolysis is the only way to quantitatively recover GDGTs from exponentially growing cells. Because acid hydrolysis removes polar head groups, it is not suitable for analysis of intact lipids. In contrast to the culture, high recovery was achieved for intact and core lipids from environmental samples, including water column particles, sediments, and soils. The high recovery from environmental samples and the large portion of core GDGTs relative to intact GDGTs suggest that lipids in the environment are largely derived from dead or dying cells. Because previous studies measured core GDGTs without acid hydrolysis, lipids contained in living cells were not detected and thus these results do not reflect viable biomass. We present a method to accurately quantify living and relict GDGTs by choosing the most efficient extraction protocol and measuring core GDGTs in extracts before and after acid hydrolysis.