Occurrence and distribution of extractable glycerol dialkyl glycerol tetraethers in podzols

The abundance and distribution of glycerol dialkyl glycerol tetraethers (GDGTs) were compared in two contrasting environments: a temperate podzol located 40 km north of Paris (France) and a tropical podzol from the Amazon Basin (Brazil). Five samples were collected from the profile of the temperate podzol, including the litter layer. Five others were obtained from three profiles from the tropical soil sequence, which is representative of the transition between a latosol and a well-developed podzol. GDGTs can occur as core lipids (i.e. as extractable “free” lipids) within the soil organic extract. They may also be present in ester- and/or glycosidic ether-bound form (i.e. as extractable “bound” lipids) in the extract. To date, most studies of GDGTs in soil have considered extractable “free” GDGTs but have not investigated extractable “bound” GDGTs. The extractable “free” and “bound” GDGT pools were therefore both investigated. Accelerated solvent extraction (ASE) was used to extract total extractable (i.e. “free” and “bound”) GDGTs. Half of these extracts was then submitted to acid methanolysis to release ester- and/or glycosidic ether-bound GDGTs, allowing subsequent analysis of total extractable GDGTs using high performance liquid chromatography–mass spectrometry (HPLC/MS). The other half was directly analysed for “free” GDGTs. Branched GDGTs derived from soil bacteria were present in all the samples, along with GDGTs of archaeal origin, although the latter were detected in much lower abundance than bacterial GDGTs. A higher degree of methylation was observed for the alkyl branched GDGTs from the temperate samples than for the tropical ones, in agreement with differences in air temperature. The amount of bacterial and archaeal GDGTs in the surficial sample from the well-developed part of the tropical podzol was greater than in the latosol and transition profiles. The difference could be related to a lower pH in the developed podzol than in the other two profiles. Furthermore, branched GDGTs were detected in the litter layer from the temperate podzol, which may indicate the presence of branched GDGT-producing bacteria in the litter, probably in anoxic microsites. Last, GDGT abundance and distribution were comparable in the extractable “free” and total extractable lipid fractions of most of the soil samples, suggesting that GDGTs mainly occur as “free” lipids in the extracts of all these soils and that similar data concerning past air temperature and soil pH could be derived from both extractable “free” and total extractable GDGT distributions.