Occurrence and distribution of non-extractable glycerol dialkyl glycerol tetraethers in temperate and tropical podzol profiles

Abstract

Glycerol dialkyl glycerol tetraethers (GDGTs) are high molecular weight lipids present in the membranes of archaea and some bacteria. Isoprenoid GDGTs with acyclic or ring containing dibiphytanyl chains are known to be synthesised by archaea. In soil, another type of GDGT, which can be distinguished from tetraethers of archaeal origin by way of the branched nature of the alkyl chain, was discovered recently. Alkyl branched GDGTs were suggested to be produced by anaerobic bacteria and can be used to reconstruct past air temperatures and soil pH. Lipids in soils can take two broad forms: extractable, i.e. recoverable via solvent extraction, and non-extractable, linked to the mineral or organic matrix. The present study aimed at comparing the abundance and distribution of these two pools of GDGTs in two contrasting podzol environments: a temperate podzol 40 km north of Paris (France) and a tropical podzol from the Amazon basin (Brazil). Five samples were collected from the whole profile of the temperate podzol. Five additional samples were obtained from three profiles of the tropical soil sequence, which are representative of the transition between a latosol and a well developed podzol. For the first time, we showed that substantial amounts of non-extractable GDGTs can be released after acid hydrolysis of solvent-extracted soils, non-extractable GDGTs representing 25 ± 15% of the total (i.e. extractable + non-extractable) bacterial GDGTs and 29 ± 17% of the total archaeal GDGTs in podzol samples. This implies that extractable GDGTs can be incorporated into the organic and/or mineral matrix of soil. In addition, we observed that extractable and non-extractable GDGTs could present different distribution patterns, notably suggesting that some extractable GDGTs might be preferentially transferred to the non-extractable pool and/or might be preferentially degraded by soil microorganisms. The relative abundances of bacterial and archaeal GDGTs were compared along the temperate soil profile and the tropical soil sequence. The relative amount of bacterial vs. archaeal GDGTs was shown to be much higher in the extractable than in the non-extractable fraction in the surficial horizons of the temperate podzol and in the well-developed part of the tropical podzol, implying that extractable archaeal GDGTs could be preferentially transferred to the non-extractable lipid pool compared to extractable bacterial GDGTs in these horizons. This might be due to the fact that the types of polar head groups associated with bacterial GDGTs differ from, and are more labile than, those associated with archaeal GDGTs.