Absence of short-term temperature adaptation in core and intact branched tetraether membrane lipid distribution in a mid-latitude highland peat

Abstract

Branched glycerol dialkyl glycerol tetraethers (GDGTs) are bacterial membrane lipids which are abundant in soils and peat and used as a proxy for temperature and pH. Several types of GDGTs exist, whose distribution can be quantified using the Methylation index of Branched tetraethers (MBT) and the Cyclisation ratio of Branched Tetraethers (CBT). These indices are used as the MBT-CBT proxy to give an indication of the annual mean air temperature (MAT). In this research, peat samples from a small bog in Moor House, Northern England were incubated for 2 months at different temperatures to see if an adaptation in the GDGT distribution could be found. To this end three different lipid types were measured using HPLC-APCI/MS: core lipids (CLs), glycolipids and phospholipids. The analysis of the different GDGTs has demonstrated that the lipid distributions, both core and intact polar lipids do not show any correlation with incubation temperatures. Instead they all reflect MAT at the sampling site. This indicates that the turnover time of the lipids is longer than the incubation time. Because of the absence of any adaptation in the tetraether membrane lipid distribution, the samples of the individual experiments were considered replicate samples and used to estimate the relative abundance of CLs, glycolipids and phospholipids in this peat. It was shown that the branched GDGTs in this peat mostly consist of CLs (85%) plus circa 10% glycolipids and 5% phospholipids. The current samples show a different lipid composition compared to those found in soils, with soils containing more phospholipids and peats more glycolipids. This might be due to differences in unknown factors during decomposition or production.