Effects of temperature and pH on archaeal membrane lipid distributions in freshwater wetlands

J.Blewett^a,b, B.D.A.Naafs^a,b A.V.Gallego-Sala^c, R.D.Pancost^a,b

Organic Geochemistry Unit, School of Chemistry, and School of Earth Sciences, University of Bristol, Bristol, UK.

Abstract

Freshwater wetlands harbour diverse archaeal communities and associated membrane lipid assemblages, but the effect of environmental factors (e.g. pH and temperature) on the distribution of these lipids is relatively poorly constrained. Here we explore the effects of temperature and pH on archaeal core-lipid and intact polar lipid (IPL) derived core lipid distributions in a range of wetlands. We focus, not only on the commonly studied isoprenoidal glycerol dialkyl glycerol tetraethers (isoGDGTs), but also widen our analyses to include more recently identified, but relatively widespread, archaeal lipids such as isoGDGT isomers, methylated isoGDGTs (Me-GDGTs), and butanetriol and pentanetriol tetraethers (BDGTs and PDGTs). Based on multivariate analysis and a globally distributed set of wetlands, we find that the degree of isoGDGT cyclisation does increase along with temperature and pH in wetlands. However, and unlike in some other settings, this relationship is obscured in simple scatterplots due to the incorporation of isoGDGTs from highly diverse archaeal sources with multiple ring-temperature or ring-pH relationships. We further show that the relative abundance of early eluting to later eluting isoGDGT isomers increases with pH, representing a previously unknown and seemingly widespread archaeal membrane homeostasis mechanism or taxonomic signal. The distribution and abundance of crenarchaeol, a marker for Thaumarchaeota, demonstrates that in wetlands these Archaea, likely involved in ammonia oxidation, are restricted primarily to the generally drier soil/sediment surface and typically are more abundant in circumneutral pH settings. We identify Me-GDGTs and Me-isoGMGTs (homologs of isoGDGTs and isoGMGTs, but with additional methylation on the biphytanyl chain) as ubiquitous in wetlands, but variation in their abundance and distribution suggests changing source communities and/or membrane adaptation. The high relative abundance of BDGTs and PDGTs in the perennially anoxic part of the peat profile (catotelm), as well as their elevated abundance in a circumneutral pH wetland, is consistent with an important input from their only known culture source, belonging to the methanogenic Methanomassiliicoccales. Our results underline the diversity of archaeal membrane lipids preserved in wetlands and provide a baseline for the use of archaeal lipid distributions in wetlands as tracers of recent or ancient climate and biogeochemistry.

Keywords: Temperature, pH, GDGTs, BDGTs, Me-GDGTs, Crenarchaeol, Isomers, Archaea, Wetlands, Biogeochemistry.