Production of Heterocyst Glycolipids and Glycerol Dialkyl Glycerol Tetraether membrane lipids in the water column of a stratified tropical lake, Malawi, Africa
Temperature is a critical component of paleoenvironmental reconstructions, yet it is notoriously difficult to measure in terrestrial archives. Presented here is an investigation of the sources and distributions of archaeal and bacterial glycerol dialkyl glycerol tetraethers (GDGTs) and cyanobacterial heterocyst glycolipids (HGs) in Lake Malawi. The study aims to evaluate the potential for these lipids to function as paleotemperature proxies in tropical lacustrine environments. GDGTs and HGs were extracted from settling particulate matter (SPM) collected at bi-monthly intervals from 2011 – 2013. Sediment traps used to collect SPM were moored in both the north and south basins of Lake Malawi in order to evaluate spatial trends, in addition to temporal trends, in lipid production and export across the lake.
Distributions of isoprenoid GDGTs indicate that Thaumarchaeota are the dominant GDGT-producing archaea in the surface waters of Lake Malawi. However, TEX86-based temperatures do not track lake surface temperatures at either the northern or southern sediment trap locations. TEX86 in the north basin instead reflects surface water temperatures at the time of maximum Thaumarchaeota activity, while TEX86 in the south basin records patterns in seasonal upwelling that possibly drive shifts in the membrane composition of Thaumarchaeota or in the dominant planktonic archaeal community. Branched GDGTs are likely produced by distinct groups of bacteria within the water column, complicating the interpretation of temperatures reconstructed from their distributions. The inability of available branched GDGT calibrations to produce reasonable temperatures underscores the need for comprehensive studies of autochthonous branched GDGT production in lakes. Bulk sedimentation is the primary driver of branched and isoprenoid GDGT export in the metalimnion.
HGs are present throughout the time-series, but maximum fluxes occur in December. HGs in SPM are sourced from actively living cyanobacteria populations, indicating rapid export of the lipids through the water column. Temperatures reconstructed with published HG-based indices do not match the seasonal variability in surface temperatures, however the fractional abundances of HG diols with C26 and C28 side chains do appear related to lake temperatures in this system. The production of C28 HG keto-ols may also be associated with heterocyst differentiation
