Thaw 11

Production of Heterocyst Glycolipids and Glycerol Dialkyl Glycerol Tetraether Membrane Lipids in the Water Column of a Stratified Tropical Lake, Malawi, Africa 

Temperature is one of the most critical climate variables for paleoenvironmental, yet it is notoriously difficult to measure in terrestrial archives. Several terrestrial temperature proxies have been proposed based on the distributions of microbial membrane lipids preserved in lake sediments, including archaeal and bacterial glycerol dialkyl glycerol tetraethers (GDGTs) and cyanobacterial heterocyst glycolipids (HGs). Presented here is a rigorous examination of the production and export of GDGTs and HGs in the surface waters of the stratified tropical Lake Malawi that aims to evaluate these lipids’ potential as paleotemperature proxies.

Settling particulate matter (SPM) was collected in the metalimnion of Lake Malawi’s North Basin (NB) and South Basin (SB) from 2011 to 2013 and analyzed for its lipid content. Fluxes of isoGDGTs and brGDGTs are driven by bulk sedimentation processes, which vary across the lake due to differences in basin morphology and hydrological regimes. Distributions of core lipid isoGDGTs suggest that Thaumarchaeota are the dominant GDGT-producing archaea in the surface waters of the lake, though there is likely a second archaeal community living at depth. During the dry, windy season, isoGDGTs in the SB reflect a mixture of the two sources resulting in biased reconstructed temperatures, as quantified with the TEX86 index, towards warmer temperatures. TEX86-based temperatures in the NB do not appear significantly impacted by the upwelling of non-Thaumarchaeotal lipids, however they exclusively reflect lake surface temperatures (LST) at the time of maximum Thaumarchaeota activity, possibly due to a long residence time of the lipids in the water column. BrGDGTs are also likely produced by multiple distinct groups of organisms, complicating interpretation of temperatures reconstructed from their distributions. The inability of any published calibrations to produce reasonable temperatures with these lipids underscores the need for further studies on autochthonous brGDGTs in lakes.

HGs are present in all samples, but maximum fluxes occur just after the principal phytoplankton bloom each year. In contrast to GDGTs, fluxes and distributions of HGs in SPM are related to actively living cyanobacteria populations. Published HDI and HTI indices do not record seasonal variability in LST, however the relative abundance of HG diols with C26 and C28 side chains tracks LST remarkably well.