Dr. Sebastian Kopf
CU Boulder
 

Topic: Insane in the membrane

Abstract: Glycerol dialkyl glycerol tetraether lipids (GDGTs) are large, chemically robust components of microbial cell membranes that hold tremendous geologic potential because they are comparatively easily preserved in the rock record without alteration to their core structure during burial and diagenesis. Several existing paleoproxies such as the widely used TEX86 paleothermometer are based on changes in the relative abundance of tetraethers. The more recently discovered bacterial tetraethers (brGDGTs) likewise have a strong empirical correlation between their structural properties and temperature / pH in modern terrestrial environments. Methodological advances in sample preparation and analysis over the past decade have accelerated the pace of discovery and highlighted the vast global distribution and potential of brGDGTs across terrestrial, aquatic, and sedimentary systems. Calibration efforts in modern environments have established global and regional temperature proxy calibrations for soils, peats, and lakes. However, one result of the rapid expansion of paleoclimate applications was that generation of brGDGT records had greatly surpassed our fundamental understanding of a mechanistic link between this proxy and the inferred environmental parameters. Following up on previous laboratory and environmental studies that had implicated the globally abundant bacterial phylum of the Acidobacteria in the production of brGDGTs, we tested the hypothesis that brGDGT production may require specific environmental triggers commonly encountered in nature but not necessarily in the lab. We discovered that O2 limitation can indeed trigger brGDGT production showing that previously suspected redox constraints may indeed play an important role in the environmental production of brGDGTs and the interpretation brGDGT-based temperature and pH reconstructions. Following up on this work, we investigated several other Acidobacteria that harbor low affinity terminal oxidases in their genomes (thus suggesting a capacity for growth at low O2) and found a prolific brGDGT producer that makes a large portion of its membrane (~1/4) out of tetraethers in what is a highly unusual fatty acid/ether lipid hybrid membrane that defies the traditional biochemical divide between ester lipids in Bacteria+Eukaryotes vs. ether lipids in Archaea. We further demonstrated that the degree of brGDGT methylation in this organism in response to temperature variations agrees with empirically developed brGDGT-based temperature indices. In contrast, we found that the degree of brGDGT cyclization in response to pH does not match environmental trends, supporting the hypothesis that changes in bacterial community composition rather than physiological adaptation may underlie the link between cyclization number and pH observed in environmental samples. Additionally we identified several brGDGT isomers that seem to respond to changes in O2 thus providing a potentially new proxy for O2