A distinct characteristic of the Archaea is their unique membrane lipids that set them apart from the other two domains of life. Archaeal lipids contain ether linkages between glycerol and isoprenoid chains and certain archaea however produce membrane spanning lipids, comprising 40 carbon isoprenoid chains, commonly referred to as GDGTs. Archaeal GDGTs are ubiquitous lipids in both marine and terrestrial environments and are used extensively in geological and environmental studies.
One GDGT-forming phylum of archaea of particular interest to geochemists is Thaumarchaeota (recently renamed Nitrososphaerota). This globally distributed phylum makes GDGTs with 0 to 4 cyclopentane rings and a very specific GDGT called crenarchaeol which contains 4 cyclopentane rings and one cyclohexane ring. To date, crenarchaeol has been detected only in Thaumarchaeota and hence is considered to represent a specific biomarker for members of this phylum. In Thaumarchaeota the distribution of GDGT lipids depends on growth temperature, the basis of the widely applied TEX86 paleotemperature proxy. This paleotemperature proxy has been applied to estimate sea surface temperatures as far back as the middle of the Jurassic era (~160 million years ago). Paleotemperature reconstructions are essential for future climate modeling. Indeed, to better predict and understand our future with a warmer and rapidly changing climate we can reconstruct different environmental parameters over past climate fluctuations and sudden warming events.
It is not just the lipids of Thaumarchaeota that fascinate geochemists. We are also interested in a wide range of archaea including extremophiles, such as halophiles and thermophiles. Many of these produce distinct lipids that can serve as biomarkers in geological studies. Our ongoing research into archaeal lipidomes both in culture and in samples from the environment is providing insights into the ecology of archaea as well as related topics such as membrane adaptation and lipid biosynthesis.
Here I will present one such study into the archaeal symbiont Ca. Nanohaloarchaeum antarcticus, which is dependent on its archaeal host Halorubrum lacusprofundi for lipids. We explored the lipidome dynamics of the Ca. Nha. antarcticus – Hrr. lacusprofundi symbiotic relationship during co-cultivation. We did this using a comprehensive untargeted lipidomic methodology, which processes Orbitrap mass spectrometry using MZmine software and the GNPS platform. Our study revealed that Ca. Nha. antarcticus selectively recruits 110 lipid species from its host, i.e., nearly two-thirds of the total number of host lipids. Lipid profiles of co-cultures displayed shifts probably associated with changes in membrane fluidity and improved resistance to membrane disruptions, consistent with compensation for higher metabolic load and mechanical stress on host membranes when in contact with symbionts. Our work emphasizes the strength of employing untargeted lipidomics approaches to provide details into the dynamics underlying an archaeal symbiont-host system.