- Why do bacteria and archaea make the lipids they do?
- The fossil signature of microbial biomarkers
- Further reading
A key area of our research is the field of geomicrobiology, the investigation of microbial signatures in geological materials and the environment. Such research necessarily overlaps other research themes, such as biogeochemistry, environmental chemistry and paleoclimate, but is distinguished by a focus on the microorganisms themselves rather than the processes they mediate.
Both Bacteria and Archaea synthesise a range of specific compounds, including specific phospholipid fatty acids, hopanoids and ether lipids; our interests revolve around the quantification and isotopic characterisation of these compounds, allowing the profiling of microbial signatures in modern and ancient rocks.
A single-celled organism’s membrane is a crucial interface with its surroundings: it must be physically robust but also act as an effective barrier through which only certain nutrients and carbon substrates can pass. Consequently, organisms’ membrane lipid compositions often appear to be finely attuned to the environment in which it lives.
To explore such interactions, we currently investigate the range of environments that occur on Earth’s surface, including subglacial sediments, subaerial geothermal systems in New Zealand and Iceland and deep ocean hydrothermal systems. We work with a wide range of UK and international collaborators to obtain such exotic materials, and have conducted field work from Svalbard to the Bransfield Strait near Antarctica.
We have discovered new compounds, presumably adaptations to these unusual environments, and we have determined the pH and temperature controls on lipid variability.
Like all lipids, those of Bacteria and Archaea are well preserved in ancient sediments, and in fact our colleagues in the organic geochemistry community use them to study the emergence of the earliest forms of life. We typically do not look at rocks that old, but we have examined bacterial and archaeal lipids preserved in rocks ranging in age from the Ordovician (450 Million years old) to silica sinters that have only recently precipitated from hydrothermal springs.
These lipids provide fascinating snapshots of ancient microbial life. Lipid signatures in ancient cold seep and hydrothermal deposits constrain the underlying ecology and, thus, geochemistry that fuelled such systems. The microbial responses to past geological events can be used to interrogate the nature of biotic turnover; for example, the concentrations of cyanobacterial biomarkers (the 2-methylhopanoids) reveal changes in the photosynthetic community across the Permo-Triassic Boundary, the largest mass extinction in Earth history.
However, the geological record also reveals some rather unexpected lipid signatures, suites of unusual compounds that briefly appear and then disappear. Beyond representing fascinating evolutionary experiments, they are powerful tools for fingerprinting rocks of a certain age… and the petroleum derived from it.
Our research in this field is vibrant and ongoing. Current projects include an investigation of how geothermal organisms’ lipid compositions respond to changes in pH and temperature and the role of archaea and bacteria in the formation of spectacular carbonate build-ups and barite chimneys in the deep sea. From these efforts, we are finding new and unexpected compounds and we are developing new proxies for modern and ancient biogeochemical processes.
Kaur, G., Mountain, B.W. and Pancost, R.D. (2008) Microbial membrane lipids in active and inactive sinters from Champagne Pool, New Zealand: Elucidating past geothermal chemistry and microbiology. Organic Geochemistry 39, 1024-1028.