CARBON ISOTOPIC SIGNATURES OF MICROBIAL LIPIDS IN GEOTHERMAL DEPOSITS: ELUCIDATING THERMOPHILIC ECOLOGY

Understanding the origins and diversification of life represents one of the most challenging scientific endeavours. In such efforts, constraining the limits of life on Earth is vital, as is an understanding of the ecology of those organisms that can survive and even thrive in environments characterised by extremes of temperature, salinity or pH. Of particular interest are geothermal systems, populated by diverse and deeply-branching thermophilic bacteria and Archaea. Recently we have demonstrated that microbial lipids are powerful tools in assessing and reconstructing the microbiology of terrestrial hot springs. In this proposal we focus on elucidating the carbon isotopic signatures of the lipids of geothermal organisms. The use of lipid biomarkers in combination with stable isotope analysis is crucial to understanding microbial ecology, providing a direct link between microbial identity and biochemical processes.

Lipids are found in all living organisms, typically serving as energy sources and structural components of cell membranes. Often highly diagnostic, well-preserved in the geological record, and entrained with information on biological diversity, environmental conditions and post-depositional alteration history, these compounds are particularly attractive for early life and astrobiological investigations. Our past studies of biomarkers in geothermal deposits reveal a profound diversity of encapsulated lipids, which can be utilized to profile microbial community composition. However, a potential of microbialite-preserved lipids that was untapped by our previous work is their carbon isotopic composition; strong and highly unusual variations in lipid carbon isotope values were observed in a small subset of our data but lacked crucial contextual data (e.g. the carbon isotopic composition of dissolved inorganic carbon, DIC, in the pools). This remains an untapped source of information on thermophilic physiologies and ecology in the New Zealand Taupo Volcanic Zone and the silica deposits formed there. Such deposits are essential archives of past life and elucidating the controls on organic matter formation in such settings will allow a better interpretation of the OM assemblages preserved in them.

We will map the range of carbon isotope values and evaluate their reproducibility in various geothermal sites with differing physicochemical conditions and in different biofacies. By embedding this data in the context of thermal spring DIC carbon isotope values and recent microbiological community profiling based on molecular (DNA and RNA) approaches, we will resolve the origin of the unusual carbon isotopic compositions. This will greatly expand on our capacity to interpret biomarker carbon isotope values preserved in ancient silica sinters, providing crucial information on sources of lipid biomarkers, metabolism and trophic structure.

In this work, we will target a range of diagnostic compounds from well-characterized geothermal settings in the Taupo Volcanic Zone in New Zealand and explore the controls on the variation of carbon isotopic signatures, coupling our past biomarker-based interpretations to carbon isotopic analyses in order to achieve a better understanding of the biodiversity of geothermal environments and to unravel biogeochemical and ecological function. This work will advance our interpretation of biosignatures preserved in the rock record, providing insight into the evolution and ecology of the earliest life-forms on Earth and informing the search for life elsewhere in the Solar System.

Our basic objective to investigate organic signatures in geothermal systems will provide fundamental knowledge for microbiologists, Earth scientists and geochemists. Investigations into thermophilic ecology and biogeochemical processes through isotopic compositions of lipid biomarkers will be invaluable in the field of microbiology - diagnostic lipid biomarkers together with carbon isotopic compositions are powerful tools in assessing composition and metabolism of diverse microbial communities. In addition, the assessment of preservation of lipid biomarkers and their isotopes in mineral deposits will enhance interpretation of biosignatures in the geological record with biomarker investigations of ancient deposits proving useful in palaeoecological studies. Overall, this work will facilitate interdisciplinary research that will result in enhanced understanding across the academic community of the evolution and ecology of geothermal systems.

Planetary scientists have discovered extraterrestrial hydrothermal systems, and such sites have become high priority targets in the search for life beyond Earth. Terrestrial analogues of these sites therefore play a key role in astrobiology. Our study will enhance the understanding of the ecology of terrestrial geothermal systems, providing vital information for planning exploration missions and the interpretation of the data they retrieve.

Further impact will arise from potential biotechnological benefits in characterizing these unusual biochemicals. For example, since these lipids control and maintain the integrity of the membranes of thermophiles and acidophiles, they could also serve as the platform for designing particularly stable liposomes for drug delivery. Thus the impact of this work is fundamental, interdisciplinary and potentially far reaching.

Outside of academia, extreme environments and their unusual inhabitants are of great interest to the general public. The University of Bristol has numerous public engagement and outreach activities, including the Bristol ChemLabS Outreach programme (engaging with ca. 30,000 people per year) and the Bristol Festival of Nature (engaging with ca. 30,000 people over one weekend). By linking to these established programmes and developing our own engagement agenda (including workshops, game development, lecturettes and articles in science education magazines), we will communicate this research to the general public.