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Project Progress

Isotopic analyses of nucleic acids. At the time of the last report, Alex Sessions was completing his doctoral thesis and Ann Pearson was working at Woods Hole Oceanographic Institution (WHOI) as a postdoctoral research associate, obtaining molecular biological advice through collaboration with Dr. Katrina Edwards, a member of the resident scientific staff at WHOI. Almost within weeks, Sessions passed his final oral exam and transferred to postdoctoral status and Pearson decided (most sensibly) to accept the offer of an assistant professorship in biogeochemistry at Harvard. Her move dissolved our collaboration with Edwards and temporarily arrested our progress on analyses of nucleic acids. More recently, samples have begun to flow from Cambridge to Woods Hole. Specific analyses have yielded isotopic compositions for the 16S ribosomal ribonucleic acid (RNA) from Cenarcheum symbiosis and a bacterium growing in association with the sponge Axinella mexicana. A report summarizing the capabilities of this technique will be submitted for publication in the fourth quarter of 2002.

Bacterial fractionations of hydrogen isotopes. The collaboration between Sessions and Linda Jahnke (NASA/Ames Astrobiology group) led to an extensive paper describing the sources of H used in biosynthesis of lipids by the aerobic methanotroph Methylococcus capsulatus Bath and the isotopic fractionations associated with the biosynthetic pathways. A revised version of that manuscript has now been accepted for publication in Geochimica et Cosmochimica Acta.

Hydrogen isotopic analyses of Australian oils. Sessions and Roger Summons (a member of the staff of the Australian Geological Survey Organisation at the time of the last report, now a member of the faculty at MIT) have completed analyses of an extensive series of oils from Australia. The results suggest that hydrogen isotopic compositions of sedimentary hydrocarbons are minimally affected by diagenetic processes and can be used to resolve mixtures of sedimentary compounds with distinct sources.

Molecular Organic Geochemistry. New team member Roger Summons conducts research on the chemical and isotopic characterization and biogeochemical significance of lipids from cultured microbes, environmental samples and their fossil analogues in ancient sedimentary environments. In collaboration with Linda Jahnke (NASA Ames) and a research group at the University of Regensburg, Germany (Prof Dr Karl Stetter, Dr Robert Huber, Dr Harald Huber and Dr Manuela Baumgartner and their graduate students), he is currently analyzing the lipids of a number of new isolates of thermophilic and hyperthermophilic bacteria (Aquificales new sp., Thermovibrio ruber) and archaea (e.g. Ignicoccus sp., Nannoarchaeum equitans, Methanococcus sp.). Ignicoccus sp. and Nannoarchaeum equitans represent new phyla, and their physiologies are unknown. Through this work, Summons and colleagues have greatly extended our understanding of the variety and complexity of extremophile lipid biosignatures. In complementary studies with Kathleen Londry of the University of Manitoba, he is re-examining the carbon isotopic fractionation associated with methanogenesis by Methanosarcina barkerii grown on different carbon sources and under different conditions. Preliminary research has begun on organic-rich, low maturity sediments that span both the Precambrian-Cambrian (Oman) and Permo-Triassic (Australia) boundaries. The aim of this aspect is to search for biomarkers that might be diagnostic for biogeochemical processes, such as methanogenesis and methane oxidation or productivity collapse associated with negative carbon isotopic excursions. In complementary studies of ancient hydrothermal environments, collaborating with Malcolm Walter of Macquarie University and colleagues at Geoscience Australia, he has submitted a paper on geochemical signatures for the hydrothermal alteration of organic matter associated with the formation of a sediment-hosted lead-zinc sulfide deposit. The polycyclic aromatic hydrocarbon (PAH) and other hydrocarbon distributions in this deposit reflect a thermal halo, and they can be used to map the source and cooling profile of hydrothermal brines that moved through the sediment system and created the mineralization.