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Project Progress

Task 1. Energetics of Hydrothermal Ecosystems (dm)

We have continued to apply thermodynamic calculations toward establishing the viability of both biological and abiotic reactions relevant to astrobiological issues. For example, we recently focused on determining the energetics of overall metabolic reactions of both thermophilic and hyperthermophilic prokaryotes. While we have determined the â??GRâ??for critical metabolic reactions such as reduction-oxidation, hydrolysis, and disproportionation, we focused on the Knall-gas reaction, anaerobic sulfur and nitrate reduction, and autotrophic methanogenesis. We determined the overall Gibbs free energy as a function of temperature for a wide range of chemical compositions representative of near-surface and deep hydrothermal systems. To date we have calculated the values of â??GRâ?? as functions of temperature for 188 established metabolic redox reactions plus an additional 182 reactions that chemically link metabolic processes to the composition of natural waters. It is anticipated that these data will be useful in the design and optimization of culture media, the quantization of microbial energetics, and the placement of hyperthermophillic and thermophillic prokaryotes in the context of their geochemical and ecological environments.

We have also applied thermodynamic calculations to the analysis of the potential for the abiotic synthesis of aliphatic and aromatic hydrocarbons detected in martian meteorites, e.g., ALH84001. Our calculations show that polycyclic aromatic hydrocarbons (PAHs) and normal alkanes could form metastably from CO, CO2, and H2 below approximately 250-300°C during rapid cooling of trapped magmatic or impact-generated gases. Depending on the temperature, bulk composition, and oxidation-reduction condition, PAHs can form at low bulk H/C ratios, higher CO/CO2 ratios, and higher temperatures than normal alkanes. We conclude that it is possible that all of the hydrocarbons detected within ALH84001 could have formed abiotically via a reaction of iron carbonate liberating CO during a thermal conversion to magnetite.

Theoretical Foundation for Organic Synthesis Experiments (dm)

We have been seeking to explain the ubiquity and origins of intermediary metabolism. We note that the core of intermediary metabolism in autotrophic organisms lies the citric acid cycle. In a certain group of chemoautotrophs, the reductive citric acid cycle is an engine of synthesis, taking in CO2 and synthesizing the molecules of the cycle. We have examined the chemistry of a model system of C, H, and O that starts with carbon dioxide and reductants and uses redox couples as the energy source. To inquire into the reaction networks that might emerge, we start with the largest available database of organic molecules, Beilstein on-line, and prune by a set of physical and chemical constraints applicable to the model system. From the 3.5 million entries in Beilstein we emerge with 153 molecules that contain all 11 members of the reductive citrate cycle.

A small number of selection rules generates a very constrained subset, suggesting that this is the type of reaction model that will prove useful in the study of biogenesis. The model indicates that the metabolism shown in the universal chart of pathways may be central to the origin of life, is emergent from organic chemistry, and may be unique.
Task 2.