Curtin Applied Geology Seminars

Wed 26th March

12 – 1 pm

Rm 312.222

Jeffrey Dick

Curtin University, Dept. Chemistry

Bringing Geochemistry to Life

Abstract

Among the thermodynamic concepts used in theoretical geochemistry are oxidation-reduction reactions, Gibbs energy changes, and equilibrium
models. These are powerful tools to quantify the conditions and products
of reactions involving rocks and fluids, and often are used to study
mineral paragenesis and other processes leading to compositional
zonation in geological systems. Geobiology also abounds with examples of compositional zonation, for instance, the biofilms growing in chemical
and thermal gradients in hot springs in Yellowstone National Park.
Inspired by the geochemical approach, this study uses models based on
chemical reactions to describe the compositions of proteins and the
relative abundances of organisms in microbial communities.

Environmental DNA sequences (metagenomes) can be used to predict protein sequences, which make up much of the cellular biomass. In the first part of this study, the spatial variation in the compositions of proteins in the source and cooling outflow channel of a hot spring, “Bison Pool”, is modeled using relative stability calculations. The model is calibrated by proposing a gradient of oxidation-reduction potential connected to temperature in the hot spring. The calibrated oxidation potential increases with decreasing temperature, but overall is more reducing compared to various inorganic redox proxies. Next, a metastable equilibrium model for predicting the relative abundances of coexisting microbial phyla is compared with the observed abundances from the metagenome. This model is also calibrated by adjusting the effective oxidation-reduction potential, in order to minimize the energetic
distance between theoretical relative abundances of phyla and the
observed abundances. Deviations of the metastable equilibrium model from the observed relative abundances can be interpreted as resulting from a contribution of additional energy sources or sinks such as
photosynthesis or low growth efficiency.

This approach uses thermodynamic relations to integrate aspects of
inorganic geochemistry and biochemistry in a way that has not previously been explored. The equilibrium predictions are necessarily independent of the timescale of reaction, but because the model takes account of differences in protein composition (derived from gene sequences), evolutionary differences are implicit in the comparisons. These results support the notion that chemical reactions can be used as a basis for describing the emergence of ecosystem patterns occurring over geological time and in an environmental context.