Isotopic Signatures of Sulfur Cycling Organism Physiology and Ecology

Introduction

Dissimilatory S metabolisms impart large S and O isotope fractionations, which are modulated by a suite of (bio)chemical reactions and physical processes (i.e., diagenesis) and ultimately preserved in the reduced and oxidized products of sedimentary S cycling (e.g., pyrite, carbonate-associated sulfate).

Importance of Multi-Isotope Composition

The multi-isotope composition of such compounds encodes valuable information about microbial activity, environmental conditions, and elemental cycles. Robust interpretation of these signals requires mechanistic understanding of both the metabolic isotope fractionation itself and the impacts of diagenesis on its ultimate preservation.

Current Limitations

Such understanding is currently limited by:

1. Simplified models of isotopic fractionation in dissimilatory S metabolisms.
2. Difficulty in capturing spatio-temporally heterogeneous diagenesis in reaction-transport models.

Proposed Development

We pioneered metabolic-isotopic models (MIMs), which account for the thermodynamics and kinetics of enzymatic reactions and turn empirical multi-isotope correlations into causal relationships. Here, I propose to develop and apply novel, experimentally validated MIMs of the three most ecologically important S metabolisms:

• Sulfate reduction
• Reduced S oxidation
• S disproportionation

Methodology

By embedding these MIMs in a hierarchy of ecosystem models of increasing dimensionality and sophistication, and comparing the results to microfluidic experiments and environmental data, we will gain quantitative, nuanced insight into:

1. The controls on multi-isotope fractionation in metabolic S cycling.
2. Its heterogeneous manifestation in aqueous and solid compounds.
3. The use of these compounds' isotopic compositions to robustly probe S cycling on microscopic to global scales, microenvironmental conditions, and depositional parameters, in both modern and ancient settings.

Conclusion

With S as a test case, we blaze a path to similar treatment of the processes that govern the isotopic composition of many other natural materials.