Novel insights into sulfur and energy metabolism and microbe-mineral interactions in a model phototrophic bacterium

Microbes that synthesize or degrade insoluble sulfur-minerals are instrumental in environmental sulfur cycling and catalyze industrial processes such as biomining and sulfide remediation. However, the mechanisms of microbe-mineral interactions are poorly understood, and challenges in culturing these environmental microbes can obstruct systematic, ‘-omic’ analyses. The model organism of the phototrophic green sulfur bacteria, Chlorobaculum tepidum, both synthesizes and degrades extracellular sulfur globules (S⁰) as an intermediate of sulfide and thiosulfate oxidation, providing a platform to study microbe-mineral interactions within a single system.

This work aims to gain insight into mechanisms of S⁰ synthesis and oxidation and microbe-mineral interactions through proteomic analyses. To address known challenges with growth variability, a systematic, quantitative approach to reproducibly culture and characterize Cba. tepidum under a range of conditions was first established. These studies, which spanned a factorial space of S⁰-synthesizing and S⁰-degrading states and a range of light fluxes (the energetic landscape), provided unexpected insights into Cba. tepidum sulfur and energy metabolism. For example, at low light flux, kinetic limitations on electron donor utilization were observed which are not obvious consequences of current electron transport models for Cba. tepidum.

Proteomic analyses identified over 40 proteins that associate with purified biogenic S⁰, including uncharacterized proteins and proteins involved in sulfur oxidation pathways. Beyond an improved mechanistic understanding of microbe-mineral interactions, this knowledge informs efforts to functionalize nanoparticles and other surfaces for biocompatibility.