Sulfur is a major byproduct of crude oil and natural gas refining, where it is estimated that over 1.5 million tons of waste sulfur were produced in 2014 alone. In developing strategies for managing this waste sulfur, an understanding of the environmental microbes that interact with and transform insoluble forms of sulfur is essential. Currently, mechanisms of microbial attachment to and transformation of insoluble sulfur are poorly understood. ‘omic’ analyses have the potential to provide insight into these mechanisms, but challenges with culturing these and other environmental microbes can obstruct these high-throughput approaches.
The model organism of the phototrophic green sulfur bacteria, Chlorobaculum tepidum, both produces and degrades (oxidizes) extracellular, insoluble sulfur globules (S0) as an obligate intermediate of its energy metabolism. Thus, Cba. tepidum provides a platform to study microbial production and degradation of insoluble sulfur, as well as microbe-sulfur interactions, within a single system. Through the application of factorial experimental design and proteomic analyses, this work seeks to obtain novel insights into microbial interactions with insoluble sulfur.
To address known challenges with growth variability, a systematic 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 oxidation 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-sulfur interactions, these protein identifications inform efforts to functionalize nanoparticles and other surfaces for biocompatibility.