THE FORM AND OXIDATION STATE OF IRON AND SULFUR COMPOUNDS INFLUENCES THE COMPOSITION OF GROUNDWATER MICROBIAL COMMUNITIES

Much of the world’s global freshwater supply lies in shallow aquifers that are both easier to replenish than deep subsurface aquifers but also more prone to contamination by surface processes and activities. Hydrologic changes in flow pathways, the input of labile organic carbon, and mixing with deep, anoxic, and saline fluids can all impact water chemistry in shallow aquifers and consequently microbiology. As fluids grow anoxic, residual iron and sulfur compounds become increasingly important to microbial metabolism. In the present study, we explored the influence of differences in the form and oxidation state of iron and sulfur compounds in experimental microcosms amended with both simple and complex carbon sources. To demonstrate the feasibility of this approach, we worked with low diversity microbial communities in hyperalkaline fluids from a shallow subsurface aquifer in the California Coast Range. Results of this work showed that some substrates (e.g., polysulfides, FeOOH) promoted high growth yields and, when coupled with acetate a common end-product of microbial fermentation, the communities closely resembled that of natural populations in the aquifer. On the other hand, when the microcosms were amended with complex organic carbon sources, or met with unfavorable substrates (e.g., sulfite, magnetite), growth yields were much lower and did not resemble native communities. We consider these results in terms of both the bioenergetic yield of different red-ox couples and the physiological capacity of microbes to metabolize the various substrates. These results point to the inherent value of measuring iron and sulfur compounds in various forms and oxidation states in natural groundwaters as they are commonly missed in routine surveys. We further suggest that the combination of observational and experimental approaches presented here could serve as a valuable template towards understand the biogeochemical impact of microbial communities in shallow groundwaters.