Bacteria of a feather floc together: Microbial interactions and function within iron-oxidizing bacterial communities

Abstract

The microaerophilic iron-oxidizing bacteria are highly charismatic, forming bright orange structures called iron mats in low flow creeks and streams, in road-side ditches, and marshes. Yet, the study of the microbial communities that exist in the microniches formed by these iron mats has been heretofore lacking. This dissertation addresses ongoing questions regarding the microbial community and its interactions as they take place in iron-oxidizing bacterial communities. We present evidence of the presence of microorganisms previously unobserved in iron mats, including functional groups associated with nitrate-reducing iron-oxidation and methane cycling archaea, adding to the understanding of what major biogeochemical cycles are linked to and within the iron mat system. Furthermore, the possibly syntrophic relationship between iron-oxidizing and sulfate-reducing bacteria can now be studied in the laboratory, as we present in this work a methodology for co-culturing these functional groups. The ability of the iron mat microbial community to respond to anthropogenic stressors was tested using an in situ sampling procedure in a hydrocarbon exposed creek. The alpha diversity of exposed mats was found to decrease compared to unexposed mats, and the overrepresentation of common taxa was found to be tied to seasonality. However, the iron mat communities appear to functionally respond to hydrocarbon exposure, as genes associated with benzene degradation were more abundant in exposed mats and taxa associated with benzene degradation (e.g., Hydrogenophaga spp.) were also found. In order to elucidate whether the evolution of microorganisms exposed to hydrocarbons was also influenced, we exposed Hydrogenophaga taeniospiralis 2K1 to two different ecologically relevant regimes that are analogous to press and pulse treatment - an intensifying press and repeated pulse - with benzene. We found that both regimes could lead to speciation-level differences in the whole genome sequences of the cultures within 100 generations. Cumulatively, this work serves to further our understanding of the iron mat community, addressing questions regarding structure, function, microbial interactions, and sensitivity to abiotic perturbation.