Microbially-mediated iron cycling in the rhizosphere of wetland plants

An iron coating known as plaque forms on the roots of wetland plants due to oxygen leaking from the roots reacting with the reduced iron [Fe(II)] in the anaerobic soil solution. Although plaque formation in circumneutral wetlands has been largely considered to be an abiotic process, my dissertation considered if the wetland plant rhizosphere is a suitable habitat for both Fe(II)-oxidizing bacteria (FeOB) and Fe(III)-reducing bacteria (FeRB). I hypothesized that the presence of both rhizosphere FeOB and FeRB promotes a dynamic microbially-mediated rhizosphere iron cycle. My research was divided into three components: (1) a survey for rhizosphere FeOB and FeRB, (2) isolation and characterization of four novel strains of rhizosphere FeOB and (3) Fe(III) plaque reduction experiments. FeOB were found to be ubiquitous in the wetlands ecosystems, and FeRB were dominant members of the rhizosphere microbial community, accounted for an average of 12% of the total cell number. Four strains of lithotrophic FeOB were isolated from the roots of wetland plants and, based on 16s RNA sequencing and rep-fingerprinting, were nearly identical to each other (>99.5% similarity between the strains). All sequences were within a novel and distinct lineage within the γ-proteobacteria. By comparing root plaque and bulk soil with respect to Fe mineralogy and Fe(III) reduction potential, the roots had a significantly higher percentage of poorly crystalline Fe (66 ± 7%) than the bulk soil (23 ± 7%); the percentage of poorly-crystalline Fe in the roots and soil was correlated with the percentage of FeRB (r 2 = 0.583, n = 5 sites, p < 0.05). The root plaque had a much higher Fe(III) reduction potential than the roots with ca. 75–80% of the Fe(III) being reduced in 350-hrs compared to bulk soil rarely exceeded 30–40%. Higher amounts of amorphous Fe(III) and a higher Fe(III) reduction potential in the rhizosphere compared to the soil indicate that the rhizosphere is a site of active Fe cycling. This is further supported by the co-existence of FeOB with FeRB in the rhizosphere. Such an Fe cycle could have important implications on biogeochemical processes such as methanogenesis, phosphorus availability, and the mobility of trace metals.