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Mechanisms promoting a wetland rhizosphere ferrous wheel.
Megonigal, Patrick*,1, Weiss, Johanna2, Neubauer, Scott1, Emerson, David3, 1 Smithsonian Environmental Research Center, Edgewater, MD, USA2 George Mason University, Fairfax, VA, USA3 American Type Culture Collection, Manssas, VA, USA
ABSTRACT- Competition between anaerobic microorganisms for electron donors suppresses methane emissions from wetlands. Fe(III) reducing bacteria outcompete methanogens for H2 and acetate, but relatively little is known about the mechanisms by which Fe(III) oxides are deposited and reduced in wetlands. We investigated Fe(III) oxide deposition in the rhizosphere of wetland plants and determined that the process does not proceed strictly by chemical oxidation as once believed. Chemolithotrophic Fe(II)-oxidizing bacteria were ubiquitous in wetlands and mediated up to 60% of Fe(III) deposition in laboratory studies. Furthermore, the mineralogy of rhizosphere Fe(III) oxides favored rapid reduction by Fe(III)-reducing bacteria. Labile, poorly-crystalline Fe(III) oxides were 66% of the rhizosphere Fe pool, compared to 23% of the bulk soil Fe pool. As a result, the rhizosphere Fe(III) oxides were consumed in respiration at twice the rate of those in the bulk soil during anaerobic incubations. This difference persisted when the incubations were spiked with an Fe(III)-reducing bacterium (Geobacter metallireducens) and an electron-shuttling compound (AQDS), and the same pattern was observed during a year-long field experiment in which 90% of rhizosphere Fe(III) oxides were reduced compared with 40% in the bulk soil. We concluded that the wetland plant rhizosphere is a "hot spot" of Fe cycling because roots are ideal sources of all the substrates required by both Fe(II)-oxidizing and Fe(III)-reducing bacteria, including labile Fe(III) oxides, labile organic C, O2, and Fe(II) (from Fe oxide reduction or diffusion from wetland porewater). This work helps explain why the presence of plants reduces the proportion of organic carbon that is metabolized by methanogens.
Key words: rhizosphere, microbial ecology, iron reduction, iron oxidation