Iron is an essential element for almost all known life on Earth. It is one of the most abundant elements in the planet’s crust and, as a result of microbial and abiotic redox transformations between its two environmentally relevant valence states (FeII and FeIII), is intricately tied to all other major element cycles (such as that of oxygen, carbon, nitrogen and sulfur). In environments lacking oxygen, microbial Fe(II) oxidation can be catalyzed by two distinct types of microorganisms: nitrate-reducers and anoxygenic phototrophs. These organisms are known to spatially co-exist in sedimentary habitats and have the potential to compete for Fe(II), which can in turn influence microbial community composition, biogeochemical cycling and geochemical profiles of many redox species (e.g. FeII, FeIII, etc.) in sediments. However, the competition dynamics between these different types of Fe(II)-oxidizers have never been directly studied. It is therefore unclear whether they can truly co-exist or which organisms would dominate under different geochemical conditions. Utilizing a model freshwater co-culture system containing an anoxygenic phototroph (Rhodobacter ferrooxidans SW2) and a nitrate-reducing Fe(II)-oxidizing enrichment culture (culture KS which contains an Fe(II)-oxidizing Gallionella species with a flanking community of heterotrophic nitrate-reducers), we have determined that anoxygenic phototrophs do not outcompete nitrate-reducing Fe(II)-oxidizers when conditions for both metabolisms are optimum (i.e sufficient nitrate and light are provided). More surprisingly, Fe(II) oxidation by the phototroph is inhibited even when nitrate concentrations are limiting and favour phototrophic Fe(II) oxidation. Additional batch experiments have confirmed that this is not a result of metabolic flexibility on the part of the phototroph and that active cells (not simply biomass) are essential for this inhibition to occur. The environmental relevance of this finding was determined via the manipulation of light and nitrate concentrations in microcosms containing freshwater pond sediment where both types of metabolisms are known to be present. These results have key implications for determining the microbial drivers of iron cycling in sediments, and also suggest that the activity of phototrophic Fe(II)-oxidizers in both modern and ancient environments may be strongly influenced by nitrate concentrations and nitrate-reducing microorganisms in particular.

Influence of inter-species competition on the kinetics and extent of microbial anaerobic Fe(II) oxidation

255th National Meeting (2018)

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