The importance of iron redox reactions and mineral transformations for the fate of phosphorus in the environment
Final Report Abstract
Phosphorus (P) is an essential but limiting macronutrient for all living organisms that significantly affects global biogeochemical cycles, including the regulation of global primary productivity. The environmental P cycle is strongly coupled to Fe redox cycling since Fe minerals are most abundant in nature and have strong P adsorption capacities. Fe(III) (oxyhydr)oxides are readily subject to biotic or abiotic reductive dissolution under anoxic conditions, leading to P mobilization or sequestration into the Fe(II)-phosphate mineral vivianite. Meanwhile, aqueous/solid Fe(II) species can easily undergo microbial and chemical oxidation in redox transition zones of natural environments, resulting in P retention on newly formed Fe(III) (oxyhydr)oxides. Results from our project - based on four working packages (WP) - illuminated the critical roles of Fe redox reactions and mineral (trans)formations to P cycling in complex environmental settings. Specifically, in WP1 microbial Fe(III) reduction: (1) the reductive transformation of abiogenic and biogenic Fe(III) (oxyhydr)oxide minerals led to further fixation of adsorbed P as Fe(II) phosphate precipitates; (2) the sequestration rates and extents were related to the Fe(III) (oxyhydr)oxides’ crystallinity and sources (i.e., biogenic and poorly crystalline minerals had higher reduction reactivity than their abiotic counterparts). In WP2 microbial Fe(II) oxidation: The extents and rates of vivianite oxidation were determined by different microbial Fe(II) oxidation mechanisms. Microbial vivianite oxidation did not lead to the release of aqueous PO43− and only phototrophic Fe(II)-oxidizers could achieve >95% oxidation compared to 50% oxidation by air oxygen (O2), suggesting that microbial oxidation is the main pathway for high vivianite oxidation. In WP3 microbial sulfate reduction: Microbial sulfate reduction resulted in the transformation of the formed Fe(II)-(Fe(III))-minerals into iron sulfide phases and aqueous P release (~ 80% of total P), indicating an important role for S cycling in releasing P compared to Fe cycling. In WP4 Fe redox cycling: In redox transition zones of natural environments, continuous Fe redox cycling resulted in the increasing formation of highly crystalline Fe (oxyhydr)oxides and P incorporation to form occluded P species, which was also influenced by P concentrations. The results from these four WPs deepen our current understanding on the coupled Fe, P and S biogeochemical cycles in natural environments and provide new insights to comprehensively understand global P cycling on modern and ancient Earth, which is of interests for geologists and environmental or soil scientists, and environmental agencies.
Publications
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Mobility of phosphorus regulated by microbial co-reduction of iron(III) and sulfate in mixed cultures. Goldschmidt2023 abstracts. European Association of Geochemistry.
Wan, Biao; Joshi, Prachi; Shuster, Jeremiah; Fischer, Stefan; Kappler, Andreas & Mansor, Muammar
