Microbial Anaerobic Ammonium Oxidation Under Iron Reducing Conditions, Alternative Electron Acceptors

Abstract:

Autotrophic Acidimicrobiaceae-bacterium named A6 (A6), part of the Actinobacteria phylum have been linked to anaerobic ammonium (NH₄⁺) oxidation under iron reducing conditions. These organisms obtain their energy by oxidizing NH₄⁺ and transferring the electrons to a terminal electron acceptor (TEA). Under environmental conditions, the TEAs are iron oxides [Fe(III)], which are reduced to Fe(II), this process is known as Feammox. Our studies indicate that alternative forms of TEAs can be used by A6, e.g. iron rich clays (i.e. nontronite) and electrodes in bioelectrochemical systems such as Microbial Electrolysis Cells (MECs), which can sustain NH₄⁺removal and A6 biomass production.

Our results show that nontronite can support Feammox and promote bacterial cell production. A6 biomass increased from 4.7 x 10⁴ to 3.9 x 10⁵ cells/ml in 10 days. Incubations of A6 in nontronite resulted in up to 10 times more NH₄⁺ removal and 3 times more biomass production than when ferrihydrite is used as the Fe(III) source. Additionally, Fe in nontronite can be reoxidized by aeration and A6 can reutilize it; however, Fe is still finite in the clay. In contrast, in MECs, A6 harvest electrons from NH₄⁺ and use an anode as an unlimited TEA, as a result current is produced. We operated multiple MECs in parallel using a single external power source, as described by Call & Logan (2011). MECs were run with an applied voltage of 0.7V and different growing mediums always containing initial 5mM NH₄⁺. Results show that current production is favored when anthraquinone-2,6-disulfonate (AQDS), an electron shuttled, is present in the medium as it facilitates the transfer of electrons from the bacterial cell to the anode. Additionally, A6 biomass increased from 1 x 10⁴ to 9.77 x 10⁵cells/ml in 14 days of operation.

Due to Acidimicrobiaceae-bacterium A6’s ability to use various TEAs, MECs represent an alternative, iron-free form, for optimized biomass production of A6 and its application in NH₄⁺ oxidation, an essential process in water quality control. Future work seeks to scale up MECs in order to achieve rates of microbial NH₄⁺ oxidation comparable to existing technologies.