Heterotrophic Organisms with Ammonia Oxidation pathways
Methane Oxidizing organisms (Methanotrophs) are close cousins of Nitrosomonas and the enzymes produced for C1 oxidation can also oxidize ammonia. The bacteria do not gain any energy from this reaction and high ammonia concentrations can actually block methane degradation.
Paracoccus denitrificans is an interesting common wastewater organism that is capable of both sulfide and ammonia oxidation while also using nitrate/nitrite as alternative electron acceptors. P. denitrificans metabolism on ammonia was first investigated in the late 1980s and the ability to oxidize ammonia to nitrite coupled with simultaneous denitrification was noted in lab cultures. In real wastewater systems, this organism does aggressively denitrify, but often pursues higher energy yields using conventional heterotrophic pathways.
Similar to P. denitrificans, several Pseudomonas sp. have similar ammonia oxidation and denitrification pathways. To what extent these are responsible for ammonia oxidation in working wastewater systems is unknown. Other genera with potentially similar pathways include Alcaligenes and Bacillus.
What does this mean for wastewater operations
- Most ammonia and nitrite oxidation is accomplished via chemolithotrophic pathways by AOB & NOB cultures.
- When it comes to heterotrophic nitrification, just because an organism has the genes for ammonia oxidation pathways does not mean that the genes are turned on and the organism is oxidizing ammonia.
- Paracoccus denitrificans was one of the first heterotrophic organisms noted to have ammonia oxidation and denitrification pathways in a single organism. Paracoccus bacteria are commonly found in wastewater treatment systems – and are especially useful in improving nitrate/nitrite removal rates.
- Pseudomonas tend to also be aggressive denitrifiers and have been found to possess ammonia and nitrite oxidation genes. What we don’t know is how much Pseudomonas and other similar genera are responsible for primary ammonia oxidation in wastewater systems.