........The bioavailable ammonia concentration and environmental pH are connected: ammonia exists in two different forms in the environment, non-protonated ammonia (NH3) and its protonated form ammonium (NH4+) that are in a pH-dependent equilibrium (pKa = 9.27). In acidic conditions, the amount of available NH3 is extremely low. A 1974 study by Suzuki and colleagues demonstrated that the AOB
N. europaea is only able to use ammonia, rather than ammonium, which has long been considered the key reason why AOB are unable to grow in acidic environments (Suzuki, Dular and Kwok
1974). This study is still the cornerstone of ammonia oxidation research although it has not been repeated with other strains of AOB, AOA nor comammox
Nitrospira. However, the inability to use ammonia in low pH does not exclude survival and activity of AOB in these conditions: AOB are able to grow in acidic conditions using urea as substrate and by growing in biofilms and aggregates (de Boer
et al.1991; Burton and Prosser
2001). The notion that ammonia oxidation is problematic at low pH was overturned by the discovery of the first obligately acidophilic ammonia oxidiser,
N. devanaterra (Lehtovirta-Morley
et al.2011). Originally isolated from an acidic agricultural soil, this archaeon grows autotrophically in the pH range of 4–5.5 in laboratory culture with ammonium chloride as its sole energy source. Although
Nitrosotalea is the only obligately acidophilic ammonia oxidiser described to date, it is unlikely to be the only microorganism performing nitrification in acidic soils. Cultivation of acid-tolerant γ- and β-proteobacterial AOB has been reported (de Boer
et al.1991; Hayatsu
et al.2017). In addition to AOB that tolerate acidic conditions and are found in acidic soils, the archaeal genus
Nitrososphaera is abundant in many acidic soils globally and has been reported to grow in acidic soil microcosms (Gubry-Rangin
et al.2011; Wang
et al.2014).....