Nitrification, the oxidation of ammonia to nitrate, is an essential process in the biogeochemical nitrogen cycle. The first step of nitrification, ammonia oxidation, is performed by three, often co-occurring guilds of chemolithoautotrophs: ammonia-oxidizing bacteria (AOB), archaea (AOA), and complete ammonia oxidizers (comammox). Substrate kinetics are considered to be a major niche-differentiating factor between these guilds, but few AOA strains have been kinetically characterized. Here, the ammonia oxidation kinetic properties of 12 AOA representing all major cultivated phylogenetic lineages were determined using microrespirometry. Members of the genus Nitrosocosmicus have the lowest affinity for both ammonia and total ammonium of any characterized AOA, and these values are similar to previously determined ammonia and total ammonium affinities of AOB. This contrasts previous assumptions that all AOA possess much higher substrate affinities than their comammox or AOB counterparts. The substrate affinity of ammonia oxidizers correlated with their cell surface area to volume ratios. In addition, kinetic measurements across a range of pH values supports the hypothesis that—like for AOB—ammonia and not ammonium is the substrate for the ammonia monooxygenase enzyme of AOA and comammox. Together, these data will facilitate predictions and interpretation of ammonia oxidizer community structures and provide a robust basis for establishing testable hypotheses on competition between AOB, AOA, and comammox.

硝化作用是氨氧化成硝酸盐，是生物地球化学氮循环中的一个重要过程。硝化的第一步，氨氧化，由三个经常同时出现的化学自养生物协会执行：氨氧化细菌 (AOB)、古生菌 (AOA) 和完全氨氧化剂 (comammox)。底物动力学被认为是这些行会之间的主要生态位分化因素，但很少有 AOA 菌株被动力学表征。在这里，代表所有主要栽培系统发育谱系的 12 AOA 的氨氧化动力学特性是使用微呼吸测定法确定的。Nitrosocosmicus属的成员在任何表征的 AOA 中，对氨和总铵的亲和力最低，这些值与先前确定的 AOB 的氨和总铵亲和力相似。这与之前的假设形成对比，即所有 AOA 都具有比其 comammox 或 AOB 对应物更高的底物亲和力。氨氧化剂的底物亲和力与其细胞表面积与体积比相关。此外，在一系列 pH 值范围内的动力学测量支持这样的假设，即与 AOB 一样，氨而不是铵是 AOA 和 comammox 的氨单加氧酶的底物。总之，这些数据将有助于对氨氧化剂群落结构的预测和解释，并为建立关于 AOB、AOA 和 comammox 之间竞争的可检验假设提供坚实的基础。