Higher Contribution by Comammox Bacteria Than AOA and AOB to Nitrification in the Sediments of Lake Taihu

Nitrification is the initial reaction and maintains the nitrogen balance in natural ecosystem. The discovery of complete ammonia oxidation (comammox) has revolutionized the traditional theory of 'two-step nitrification'. However, the comammox bacteria and contribution in lake sediment are little known, therefore Lake Taihu was selected for the study, which is a typical freshwater lake with heterogeneity in China. The sediments from five areas with different nutrient levels were selected to investigate the phylogeny and distribution of comammox bacteria by amoA gene at different spatial scales, as well as their correlation with environmental factors and the contribution in lake sediment. The result showed that comammox bacteria amoA gene abundance ranged from 5.29 x 10(5) to 5.49 x 10(11) copiesg(-1) dry sediment, which was generally higher than that of AOA and AOB. The abundance of comammox bacteria was significantly correlated with the N concentration (p < 0.05). Moreover, through high-throughput sequencing of amoA gene, comammox Nitrospira were widely distributed in the phylogeny of clade A and clade B, approximately 50.88%-76.19% and 18.32%-36.68% of clade A.1 and A.2, respectively. The potential rate of comammox bacteria was ranged from 0.131 to 3.793 mg Nkg(-1) d(-1) in the sediment by microcosm incubation, followed by AOB and AOA. According to the calculation, the contribution of comammox bacteria to the overall nitrification was ranged from 45.06% to 91.43%. In this study, comammox possessed a high rate in the lake sediments, which should be paid more attention on in the future.