Discovery of <i>Candidatus</i> Nitrosomaritimum\nas a New Genus of Ammonia-Oxidizing Archaea Widespread in Anoxic Saltmarsh\nIntertidal Aquifers

Ammonia-oxidizing archaea (AOA) are widely distributed\nin marine\nand terrestrial habitats, contributing significantly to global nitrogen\nand carbon cycles. However, their genomic diversity, ecological niches,\nand metabolic potentials in the anoxic intertidal aquifers remain\npoorly understood. Here, we discovered and named a novel AOA genus, <i>Candidatus</i> Nitrosomaritimum, from the intertidal aquifers\nof Yancheng Wetland, showing close metagenomic abundance to the previously\nacknowledged dominant <i>Nitrosopumilus</i> AOA. Further\nconstruction of ammonia monooxygenase-based phylogeny demonstrated\nthe widespread distribution of <i>Nitrosomaritimum</i> AOA\nin global estuarine-coastal niches and marine sediment. Niche differentiation\namong sublineages of this new genus in anoxic intertidal aquifers\nis driven by salinity and dissolved oxygen gradients. Comparative\ngenomics revealed that <i>Candidatus</i> Nitrosomaritimum\nhas the genetic capacity to utilize urea and possesses high-affinity\nphosphate transporter systems (<i>phnCDE</i>) for surviving\nphosphorus-limited conditions. Additionally, it contains putative <i>nosZ</i> genes encoding nitrous-oxide (N<sub>2</sub>O) reductase\nfor reducing N<sub>2</sub>O to nitrogen gas. Furthermore, we gained\nfirst genomic insights into the archaeal phylum <i>Hydrothermarchaeota</i> populations residing in intertidal aquifers and revealed their potential\nhydroxylamine-detoxification mutualism with AOA through utilizing\nthe AOA-released extracellular hydroxylamine using hydroxylamine oxidoreductase.\nTogether, this study unravels the overlooked role of priorly unknown\nbut abundant AOA lineages of the newly discovered genus <i>Candidatus</i> Nitrosomaritimum in biological nitrogen transformation and their\npotential for nitrogen pollution mitigation in coastal environments.