The community and function shift of ammonia oxidizers across terrestrial and marine (soil/sediment) boundaries in two coastal bay ecosystems

Terrestrial–marine boundaries, such as those in intertidal coastal ecosystems, represent delineated changes from land to sea. Nitrification activity along terrestrial-soil and marine-sediment boundaries will be strongly influenced by environmental conditions that change sharply. This makes them ideal environmental gradients to investigate niche separation and functional alteration between ammonia oxidising archaea (AOA) and bacteria (AOB). This study investigated ammonia oxidisers across a terrestrial/marine boundary in two intertidal coastal bays (Clew and Rusheen Bay) on the Atlantic coast of Ireland via in situ field studies and ex situ laboratory microcosms. Within each bay 5 sites spanning soil, soil/sediment interface and marine sediments were targeted. AOA>AOB in soil but AOB>AOA in the sediments of both bays, while the soil-sediment interface had equal abundances of AOA and AOB. AOA and AOB communities from soil, soil/sediment interface and sediment clustered separately. The ratio of AOA to AOB negatively correlated with pH and salinity, but positively with total organic carbon. To further investigate the hypothesis that AOA were the dominant active ammonia oxidisers in soil and AOB in sediments, triplicate soil, interface and sediment microcosms were amended with ammonia and incubated for 28 days at 17C. Control microcosms containing acetylene, an inhibitor of ammonia oxidation, were also included. NH₄⁺ was consumed and accompanied by an increase in NO₃^-, but not in the acetylene controls. In the soil microcosms, the increase in NO₃^- was accompanied by an increase in AOA amoA genes and transcripts. Active AOA amoA transcripts clustered with Nitrosopumilus maritimus and soil-sediment clusters 1 and 3. In the sediment microcosms AOB amoA genes and transcripts numbers increased with active AOB amoA gene transcripts clustering with Nitrosomonoas sp.