A mini-review on distribution patterns of AOA across oxygen gradients in aquatic systems

Abstract

Ammonia-oxidizing archaea (AOA) are central players in the marine nitrogen cycle and rank among the most abundant microorganisms in the ocean. While classically considered obligate aerobes, AOA are consistently detected in oxygen minimum zones and oxygen-deficient zones, where dissolved oxygen reaches nanomolar or undetectable levels. This review compiles global evidence on absolute and relative AOA abundances across oxygen minimum zones, oxygen-deficient zones, nanoxic, and anoxic zones, linking their distributions to oxygen availability. This review found that AOA abundances often peak within oxygen minimal or deficient cores or its transition zones, compared to fully oxygenated waters, indicating their ability to persist and even thrive under severely oxygen-limited conditions. Strikingly, absolute AOA abundances show consistency at similar oxygen concentrations across the investigated locations, while there are also some fluctuations within the same regions. Candidatus Nitrosopelgicus exhibits higher abundances in well-oxygenated surface levels, whereas Nitrosopumilus more predominantly thrives at deeper, less oxygenated depths across various locations. Seasonal dynamics in oxygen levels emerge as a crucial determinant of AOA abundance patterns. Depth-stratified ecotypes (WCA, WCB) further illustrate how community composition is structured along oxygen gradients, though the exact physiological distinctions between these groups remain unclear. Together, these findings emphasize both global consistencies and regional variability in AOA ecology, pointing to key knowledge gaps about their metabolic versatility and their role in shaping biogeochemical cycles in a future ocean with expanding oxygen minimum zones.