Marine nitrogen cycling plays a critical role in regulating ocean productivity, biogeochemical fluxes, and the global carbon cycle. This mini-review synthesizes recent advances in understanding nitrogen transformations, from fixation by diverse diazotrophs to losses via denitrification, anammox, and DNRA, highlighting their ecological and biogeochemical significance. Recent studies have revealed the complex interactions between microbial communities and environmental drivers, including nutrient availability, oxygen minimum zones, stratification, and climate-induced warming. Technological innovations such as metagenomics, metatranscriptomics, proteomics, autonomous sensors, and compound-specific isotope analysis have provided unprecedented insights into nitrogen cycling dynamics, allowing high-resolution spatial and temporal assessments. The review also emphasizes the impacts of anthropogenic stressors, including eutrophication, pollution, and emerging contaminants like microplastics, on nitrogen fluxes and microbial function. Despite these advances, substantial uncertainties remain regarding global nitrogen budgets, the ecology of understudied diazotrophs, and the long-term responses of nitrogen cycling processes to climate and human pressures. Future research priorities include expanding long-term autonomous observations, integrating microbial process rates into biogeochemical and Earth system models, and investigating interactions with emerging pollutants. Improved understanding of these processes is essential for accurately predicting ocean productivity, carbon sequestration, and feedbacks to climate change. Collectively, this review underscores the need for interdisciplinary approaches that combine molecular biology, biogeochemistry, and advanced observational technologies to resolve key knowledge gaps and enhance our capacity to manage and predict the functioning of marine nitrogen cycles in a rapidly changing ocean environment.