Urea inputs to aquatic ecosystems have increased rapidly due to intensified fertilizer use, wastewater discharge, and aquaculture expansion, altering nitrogen composition and accelerating eutrophication. However, the physiological, ecological, and evolutionary mechanisms linking urea enrichment to harmful algal blooms (HABs) remain incompletely synthesized. Here, we integrate a bibliometric survey of 818 papers from 2003 to 2024 with a quantitative analysis of 32 physiological studies and a comparative genomic assessment of urea metabolic pathways. Urea concentrations range from the nanomolar scale in open oceans to >20 μ M in eutrophic rivers and estuaries—levels sufficient to stimulate urea-preferring taxa. Many HAB-forming dinoflagellates and cyanobac teria exhibit enhanced growth, photosynthetic efficiency, and in some cases, increased toxin production when supplied with urea. Comparative pathway analysis shows that bloom-forming taxa predominantly utilize the energy-efficient urease pathway, whereas non-blooming taxa more commonly rely on the ATP-consuming urea amidolyase (UAL) pathway. Several bloom-forming species are capable of using both pathways, suggesting metabolic flexibility under fluctuating nitrogen regimes. These findings support the hypothesis that anthropo genic urea enrichment acts as a selective pressure favoring taxa capable of rapid and energetically economical nitrogen assimilation. We identify key research gaps—including evolutionary origins of UAL, regulatory diver gence between pathways, and interactions with warming and stratification—and provide recommendations for monitoring and management. This synthesis highlights urea as a critical yet underappreciated component of global nitrogen pollution and HAB dynamics.

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