Microbial regulation of global macroalgal blooms (green tides): From holobiont interactions to bloom dynamics and biogeochemistry

Abstract Green tides caused by Ulva species have become one of the most serious marine ecological disasters, now impacting many coastal nations around the world. Although climatic and environmental drivers of these macroalgal blooms are well recognized, growing evidence identifies Ulva ‐associated microbiota as potential pivotal regulators of bloom initiation, development, and demise. Here, we synthesize current advances that reframe green tides as emergent outcomes of the Ulva –microbiota holobiont. During bloom initiation, microbiota produce signaling molecules and phytohormone‐like compounds that regulate spore colonization, germination, and morphogenesis. At the bloom stage, microbial partners sustain rapid macroalgal proliferation by mediating nitrogen fixation, phosphorus mobilization, and iron acquisition, enabling large‐scale biomass accumulation. Conversely, during bloom decline, algicidal and decomposer taxa accelerate Ulva mortality and biomass breakdown, recycling nutrients and driving carbon, nitrogen, phosphorus, and sulfur cycling. Collectively, these findings highlight that microbiota exert far more intricate and influential roles in green tide dynamics than previously recognized. Deciphering these holobiont interactions not only advances our understanding of the ecology of green tide but also offers microbiome‐informed strategies to mitigate future green tide outbreaks.