Trichodesmium, one of the most abundant autotrophic nitrogen-fixing cyanobacteria in marine environments, serves as a critical source of "new nitrogen" in marine ecosystems, contributing approximately 80 Tg N per year to the global ocean. By facilitating two key biochemical processes-photosynthesis and nitrogen fixation-Trichodesmium plays an essential role in sustaining oceanic primary productivity and driving the biogeochemical cycling of both carbon and nitrogen. However, oxygen generated during photosynthesis acts as a potent inhibitor of nitrogenase activity due to its oxidative properties, hindering the nitrogen fixation process. Research indicates that Trichodesmium accumulates photosynthetic carbon during the morning, which is later utilized through respiratory protection mechanisms during the day. This process reduces intracellular O₂ concentrations, thereby ensuring that nitrogen fixation can proceed even when photosynthetic activity declines. Nitrogen fixation in Trichodesmium is closely tied to phosphorus availability (P), and the community undergoes vertical migration within the water column, typically descending to the phosphocline to acquire necessary nutrients during the photoperiodic cycle. This vertical migration is closely associated with carbohydrate production during photosynthesis. During the light phase, photosynthesis generates excess carbohydrates, forming a ballast that causes the Trichodesmium colonies to sink towards the phosphocline. As they enter the dark phase, photosynthesis weakens, and respiratory protection mechanisms become more active, consuming both oxygen and carbohydrates to create hypoxic conditions favorable for nitrogen fixation. Over time, as the carbohydrate ballast is depleted, the community rises again, and photosynthesis intensifies, marking the transition back to the light phase and the subsequent production of carbohydrates. This study aims to develop a simulation model to investigate the vertical migration dynamics of Trichodesmium over the photoperiodic cycle, focusing on the factors regulating these movements. By further analyzing how vertical migration influences photosynthesis and nitrogen fixation processes, the study seeks to elucidate the underlying mechanisms of nitrogen fixation in Trichodesmium, providing a theoretical foundation for understanding global marine nitrogen cycling.

vertical migration, Trichodesmium, photoperiodic cycle, photosynthesis, nitrogen fixation