International Journal of Marine Science, 2025, Vol.15, No.4, 209-219 http://www.aquapublisher.com/index.php/ijms 216 nitrogen-fixing bacteria provide a nitrogen source, and promote the large-scale reproduction of algae. Once a water blossom is formed, its maintenance process is also affected by the combined influence of symbiotic and antagonistic forces (Cui et al., 2020). The symbiotic bacteria can prolong the peak of algae by continuously supplying vitamins and removing metabolic waste, while the amplification of antagonistic microorganisms (such as algaeicidal bacteria, algaephage viruses, etc.) gradually weakens the competitiveness of the dominant species of water blooms. Finally, when the enemy's effect gained the upper hand, the water flower population quickly declined and disintegrated, and a large amount of organic matter released by the algae were decomposed and utilized by bacteria, entering the next cycle (Yu et al., 2023). 7.2 Maintaining community diversity and ecological balance The interaction between microalgae and microorganisms is also one of the key factors in maintaining water community diversity and ecosystem stability. Reciprocal symbiosis increases the complexity of the ecosystem, providing more species with living space and resource utilization pathways, thereby enhancing community diversity. For example, the algal microenvironment, as a "hot spot", promotes the coexistence of multiple bacteria and avoids the monopoly of single algae species on nutritional resources. On the contrary, various antagonisms prevent the excessive prosperity of certain species and help maintain the dynamic balance of community structure. Viruses and predators selectively eliminate dominant algae species, giving the originally disadvantaged algae and bacteria a chance to rise, thereby improving the uniformity and steady state of the system. Therefore, the positive and negative interactions between microalgae and environmental microorganisms jointly shape the structure and function of aquatic communities and maintain a healthy balance of the ecosystem (Ashraf et al., 2022; Nizamani et al., 2024). 7.3 The role of global carbon, nitrogen and phosphorus cycles The interaction between microalgae and microorganisms plays an irreplaceable role in the Earth's material cycle. Microalgae in the ocean and freshwater fix massive CO2 through photosynthesis every year, and a considerable portion of them are decomposed by microorganisms and return to the atmosphere or water body. In this process, elements such as carbon, nitrogen, and phosphorus are continuously transformed between algae and microorganisms, realizing the close coupling between producers and decomposed people (Krohn-Molt et al., 2017). Symboyant nitrogen fixation injects new nitrogen into a nitrogen-poor environment, improving primary productivity; the mineralization and release of organophosphorus by microorganisms provides algae with phosphorus source supplies. It can be said that without the participation of microorganisms, the fixed carbon and required nutrients of algae will be difficult to circulate in the ecosystem. Similarly, without the photosynthesis of algae, aquatic microbial communities cannot prosper (Soong et al., 2019). Microalgae-microbial interaction ensures the formation and regeneration of carbon sinks, nitrogen sinks and phosphorus sinks, and has far-reaching impacts on global climate regulation and nutrient circulation balance. 8 Summary Comments The complex symbiotic and hostile interaction between microalgae and environmental microorganisms shapes the structure and function of aquatic ecosystems. On the one hand, symbiotic and mutual benefit promotes material circulation and energy conversion, allowing microalgae to grow efficiently, and show application potential in areas such as energy production and environmental governance; on the other hand, competition and antagonism naturally regulate algae populations, preventing excessive reproduction of a single species and causing ecological imbalance. It can be seen from typical cases that nutritional complementary symbiosis helps to improve the biomass yield of microalgae and pollution control efficiency, while the effects of hostile factors such as viruses and antagonists often end water blooms and trigger community succession. These studies have deepened our understanding of aquatic ecological processes and provided a scientific basis for the development of new technologies of algae synergistically. Looking ahead, there are still many scientific issues in this field that are worth in-depth exploration. For example, the analysis of the specific mechanisms of algae signaling and gene interaction from the molecular level still
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