International Journal of Marine Science, 2025, Vol.15, No.4, 209-219 http://www.aquapublisher.com/index.php/ijms 212 improve the oil production performance of microalgae. In experiments, Chlorella and Pantoea were co-cultured at a ratio of 1:5. After 8 days, the biomass and oil content of algae cells increased significantly, which were significantly higher than those of the culture group alone. Many algae-promoting bacteria can also secrete auxin (IAA), vitamins and signaling molecules to stimulate algae cell division or release dormancy, thereby further enhancing the growth rate and physiological activity of microalgae. By screening the dominant symbiotic bacteria and optimizing the proportion of algae inoculation, the mutually beneficial symbiotic effects can be maximized and the enhancement of microalgae reproduction and metabolites output can be achieved. 3.3 Microalgae - fungal symbionts and complex ecosystems The symbiotic relationship between microalgae and fungi is also of great ecological significance. The most famous example is lichen - a symbiont composed of green algae or cyanobacteria and fungi. In lichens, microalgae provide carbohydrates for fungi, and fungi provide shelter and moisture nutrients for algal cells. Through this close cooperation, both parties can survive and reproduce in extreme environments. In comparison, there are slightly fewer studies on microalgae-fungal symbiosis in aquatic environments, but in recent years, they have gradually attracted attention in ecological engineering. Because of its developed mycelium, filamentous fungi can adhere and bind to microalgae to form particles, causing microalgae to spontaneously flocculate and settle down from the culture medium. This fungi-assisted bioflocculation technique significantly reduces the harvest cost of microalgae biomass. For example, co-culture of Aspergillus fumigatus with Chlorella can form stable large particles, which can effectively remove organic pollutants and nutrients from wastewater after settlement. Some aquatic fungi can also secrete enzymes to degrade polymers such as algae cell walls and polysaccharides, and work with microalgae to accelerate the circulation of organic matter. In a complex complex ecosystem, microalgae, bacteria, fungi and other protophytes can jointly form a multi-symbiotic network, and various organisms maintain the stability and productivity of the system through the exchange of substances and information (Wrede et al., 2014; Talukder et al., 2021). This type of composite system has shown good application prospects in sewage treatment, wetland restoration, etc. 4 The Relationship Between Enemy and Antagonism 4.1 Nutrition and niche competition When environmental resources are limited, competition occurs between microalgae and microorganisms, thereby inhibiting each other's growth. This competition between nutrients and niches is particularly evident in the succession of algae blooms in eutrophied water bodies. When microalgae reproduce in large quantities, they consume essential nutrients such as nitrogen and phosphorus in the water, which makes bacteria and other algae limited by lack of nutrients; on the contrary, when heterotrophic bacteria overproliferate, they will also inhibit the photosynthesis and reproduction of algae by competing for resources such as organic carbon sources and oxygen. There is also competition for the occupation of habitats by different populations (Wu and Wang, 2024). For example, phytoplankton and attached bacteria may compete for light conditions in the water column: algae need to absorb light energy, while excessive bacterial reproduction will increase the turbidity of the water body, weaken the depth of light penetration, thereby indirectly inhibiting algae photosynthesis. Similarly, the consumption of large amounts of dissolved oxygen by microorganisms can also threaten the survival of algae (Sörenson et al., 2020). Competition between nutrition and ecological niches often leads to the replacement of community dominant species, causing a dynamic balance between the number of microalgae and microorganisms to rise and fall. By regulating the supply of external nutrients, this competitive relationship can be intervened to a certain extent, thereby achieving the purpose of controlling water blooms or regulating community structure. 4.2 Antagonistic substances and allelopathic effects In addition to competing for resources, microalgae and microorganisms also inhibit each other by releasing chemicals, the so-called allelopathic or antagonistic effect. On the one hand, many aquatic bacteria have the ability to produce algae-soluble substances. Such metabolites include proteases, organic acids, polypeptide toxins, etc., which can damage the structure of algae cells or disrupt their physiological functions, ultimately leading to the lysis and death of algae cells. For example, Rosobacteria marine secretes active molecules to exert a strong
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