International Journal of Marine Science, 2025, Vol.15, No.4, 209-219 http://www.aquapublisher.com/index.php/ijms 211 Figure 1 Biofilm development. Free-swimming bacteria initially attach to a solid surface, and colonizing bacteria further form structured aggregates called microcolonies. Biofilms are composed of numerous microcolonies, which are encased in an extracellular polymeric matrix. Biofilms permanently undergo composition/decomposition. Confocal Laser Scanning Micrographs of Klebsiella oxytocaM5aI biofilm formation (Adopted from Weiland-Bräuer, 2021) 3 Main Types of Symbiotic Relationships 3.1 Nutritional complementarity and substance exchange Nutritional complementarity is one of the most basic types of relationships in the symbiosis between microalgae and microorganisms. Photosynthesis allows microalgae to convert inorganic carbon into organic carbohydrates and release oxygen, providing energy and oxygen sources for surrounding heterotrophic microorganisms; while microorganisms use organic matter and metabolic waste excreted by microalgae to feed back nutrients such as inorganic nitrogen and phosphorus to the microalgae through mineralization and regeneration. For example, some nitrogen fixation bacteria can convert N2 in the atmosphere into ammonium salts or amino acids available to microalgae, thereby supplementing the nitrogen source in algae culture. For example, iron carriers secreted by certain bacteria help to improve the uptake rate of trace iron by microalgae, promote the synthesis of photosynthesis enzymes, and significantly accelerate the growth of algae cells. In addition, vitamin reciprocity is also a common way of nutritional complementarity - about half of microalgae cannot synthesize the B vitamins independently, and symbiotic bacteria need to provide cofactors such as vitamin B12, while bacteria obtain photosynthetic products from algae to meet their own carbon source needs. Through these substance exchanges, microalgae and microorganisms achieve resource recycling and mutually beneficial symbiosis (Li, 2024). 3.2 Microalgae - mutually beneficial symbiosis model of bacteria Various mutually beneficial symbiosis patterns have been found between microalgae and bacteria, with diverse mechanisms but the results are reflected in the mutual promotion of growth or function of both parties. A typical model is that in wastewater biological treatment systems, co-culture of algae can simultaneously remove contaminants in water and increase microalgae biomass production. Studies have shown that compared with monoalgae or monoglycculture, the algae-bacterial symbiosis system has a higher efficiency in removing nutrients such as nitrogen and phosphorus, and the growth rate and oil accumulation of microalgae have increased significantly (Hu et al., 2019; Dai and Wang, 2023). For example, in urban sewage treatment, constructing a particle symbiosis of Chlorella and activated sludge bacteria can increase the removal rate of ammonia nitrogen and phosphate to a level that is difficult to reach in the pure bacteria system, while reducing CO2 emissions and residual sludge yields (Zhang et al., 2021). In the field of bioenergy, co-cultivating beneficial bacteria can also
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