International Journal of Marine Science, 2025, Vol.15, No.4, 209-219 http://www.aquapublisher.com/index.php/ijms 214 carbon and nitrogen at the same time (Kimotho and Maina, 2023). Transcriptome and proteome analyses further reveal dynamic patterns of gene expression between both parties during interaction. Experiments show that when Chlorella co-cultured with probiotic bacteria, photosynthesis and carbon sequestration related genes in algae cells are upregulated, while bacteria enhance organic decomposition utilization and vitamin synthesis pathways (Wirth et al., 2023). Metagenomic research also found that there are a large number of unique metabolic enzymes and pathway genes in algae microorganisms, indicating that the algae flora has evolved special functions in order to adapt to the ecological niche that coexist with algae. By integrating multiomics data, researchers are gradually building a systematic model of microalgae-microbial interaction to analyze the operating mechanism of symbiotic or antagonistic relationship as a whole. 5.3 Coupling relationship between metabolic network and carbon and nitrogen cycle The interaction between microalgae and microorganisms directly affects the local, even global carbon, nitrogen and phosphorus circulation processes at the ecological level. Photosynthetic microalgae immobilize inorganic carbon and release a portion of it into water in the form of dissolved organic matter (DOM), which heterotrophic bacteria use and decompose carbon respiration into CO2. It is estimated that bacteria in the ocean consume 25%~50% of the carbon fixed by algae, reflecting the important feedback effect of microbial communities on the carbon cycle (Kuypers et al., 2018). At the same time, the decomposition of bacteria turns organic nitrogen and phosphorus in algae biomass into inorganic states, and is absorbed and utilized by algae again. A newly discovered phytoplankton diatom is symbiotic with nitrogen-fixing rhizobia, whose fixed nitrogen is directly supplied to the host algae for use, and is believed to fill the nitrogen deficiency in the oceanic seas (Wang et al., 2024). In environments such as estuaries, the oxygen released by photosynthesis of algae promotes nitrifying bacteria to oxidize ammonium to nitrates, while heterotrophic denitrifying bacteria then reduce nitrates to nitrogen to escape from the water, forming a nitrogen circulation pathway connected by algae-aerobic bacteria and anaerobic bacteria. 6 Case Analysis 6.1 Typical symbiotic system between cyanobacteria and nitrogen-fixing bacteria In the oceanic oceans, the symbiosis between plankton diatoms and nitrogen-fixing microorganisms is widely present. Traditionally, the main nitrogen fixing organism in the ocean is cyanobacteria, but in recent years, some non-cyanobacterial nitrogen fixing bacteria have also been found to symbiotically with algae to fix nitrogen. A large-scale marine diatom symbiotic with filamentous cyanobacteria mycelium Richelia, which can contribute a considerable proportion of nitrogen input in the tropical Pacific (Villareal, 2020). What is even more remarkable is the latest research finding that a heterotrophic bacteria from the order Rhizobiales can live in single-cell diatoms and directly provide nitrogen to the host through nitrogen fixation. This discovery expands people's understanding of the nitrogen fixation pathways in marine organisms, indicating that in addition to cyanobacteria, certain conventional bacteria can also participate in the marine nitrogen cycle through symbiosis with algae (Figure 2) (Tschitschko et al., 2024). 6.2 Case of co-culture of microalgae and heterotrophic bacteria to increase biofuel yield Algae coculture technology shows great potential to increase yield in the field of biofuels. Co-culture of oil-producing microalgae with specific heterotrophic bacteria under laboratory conditions can significantly improve microalgae biomass and oil production. After Chlorella symbiotic with a Pantoea pineapple strain, its algae cell dry weight and oil content were significantly higher than those in the culture group alone (Zhang et al., 2023). In larger-scale experiments, the algae symbiotic system also showed excellent performance: When dealing with urban sewage, the microalgae-bacterial sludge process reported by Guo et al. (2021) not only efficiently removes pollutants such as nitrogen and phosphorus in the water, but also simultaneously produces considerable microalgae biomass, and its carbon conversion efficiency and oil production potential are better than traditional methods. These cases prove that by screening and introducing probiotic bacteria, the oil production efficiency of the microalgae culture system can be significantly improved, providing a more economical and feasible way to the production of new energy such as biodiesel.
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