International Journal of Marine Science, 2025, Vol.15, No.4, 209-219 http://www.aquapublisher.com/index.php/ijms 213 lytic effect on dinoflagellate cells, which can accelerate the red tide’s regression (Wang and Coyne, 2022). For example, bacteria of Pseudomonas can produce reactive oxygen species such as hydrogen peroxide, causing algae cells to suffer oxidative stress and apoptosis. On the other hand, microalgae can also release allelopathic substances that are unfavorable to bacteria or other algae. Some cyanobacteria produce algal toxins such as microcystis toxins, which not only poison zooplankton, but also inhibit the growth of competing algae (Zak and Kosakowska, 2016). For example, compounds such as phenolic acid secreted by large aquatic plants can inhibit the photosynthesis and respiration of cyanobacteria, thereby curbing the excessive proliferation of harmful algae to a certain extent. 4.3 Parasitic and predation of microalgae by viruses, fungi and protozoa Microalgae not only face bacterial competition and chemical inhibition, but are also often directly attacked by viruses, fungi and protozoa. Aquatic viruses are one of the largest biological entities in the aquatic ecosystem, with as many as tens of millions of virus particles per milliliter of seawater, a considerable number of which are hosted by algae. These algae phage viruses usually specifically infect specific algae species, completing life history by replicating and lysing the host in algae cells, resulting in the disintegration and subsidence of algae blooms in a short period of time. For example, there have been many outbreaks of Aureococcus anophagefferens on the east coast of the United States. Research has found that a specific algae phage virus infected a large number of brown algae cells during the peak of the bloom, causing its population to collapse rapidly within a few days (Yu et al., 2023). These examples show that sudden outbreaks of viruses can end algae blooms through top-level killing mechanisms, and have a severe impact on local ecosystems. On the other hand, parasites of aquatic fungi also play a role in the regulation of algae populations. For example, fungi such as gypsum chytrium parasitize in diatoms, seizing algae cell nutrients and causing host death. In addition, miniature predators such as ciliates, meat-pods and rotifers feed on algae, and each individual ciliate can swallow tens of thousands of algae cells every day, thus forming a strong top control over algae populations (Bergman et al., 2024). Zooplankton predation of dominant algae species can effectively inhibit their overproliferation, so it is often regarded as a biological control method in water quality control. The dynamic game between microalgae-virus-predators jointly determines the rise and fall of algae populations, and has a profound impact on the material circulation and energy flow of water bodies. 5 Molecular and Ecological Mechanisms of Interaction 5.1 Signaling and group sensing mechanism The interaction between microalgae and microorganisms is not only reflected in nutrient exchange, but also regulates each other's behavior through delicate signaling processes. The "Quorum Sensing" (QS) mechanism of bacteria is a key link: when the bacterial density reaches a certain level, the concentration of self-induced signal molecules it secretes increases, which triggers the expression of related genes after being sensed by the bacteria. In the algal microenvironment, QS signaling can affect the physiological activity of algae. On the other hand, microalgae will also use signal molecules for "reverse communication". Chlamydomonas can secrete lactonease to degrade bacteria's AHL signals, thereby interfering with the bacteria's mass sensing process (Dow, 2021). In addition, metabolites such as polysaccharides and pigments released by microalgae may also act as information media to attract or reject specific microorganisms to attach. Through complex signaling networks, microalgae and microorganisms can perceive each other's existence and regulate their own physiology, achieving dynamic control of the interaction relationship. 5.2 Interaction patterns revealed by genomics and multiomics With the development of high-throughput sequencing and multiomics technologies, people have gained a deeper understanding of the molecular mechanisms of microalgae-microbial interactions. Genomics studies show that many microalgae and bacteria have undergone gene complementarity and functional differentiation in long-term co-evolution. For example, in a symbiotic system of a bialgae, the genome of the endosymbiotic nitrogen fixation bacteria was greatly simplified, and some carbon metabolism pathways were lost. The host algae lacked the nitrogen fixase gene, but the functions of both parties were just complementary, allowing the symbionts to fix
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