International Journal of Marine Science, 2025, Vol.15, No.4, 209-219 http://www.aquapublisher.com/index.php/ijms 210 Given the importance of interaction between microalgae and environmental microorganisms in terms of ecology and application, this study reviews the main types and mechanisms of symbiosis and hostility relationships between the two; introduces the basic characteristics of microalgae and environmental microorganisms, and then analyzes the symbiosis modes such as nutritional complementarity and signaling, and explains them in combination with typical cases. Finally, the ecological significance of microalgae-microbial interaction is summarized and future research directions are looked forward. 2 Basic Characteristics of Microalgae and Environmental Microorganisms 2.1 Biological characteristics and ecological functions of microalgae Microalgae generally refer to single-cell or population algae that can perform photosynthesis, including prokaryotic cyanobacteria and eukaryotic green algae, diatoms, dinoflagellates and other groups. For example, Spirulina, Chlorella, Microcystis, etc. are all common microalgae representatives. Microalgae usually have the characteristics of rapid growth and high photosynthetic efficiency, and can reproduce in large quantities under suitable conditions (Sarıtaş et al., 2024). Ecologically, microalgae are the basic primary producers of aquatic food webs, and undertake the role of converting solar energy into chemical energy and fixing CO2, providing organic matter and oxygen to the entire aquatic ecosystem (Li et al., 2022). It is estimated that about half of the world's atmospheric oxygen comes from the photosynthesis of phytoplankton microalgae in the ocean, and its carbon sink has an important impact on climate regulation. In addition, different microalgae can also produce a variety of metabolites, such as proteins, polyunsaturated fatty acids and pigments, which are of development value in the fields of aquaculture, food and medicine. 2.2 Diversity and distribution pattern of environmental microbial communities Environmental microorganisms generally refer to microorganisms such as bacteria, archaea, fungi, viruses and protozoa distributed in various environments, with extremely diverse species and functions. Take water as an example. Each milliliter of seawater contains millions of bacteria and viral particles. These microorganisms play a key role in decomposing organic matter, circulating nutrients, and regulating ecosystems. The composition of environmental microbial communities varies according to environmental conditions: in nutrient-rich water bodies, heterotrophic bacteria often dominate and utilize organic matter released by algae; and in clean, poor nutrient water bodies, autotrophic archaea and nitrogen-fixing bacteria also occupy a certain position. It is worth noting that many water microorganisms have close spatial connections with algae. For example, the "algae" area around microalgae cells converges specific bacterial communities, which are usually more diverse than the surrounding water phase. These algal microorganisms gather near algae with the help of motility such as flagella to form a stable microecological structure, providing a place for algae interactions (Garcia et al., 2022). 2.3 Theoretical basis of microalgae-microbial interaction The interaction relationship between microalgae and microorganisms is based on long-term co-evolution, reflecting the dual characteristics of synergy and antagonism. On the one hand, the "reciprocity and symbiosis" theory believes that different populations maximize resource utilization through division of labor and cooperation. For example, many green algae themselves cannot synthesize essential nutrients such as vitamin B12 and must rely on symbiotic bacteria to provide this vitamin to maintain growth. While bacteria obtain organic carbon sources from algae, they synthesize vitamins for algae. This mutually beneficial relationship is called the "nutritional complementary symbiosis mechanism" (Schlogelhofer et al., 2019). On the other hand, ecological competition theory points out that competitive exclusion will occur during the limited time of resources. Microalgae and bacteria may compete for nutrients such as nitrogen and phosphorus in water, which will inhibit the growth of the other party when one party reproduces in large quantities. In addition, the "group sensing" theory emphasizes the role of information exchange in interaction. Bacteria can release signal molecules such as N-acylhoserine lactone, perceive population density and regulate the production of algae-soluble substances. Algae are also able to secrete chemically induced substances that interfere with bacterial signals, thereby mitigating adverse effects (Figure 1) (Weiland-Bräuer, 2021; Srinivas et al., 2022).
RkJQdWJsaXNoZXIy MjQ4ODYzNA==