Microalgae, these tiny yet powerful organisms, play a vital role in aquaculture.

Firstly, microalgae play a crucial role in oxygen supply in water bodies. All algae possess chlorophyll a and carotene and can conduct highly efficient photosynthesis under light conditions. Due to the distribution of different algae in different water layers, three-dimensional oxygenation can be achieved through photosynthesis. In still water ponds, the oxygen produced by the photosynthesis of phytoplankton accounts for approximately 90% of the total dissolved oxygen in the pond water and is the main source of dissolved oxygen in intensive ponds. When the water body is lean, appropriate topdressing to enable a moderate reproduction of algae can achieve biological oxygenation; while when the water body is eutrophic, polyculture of filter-feeding fish can regulate the water quality. However, it is not the case that the more algae in the water, the richer the dissolved oxygen. Only when the microalgae populations in the water body form a stable competitive relationship with each other can the water body maintain a stable and benign state. Various algae restrict each other and cannot form dominant populations. But when improper fertilization and other reasons lead to a short-term outbreak of a certain type of algae, water blooms are prone to form in the pond, which will cause a large number of deaths of other algae and aquatic species, accelerate the corruption of the water body, the consumption of dissolved oxygen, and produce a large amount of ammonia nitrogen and nitrite that are toxic to aquatic organisms. When the concentration is high, it will cause large-scale deaths of fish and shrimp. Therefore, aquaculture farmers need to monitor various indicators of the water body in real time and respond promptly to ensure that all communities in the water body form a stable ecosystem and form a virtuous circle.

Secondly, microalgae can effectively regulate the micro-ecological environment of the water body. Free ammonia and nitrite are toxic to aquatic organisms, but appropriately adding bait microalgae in the aquaculture water body can not only directly absorb and utilize these toxic substances, but also the oxygen released by its photosynthesis can promote the nitrification of ammonia nitrogen and nitrite by nitrifying bacteria. For example, when Chlorella is introduced into the water body on the third day, the absorption efficiency of ammonia nitrogen can reach 70%. In addition, bait microalgae play an obvious role in regulating the balance of microorganisms in the water body. For instance, Vibrio, which is widely distributed in the marine environment, is rarely detected in large-scale cultivation systems of bait microalgae.

Furthermore, microalgae have significant applications in the cultivation of aquatic animal seedlings. Microalgae are rich in nutrition, and many contain high protein as well as vitamins and trace elements necessary for the growth of aquatic product seedlings, which can significantly promote various physiological functions of the seedlings. For example, vitamin E can promote the phagocytosis of macrophages in aquatic animals, increase complement activity, and enhance the immunity of juvenile aquatic animals; adding Anabaena powder to the compound feed of shrimp can significantly increase the growth rate and disease resistance of shrimp. Therefore, excellent algal species such as Anabaena and Chlorella have promising prospects in the cultivation of fish and shrimp seedlings. Adding a certain proportion of unicellular algae to the compound feed as the bait in the early stage of fish and shrimp breeding leads to a higher survival rate of the seedlings and stable and high-yield breeding. Different algal species have differences in the content of nutritional indicators. The mixed regulation of multiple algae is conducive to the balanced supply of multiple nutrients and ensures the growth, development, and smooth metamorphosis of larvae. Currently, there are more than 40 kinds of widely used bait algae in aquaculture, such as Chlorella (the first artificially cultivated microalgae in China), Dunaliella salina, Chaetoceros, Phaeodactylum tricornutum, Isochrysis galbana, Skeletonema costatum, Heterogloea, Platymonas, Nitzschia closterium, etc. Besides being directly used as bait, microalgae can also be used to feed secondary baits such as rotifers, Artemia, copepods, cladocerans, etc., which can significantly enhance the content of polyunsaturated fatty acids and various vitamins contained in the bodies of secondary bait organisms, meeting the needs of juvenile aquatic animals for high-quality secondary baits and strongly promoting the development of the seedling industry and the progress of seedling raising technology.

With the increasing emphasis on the concept of healthy breeding, it can be foreseen that in the future cultivation of seedlings of aquatic economic animals, these algae will undoubtedly play an increasingly important role. They can not only provide high-quality nutritional sources for aquatic animals but also serve as a key means to regulate the ecological environment of the water body, helping to build a green, efficient, and circular aquaculture model and promoting the sustainable development of the aquaculture industry.

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