Abstract:Shrimp production has expanded rapidly over the last few decades in China. With the increasing production, water pollution has become increasingly serious. An increase in nutrients in water results in eutrophication, characterized by low oxygen, high ammonia, high phosphorus, and high frequency of algal blooms, which in turn influence shrimp growth and yield. Phytoplankton is an important component of aquaculture ecosystems. It is a direct or indirect food source for cultured organisms, and can remove nitrogen and phosphorus and maintain water quality. Constructing a benign phytoplankton community can improve the nitrogen pollutant absorption efficiency, improve the environment, and reduce the environmental pollution caused by cultivation. Certain phytoplankton groups, such as diatoms and green algae, are desirable for their high nutritional value and contribution to water quality. The use of phytoplankton to purify and regulate aquaculture water quality could reduce the negative impacts of aquaculture, which is an environmental protection option with a low cost, low energy consumption, high benefit, and considerable development potential. Microalgal growth is significantly affected by factors such as temperature, light, and nutrient conditions. Therefore, the environmental adaptability of microalgae is the primary consideration for selecting and cultivating algal species. In situ isolation and screening of algal species can reduce the stress response of microalgae, which is conducive to their normal ecological function of regulating water quality. In an indoor industrial aquaculture system for Litopenaeus vannamei, one diatom species was dominant for a significant amount of time in the middle and late stages of aquaculture in 2019. This alga was small and covered with a silica membrane layer, which was difficult to identify with a light microscope. The shrimps effectively grew in the ponds where this alga was the dominant species. To identify the algae and explore its application in water quality regulation in aquaculture, the algal strain was isolated and purified from the indoor industrial aquaculture system, and identified by optical microscopy, electron microscopy, and 18S rRNA sequence analysis. The results identified this algal strain as Actinocyclus exiguous HY01. The cell diameter was approximately (11.4±1.0) μm. At present, A. exiguous is the smallest individual species in the genus Actinocyclus. There are numerous small pores on the mantle that were not visible under a light microscope. The pore density at the mantle center was less than that at the mantle margin. There is a pseudonodulus at the valve margin, and 3~5 labiate processes. Nitrogen is an indispensable element for the growth and metabolism of phytoplankton, and is the main component of nucleic acids, proteins, and chlorophyll in cells. Ammonia and nitrate are the two main forms of inorganic nitrogen in aquaculture water, which can be directly absorbed and utilized by microalgae. The addition of nitrogen nutrients can promote microalgal growth. However, the microalgae cell density did not increase as the nitrogen concentration increased. Excessive ammonia content is inconducive to algae growth, and even affects the absorption and transformation of other nitrogen sources. In this study, ammonia and nitrate of different concentrations were used as nitrogen sources to culture A. exiguous HY01. The results showed that both ammonia and nitrate could be used for A. exiguous HY01 growth. The optimum growth concentration of ammonia was 600 μmol/L. Under these conditions, the cell density, specific growth rate, and protein content of A. exiguous HY01 were the highest, at 4.54×108 cells/L, 0.36±0.07, and 4.45 mg/g, respectively. When the ammonia concentration was 882 μmol/L, the cell density and specific growth rate of A. exiguous HY01 were the lowest, at 2.11×108 cells/L and 0.27±0.05, respectively. When the ammonia concentration was 50 μmol/L, the A. exiguous HY01 protein content was the lowest at 1.38 mg/g. The optimum growth concentration of nitrate was 882 μmol/L. Under these conditions, the cell density, specific growth rate, and protein content of A. exiguous HY01 were the highest, at 5.92×108 cells/L, 0.40±0.01, and 11.97 mg/g, respectively. When the nitrate concentration was 100 μmol/L, the cell density, specific growth rate, and protein content of A. exiguous HY01 were the lowest. In summary, the maximum cell density, maximum specific growth rate, and protein content of A. exiguous HY01 were lower in the medium containing ammonia-nitrogen than those in the medium with nitrate-nitrogen. We speculated that A. exiguous HY01 might prefer nitrate to ammonia, but has greater tolerance to high ammonia concentrations. This might be one reason why A. exiguous HY01 was dominant for a long time in the indoor industrial aquaculture system of L. vannamei in 2019. The results are expected to provide a reference for the targeted cultivation of this alga and its application in water quality regulation in aquaculture.
