# A Review on the Use of Microalgae for Sustainable Aquaculture
## 1 Introduction  
In recent years, the depletion of wild fishery resource is driving the fast development of aquaculture worldwide [1]. In some countries, the farming of aquatic animals has surpassed the yield of wild fisheries [2]. It is expected that in the coming future, aquaculture will become the main industry providing aquatic products to human beings. However, with the continuous expansion of the scale of aquaculture and the increased production, water pollution has become a serious problem posing threats to the environmental protection and hindering the sustainable development of aquaculture [3; 4]. Moreover, in aquaculture practice, with the water deterioration, high incidence of diseases would also increase the commercial risks of the whole industry [5]. To overcome the aforementioned problems, a lot of efforts were devoted into aquaculture to control wastewater pollution and improve survival efficiency of aquatic animals. The most common and straightforward method to control the pollution of aquaculture wastewater pollution is using traditional environmental remediation technologies, such as aeration, filtration, and anaerobic-anoxic-oxic (A\({}^{2}\)O) system, to remove nutrients in wastewater [3; 6; 7]. In a real-world application, the treatment of wastewater by these technologies with high energy consumption or investment improves the total cost of aquaculture and increases the financial burden of industry [3; 8]. By traditional technologies, nutrients, including nitrogen, phosphorus, and carbon, in wastewater could not be fully utilized and recycled as resources. Some technologies may produce a large amount of carbon dioxide and sludge,causing secondary environmental pollution [9]. In aquaculture, antibiotics and medicine are commonly used as a feasible way to prevent animals' diseases and reduce aquaculture risks. However, the overuse of antibiotics or medicine may negatively impact the meat quality of aquatic animals and cause food safety problems [10]. The accumulation of residual antibiotics or medicine in the water body may also become a trigger of antibiotics-resistance and even some ecological disasters [11]. Therefore, in recent years, interests in the development of economically feasible and environmentally friendly technologies to deal with the problems occurring in aquaculture are growing worldwide. Microalgae, which could efficiently assimilate nutrients in a eutrophic water body, have been proven to be a good way for wastewater remediation [12; 13]. The great performance of microalgae for nutrients assimilation has been widely observed in the remediation of food industry effluent, agricultural waste stream, municipal wastewater, and many other types of wastewater [14; 15; 16]. In recent years, more and more studies confirmed the beneficial role of microalgae in aquaculture wastewater treatment [17; 18]. In addition to treating wastewater, microalgae could synthesize value-added components, including protein, lipid, and natural pigments. Previous studies have successfully applied various microalgal species, such as _Chloella_ sp., _Dunaliella_ sp., and _Scenedesmus_ sp., for the production of value-added biomass, which could be exploited to partly replace aquaculture feed and enhance the immunity of aquatic animals [15; 18; 19]. Last but not the least, microalgae with a high capacity of generating oxygen could act like a bio-pump for aeration in aquaculture and adjust the microbial community in a water body [20]. Thus, the water quality in aquaculture practice could be properly controlled to avoid algal bloom or oxygen depletion. Owing to aforementioned benefits, the use of microalgae for aquaculture wastewater remediation has recently emerged into the limelight. In recent years, the concept of using microalgae in aquaculture has been proposed and a lot of efforts are devoted to promote the industrial implementation of microalgae-assisted aquaculture [19; 21].