# A Review on the Use of Microalgae for Sustainable Aquaculture
## 3 Microalgae-Assisted Aquaculture
### Microalgae-Based Aquaculture Feed
#### 3.5.2 Microalgae Feed for Aquaculture  
Protein  
Protein synthesis in microalgae is impacted by both growth conditions and nutrients supply in culture medium. Table 4 indicated that protein content in dry microalgae biomass ranged between 26.5% and 53.3%. Particularly, compared with soybean protein, microalgae protein has much higher productivity, making it a feasible protein source for aquaculture. According to previous studies that used microalgae for aquaculture practice, feed conversion ratio (FCR) of fish fed by microalgae is higher than that of fish fed by traditional feed [81]. However, in some cases, due to palatability problems, microalgae feed may not perform well in aquaculture. For example, in the culture of Atlantic cod, with the increase of microalgae content (0%-30%) in fish-meal, most growth parameters, including final body weight, absolute feed intake, and specific growth rate, decreased but mortality increased [90]. Hence, in a real-world application, to ensure the sustainable operation of microalgae-assisted aquaculture, palatability of microalgae feed should be comprehensively evaluated.  
Polyunsaturated fatty acids (PUFAs), such as eicosapentaenoic acid (EPA), arachidonic acid (AA), docosahexaenoicenoic acid (DHA), and \(\alpha\)-linoleic acid (ALA), have been proven to be essential for the growth of larvae. For example, EPA, functioning as a precursor for eicosanoids synthesis that partly regulates the developmental and regulatory physiology, plays a pivotal role in the growth of aquatic animals. Compared with traditional aquaculture feed, microalgae biomass contains much higher concentrations of PUFAs. As shown in Table 4, percentages of PUFAs in microalgae could reach almost 50% under some special conditions, such as low temperature. Compared with soybean and peanut, which are usually exploited as feedstock for traditional fishmeal production, microalgae with higher contents of PUFAs have much greater potential to be used for aquaculture. Therefore, microalgae can be considered as an affordable and productive source of PUFAs for aquaculture.  
Special Pigments  
The natural pigments, such as astaxanthin, chlorophyll, and caroten, in microalgae are important to the growth of some fish species. First, biochemical characteristics of fish are partly determined by the intake of microalgae pigments. [PERSON] et al. (2006) found that rainbow trout _Oncorhynchus mykiss_ fed by _Haematococcus pluvialis_ contained higher concentration of astaxanthin (around 20 mcg/g dry weight). In most cases, rainbow trout with more astaxanthin and a better flesh color is preferred by consumers in the market. Second, some pigments play a pivotal role in the immunity of aquatic animals. It was discovered that lysozyme activity, an indicating factor of fish immunity, of large yellow croaker _Pseudosciena croca_, increased with the increase of astaxanthin and _Haematococcus pluvialis_ levels, suggesting that the fish immunity has a positive relationship with the intake of microalgae pigment (astaxanthin) [81]. Traditionally, for specific purposes, feed with pigments, of which the production, extraction, purification, and preservation cost are high, are added into aquaculture [91]. In microalgae-assisted aquaculture, biomass rich in natural pigments is directly added to feed aquatic animals, thus reducing the total cost and prevent the pigments degradation.  
Other Applications  
Besides directly using microalgae as feed, researchers also studied the microalgae-based artificial ecological system, in which herbivous fish relying on microalgae feed is preyed on by carnivorous fish. Thus, microalgae biomass can be indirectly used to feed carnivorous fish. Considering the poor performance of microalgae in the remediation of a water body with low concentrations of nutrients, recently, some studies proposed a novel concept of integrating microalgae culture with a hydroponics system, through which microalgae and leafy plants cooperate simultaneously for nutrients recovery from aquaculture.