# Digestate as Sustainable Nutrient Source for Microalgae--Challenges and Prospects
## 7 Use of Microalgae Biomass
### High-Value Products and Microalgae Biorefinery  
A biorefinery approach can increase the sustainability of microalgae production by exploiting the biomass more completely and avoiding waste [78]. As an example, the SA-BANA project (EU Horizon 2020 Research and Innovation Program under grant agreement no. 727874) aims to demonstrate a large-scale microalgae-based biorefinery concept for the production of high-value products (biostimulants, biopesticides, aquafeed additives) as well as low-value products (biofertilizers, fish feed), recovering nutrients from wastewaters (sewage, supernatant, and pig manure) in continuous mode all year round [13; 93].  
Biohydrogen production can also be an approach to create a microalgal biorefinery. During the dark fermentation process, anaerobic microorganisms convert biomass into hydrogen and CO\({}_{2}\)[94]. The arising effluent can serve as nutrient source for mixotrophic microalgae cultivation, similar to digestate [94]. The byproduct of the first stage, CO\({}_{2}\)might be introduced as a carbon source. Provided that the obtained biomass has high carbohydrate content, it would be a suitable feedstock for dark fermentation [95]. Thus, the microalgal biomass can be returned into the first stage of the process, serving as substrate for hydrogen production [94]. However, the concept of biohydrogen production using microalgae is still at an early stage.  
Another biorefinery model, based on digestate, focuses on the production of the biodegradable biopolymer PHB, which can replace commercial plastics such as polyethylene and polypropylene. It is produced by several prokaryotic microalgae (cyanobacteria) such as _Nostoc_, _Arthospira_, and _Synechocystis_ under phototrophic or mixotrophic growth and nutrient-limited conditions [96]. Recently, digestate was investigated as a low-cost substrate for cultivating _Synechocystis_ cf. _salina_ for PHB production [97]. In mineral medium, PHB accumulation was induced by nitrogen limitation, which led to consumption of phycocyanin and orange coloring of the biomass. This was not observed when digestate was used. Therefore, it is suggested that phycocyanin and PHB can be obtained as products. Besides that, the residual biomass can be used as animal feed or anaerobically converted to biogas.  
In addition to phycocyanin, microalgae are a biological source for several other valuable pigments [98]. Besides their utilization as natural colorants, they have various potential application areas such as food industry, feed additives (e.g., aquaculture), and cosmetics due to their high nutritional and bioactive value [78; 98]. It has been shown that the cultivation of _Chorella vulgaris_ and _Scenedesmus_ in diluted digestate leads to carotenoid accumulation [78]. Moreover, the limited light permeability of diluted digestate can cause higher chlorophyll and phycocyanin contents compared to cultivation in transparent mineral medium [78]. Thus, pigment extraction of microalgal biomass should receive attention to make microalgal biorefineries with digestate as nutrient source more economically feasible [78]. However, it is questionable if waste streams such as digestate can be used for the production of compounds for human use [98].  
The microbial production of exopolysaccharide (EPSs) can be applied for natural products, whose demand is rising because they are used in a wide range of biotechnological, medical, and industrial applications [66]. Further research is necessary to investigate the effect of using digestate as nutrient source on EPS production.  
Microalgae biomass grown in digestate media can be applied as environmentally friendly slow-release fertilizer [78]. This application can be an addition to the use of the solid digestate fraction as fertilizer, utilizing the nutrients of the effluent in a more complete way. In a biorefinery approach, this can serve as final vaporization of the remaining biomass after extracting other valuable compounds such as PHB, lipids, or pigments.