# Digestate as Sustainable Nutrient Source for Microalgae--Challenges and Prospects  
[PERSON]  
1 Tulln Research Site, BEST--Bioenergy and Sustainable Technologies GmbH, Konrad Lorenz Strasse 20, 3430 Tulln, Austria; [EMAIL_ADDRESS] (L.B. ); bernhard.drosg@best-research (B.D.) [PERSON]  
2 Laboratory of Algal Biotechnology, Centre Algatech, Institute of Microbiology of the Czech Academy of Sciences, Novohradska 237, 379 81 Trebot, Czech Republic; [EMAIL_ADDRESS] (K.R. ); [EMAIL_ADDRESS] (J.M.) [PERSON]  
2 Laboratory of Algal Biotechnology, Centre Algatech, Institute of Microbiology of the Czech Academy of Sciences, Novohradska 237, 379 81 Trebot, Czech Republic; [EMAIL_ADDRESS] (K.R. ); [EMAIL_ADDRESS] (J.M.) [PERSON]  
1 Tulln Research Site, BEST--Bioenergy and Sustainable Technologies GmbH, Konrad Lorenz Strasse 20, 3430 Tulln, Austria; [EMAIL_ADDRESS] (L.B. ); bernhard.drosg@best-research (B.D.) [PERSON]  
1 Tulln Research Site, BEST--Bioenergy and Sustainable Technologies GmbH, Konrad Lorenz Strasse 20, 3430 Tulln, Austria; [EMAIL_ADDRESS] (L.B. ); bernhard.drosg@best-research (B.D.) Footnote 1: email: {[EMAIL_ADDRESS]}  
Footnote 2: email: {[EMAIL_ADDRESS]}  
Footnote 3: email: {[EMAIL_ADDRESS]  
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been gaining more and more attention, which is reflected in the growing amount of produced biomass. According to [PERSON] et al., the worldwide annual production of microalgae in 2019 was 30,000 tons [3]. In Europe, around 60 companies are producing microalgae, mainly in photobioreactors and only a minor share in open ponds [4]. Worldwide, the most prominent microalgae that are cultivated commercially are _Chlorella, Arthospira_ (since 2018 also known as _Limnospira_[5] and earlier as _Spirulina_), _Haematococcus_, and _Dunialella_[4; 6]. Both _Chlorella_ and _Arthospira_ are rich in protein, polysaccharides, lipids, vitamins, and carotenoids [7; 8]. Therefore, they are used as nutritional substitute for a healthy diet. _Dunialella salina_ and _Haematococcus pluvialis_ are primarily cultivated for the production of \(\beta\)-carotene and astaxanthin, respectively, which they accumulate under stress conditions (e.g., elevated salt concentrations, nutrient limitation) [9; 10]. Both products are used as supplements in diets for humans and animals (e.g., aquaculture) [9; 10]. Microalgae can also be used to produce bioenergy. Under stress conditions (e.g., nutrient limitation), the lipid content of _Chlorella_ strains can become as high as 50%, which also makes it a potential candidate for the production of biofuel [7]. Recently, new energy-concepts using microalgae have been introduced. After extraction of lipids from microalgal biomass to produce biodiesel, the residual biomass can be fermented in two steps to produce hydrogen and methane, so-called bio-hydhane [11]. Moreover, the application of microalgae in microbial fuel cells, producing electricity, is being explored [12]. During microalgae cultivation, nutrients have to be provided besides water, carbon dioxide (CO\({}_{2}\)) and light. Currently, mainly artificial and limited nutrients are used, reducing the sustainability of the process. Moreover, the cultivation costs of EUR 10 kg\({}^{-1}\), a considerable part of which results from nutrient costs, are high [13]. Thus, the usage of alternative nutrient sources is an important step towards a more ecological and economical microalgae cultivation. By the utilization of waste streams as a nutrient source, the production costs can be reduced to under EUR 5 kg\({}^{-1}\)[14; 15]. It provides a reasonable solution, especially for low-value products, e.g., for bioenergy. Anaerobic effluents are a possible alternative nutrient source [16]. Anaerobic effluents (more specifically digestate) are, besides biogas, one of the two products of the anaerobic digestion of biodegradable feedstocks [17].