## 1 Introduction
It has to be noted that several approaches have been introduced in order to classify these heterotrophic marine microorganisms, either with a mycological or protistical orientation [31; 32]. The current study will adopt the term heterotrophic microalgae, which has been used by the scientific community in previous and recent research [33; 34; 35; 36; 37; 38; 39]. The strain _Schizochytrium limacinum_ (rearranged taxonomically to the genus _Aurantiochytrium_) has the ability to accumulate lipids up to 50% of its total dry weight, in a short period of time, with a DHA yield of 34% of the total lipid content [40]. These saline microorganisms are cultivated heterotrophically via aerobic fermentation in steered bioreactors in a culture media containing (a) a source of inorganic or organic carbon (glycose or pure glycerol), (b) a source of inorganic or organic nitrogen and (c) micronutrients that are necessary for their growth. In order to produce one ton of oil from oleaginous microorganisms, five tons of reducing sugars would be needed, considering that an ideal conversion of 100 g of glucose would yield 20 g of oil [41]. Due to the high cost of the growth medium [42] for the industrial production of DHA using _Schizochytrium limacinum_, several studies are focusing on the exploitation of alternative cheap carbon sources. These include liquid residues from potato chip processing [43], breadcrumbs [44], shochu distillery wastewater [45], coconut water as a medium additive [46] and sweet sorghum juice [47]. Although growth and lipid production were achieved utilizing these alternative carbon sources, biomass and DHA production were still low compared to industrial production standards. However, in another study, conventional carbon sources were substituted with maize starch hydrolysate, biomass and DHA, which yield reached 85.27 g L\({}^{-1}\) and 20.7 g L\({}^{-1}\), respectively [43]. With regard to crude glycerol, it has been successfully used in several culture media in order to substitute conventional and expensive carbon sources (glucose and pure glycerol). [PERSON] et al. [9] reported a biomass production of 22.1 g L\({}^{-1}\) and DHA yield of 4.9 g L\({}^{-1}\). These values improved significantly when [PERSON] et al. [48] utilized a fermentation strategy related to varying oxygen supply in relation to the developmental stages of the microorganism during the fermentation process, yielding 37.9 g L\({}^{-1}\) dried biomass and 6.59 g L\({}^{-1}\) DHA. These findings suggest that conventional carbon sources can be successfully substituted with several waste streams from the food processing and biofuel industry in order to cut down production cost and increase circularity. In addition, the use of saline waste water has been investigated to reduce the use of artificial sea water [49], while different possible nitrogen sources have been tested to replace the conventional use of the much more expensive organic nitrogen sources derived from combined amounts of yeast extract and peptone [42; 50]. Nevertheless, the combination of both waste streams from biodiesel and biogas production for the production of _Schizochytrium limacinum_, which is a high DHA value product, has not been extensively studied. It has to be noted that the DHA is a critical nutrient in aqua feeds, which is currently only commercially available in high volumes in fish oil. However, aquaculture is now seeking suitable and sustainable lipid sources for aqua feeds [51] alternative to wild fish stocks [52]. In this context, microalgae have the highest potential for fish oil replacement, since they may contain higher levels of \(n\)-3 highly unsaturated fatty acids than fish oil [53; 54; 55]. Thus, the aim of this work was to assess the effects of the simultaneous use of waste streams derived from the biofuel industry, as potential low-cost ingredients of the culture media, on the growth, biomass and DHA production of _Schizochytrium limacinum_ SR21 cultivated heterotrophically.