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The study aimed at zero-waste utilization of fish processing streams for cultivation of microalgae Galdieria sulphuraria. Wastewater from a fish processing facility, slam (mix of used fish feed and faeces), and dried pellet (sediments after enzymatic hydrolysis of rainbow trout) were investigated as potential sources of carbon, nitrogen, and phosphate for cultivation of G. sulphuraria. The pellet extract was found to support the growth of G. sulphuraria when appropriate diluted, at concentrations below 40 % (v/v). It was revealed that wastewater does not impact the growth negatively, however free amino nitrogen and carbon sources need to be supplied from another source. Therefore, only proteolyzed pellet extract (20 %, v/v) was selected for upscaling and a biomass concentration of 80 g L -1 (growth rate was 0.72 day -1 ) was achieved in a non-sterile fed-batch culture. Even though biomass was produced under non-sterile conditions no pathogens such as Salmonella sp. could be detected., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Janna Cropotova reports article publishing charges was provided by Norwegian University of Science and Technology., (Copyright © 2023 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Large amounts of food are wasted and valuable contents are not utilized completely. Methods to process such wastes into biomass of defined composition automatically and in decentralized locations are lacking. Thus, this study presents a modular design for residue utilization and continuous production of the heterotrophic alga Galdieria sulphuraria. A life cycle and economic assessment are carried out on the hypothetical design to define whether the proposed system can be ecologically and economically viable. Producing one kg of dried biomass would cost 4.38 € and be associated with 3.8 kg CO 2 eq emitted, 69.9 MJ of non-renewable energy use, and 0.09 m 2 of land occupation. Sustainability is comparable to conventional protein sources, with further improvement foreseen through avoidance of drying. These results demonstrate how circular bioeconomy potentials of residues could be realized using heterotrophic G. sulphuraria. It highlights key issues of developing an environmentally and economically sustainable concept., (Copyright © 2022 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Non-sterile heterotrophic cultivation of Galdieria sulphuraria in presence of digestate as well as straw after hydrolysis was investigated. G. sulphuraria can be grown in pure digestate at rates of 0.9 day -1 with glucose. However, a proteolytic treatment of digestate resulted in increased growth rates (1.2 day -1 ) and doubled cell concentrations. Furthermore, G. sulphuraria can utilize glucose obtained after straw hydrolysis. Biomass yields in glucose limited cultures were around 0.9 g per g glucose, while only 0.2 g biomass was formed per g glucose in glucose sufficient cultures. Biomass composition (w/w) of G. sulphuraria grown in digestate supplemented with straw hydrolysate consisted of 20% carbohydrates, 37% proteins and 3% lipids. This study revealed the potential to utilize agricultural waste streams to form algal biomass rich in proteins and may pave the way to novel utilization strategies to be implemented in rural areas., (Copyright © 2021 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Microalgae cultivation for food purposes could have high environmental impacts. The study performed life cycle assessment (LCA) of hypothetical model combining phototrophic and heterotrophic cultivations, exchanging produced gases (carbon dioxide from heterotrophic and oxygen from autotrophic) as a potential strategy to reduce the environmental impact of microalgae cultivation. The LCA indicated that the production of Galdieria sulphuraria in a combined cultivation system has environmental benefits compared with the separate phototrophic cultivation and an almost twice lower carbon footprint than phototrophic cultivation. The benefits are based on the lower volume of culture broth and consequently reduced energy demand as well as less demanding wastewater treatment of the heterotrophic cultivation. Such combination of cultivation activities could be recommended to the producers dealing with phototrophic cultivation as a sustainable strategy for the environmental impact reduction., (Copyright © 2021 Elsevier Ltd. All rights reserved.)