Biochemical Engineering Journal 165 (2021) 107815 Available online 30 September 2020 1369-703X/© 2020 Elsevier B.V. All rights reserved. Regular article Growing Spirulina (Arthrospira platensis) in seawater supplemented with digestate: Trade-offs between increased salinity, nutrient and light availability Giorgos Markou a, *, Alexandros Diamantis a , Dimitris Arapoglou a , Dimitris Mitrogiannis b , Cristina Gonz´ alez-Fern´ andez c , Adrian Unc d a Institute of Technology of Agricultural Products, Hellenic Agricultural Organization-Demeter (HAO-Demeter), Leof. Sofokli Venizelou 1, Lykovrysi, 141 23 Athens, Greece b Agricultural University of Athens (AUA), Department of Natural Resources Management & Agricultural Engineering, Iera Odos 75, 118 55 Athens, Greece c IMDEA Energy Institute, Biotechnological Processes for Energy Production Unit, 28935 M´ ostoles, Spain d Memorial University of Newfoundland, School of Science and the Environment, Corner Brook NL, A2H 5G4 Canada A R T I C L E INFO Keywords: A. platensis Digestate Light penetration Chlorophyll fuorescence Microalgal biotechnology ABSTRACT Utilization of digestate as a source of nutrients must be understood in the context of their effects on light penetration. These conficting consequences of digestate utilization were verifed by growing the cyanobacterium Arthrospira platensis (Spirulina) in artifcial seawater supplemented with 2.5 %, 5%, 10 % or 15 % (v/v) of anaerobically digested effuents (digestate) in a fed-batch mode. Therefore, the study investigates the impact of the complex interactions between nutrient availability, increased salinity, which affects negatively the photo- synthetic effciency of A. platensis and variable turbidities associated with the variable ranges of digestate ad- ditions. At the lowest digestate concentration growth was hindered by low nutrient availability. Biomass production was highest at 5% digestate (≈1300 mg/L dry biomass) but further increases in digestate concen- trations lowered biomass accumulation (≈10201150 mg/L dry biomass). On the other hand, greater concen- trations of digestate and thus more nutrients, led to greater protein content (44–55 % in higher concentrations versus 2230% in the lowest), while A. platensis cells had enhanced photosynthetic performances. The latter is likely due to the fact that addition of digestate reduced light intensity and therefore reduced the negative effect of salinity on the photosynthetic apparatus. The balance between nutrient and light availability, as associated with variable digestate rates, may be managed towards the production of A. platensis biomass with variable qualitative and quantitative biochemical parameters. 1. Introduction Biogas production through anaerobic digestion is a mature technol- ogy that valorizes organic matter and produces renewable energy. Anaerobic digestion expands rapidly worldwide generating opportu- nities for local activities in the frame of circular bio-economy. After the digestion of organic matter, the effuents consist of the residual miner- alizable organic matter rich in valuable macro- and micronutrients. This makes digestate interesting to various nutrient re-utilization pathways [1], with one particular option being the production of microalgal biomass and microalgal bio-products [2–6]. Among microalgal species, spirulina (Arthrospira platensis) has an established history mainly as food supplement (super-food) due to its high content in proteins, unsaturated fatty acids, vitamins, and minerals [7]. A. platensis is a photosynthetic cyanobacterium typically cultivated in growth media that mixes fresh- or soft brackish water with synthetic fertilizers [8]. In practice, growth media contributes signifcantly to the overall biomass production costs. Therefore, the use of low-cost culti- vation media like digestate-supplemented media might signifcantly reduce the production cost of commercial A. platensis cultivation and increasing its competitivity for the production of biomass and value-added products. Previous studies have demonstrated the ability of A. platensis to grow utilizing digestate as nutrient source [9,10]. Development of cultures of A. platensis based on seawater is projected * Corresponding author. E-mail address: gmarkou@itap.com.gr (G. Markou). Contents lists available at ScienceDirect Biochemical Engineering Journal journal homepage: www.elsevier.com/locate/bej https://doi.org/10.1016/j.bej.2020.107815 Received 11 July 2020; Received in revised form 26 September 2020; Accepted 29 September 2020