Growing Spirulina (Arthrospira platensis) in seawater supplemented with digestate: Trade-offs between increased salinity, nutrient and light availability

Abstract

Utilization of digestate as a source of nutrients must be understood in the context of their effects on light penetration. These conflicting consequences of digestate utilization were verified by growing the cyanobacterium Arthrospira platensis (Spirulina) in artificial seawater supplemented with 2.5 %, 5%, 10 % or 15 % (v/v) of anaerobically digested effluents (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 photosynthetic efficiency of A. platensis and variable turbidities associated with the variable ranges of digestate additions. 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 concentrations lowered biomass accumulation (≈1020−1150 mg/L dry biomass). On the other hand, greater concentrations 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.