2026

Hiltunen, Minna; Kannangara, Ruchira; Nakandalage, Bernadeth; Söderena, Olivia; Timilsina, Hemanta; Pirhonen, Juhani; Pulkkinen, Katja

Recycling waste to new products is a key concept in circular economy with benefits to environmental sustainability and climate change mitigation. Microalgae cultivation integrated to aquaculture is an example of circular economy that has potential to increase environmental sustainability of aquaculture via capture of nutrients and use of CO2 efflux, and production of valuable, renewable biomass and biomolecules. Here we studied the simultaneous utilization of Recirculating Aquaculture System's (RAS) water and CO2 exhaust (ca. 600–1100 ppm) on growth and biochemical composition of Chlorella vulgaris, a widely used microalga. C. vulgaris had high growth rates (μmax up to 0.95) even at the relatively low temperature of 18–19 °C used in the experiments and captured on average 98–100 % of phosphate, 82–99 % of nitrate, and 68–95 % of total nitrogen in the RAS water during 8–16 day experiments. Growth, biomass accumulation, and nutrient capture were improved by using high light intensity (ca. 90 μmol m−2 s−1) and/or RAS exhaust air. The highest lipid content (∼18 % DW) in C. vulgaris was achieved in two-stage cultivation under sulphur limitation, but nitrogen limitation by adding phosphorus (decreasing N:P ratio) into RAS water did not increase lipid content. Fatty acid composition of C. vulgaris was mainly determined by culture medium and light intensity. Overall, we produced stable and fast-growing cultures of C. vulgaris using RAS water and biogenic CO2 in conditions relevant for potential future integration of microalgal production to RAS while examining also biochemical composition of the produced biomass.