Two-phase microalgae cultivation for RAS water remediation and high-value biomass production

Abstract

The overall goal of this study was to provide solutions to innovative microalgae-based technology for wastewater remediation in a cold-water recirculating marine aquaculture system (RAS). This is based on the novel concept of integrated aquaculture systems in which fish nutrient-rich rearing water will be used for microalgae cultivation. The produced biomass can be used as fish feed, while the cleaned water can be reused, to create a highly eco-sustainable circular economy. Here, we tested three microalgae species Nannochloropis granulata (Ng), Phaeodactylum tricornutum (Pt), and Chlorella sp (Csp) for their ability to remove nitrogen and phosphate from the RAS wastewater and simultaneously produce high-value biomass, i.e., containing amino acids (AA), carotenoids, and polyunsaturated fatty acids (PUFAs). A high yield and value of biomass were achieved for all species in a two-phase cultivation strategy: i) a first phase using a medium optimized for best growth (f/2 14x, control); ii) a second "stress" phase using the RAS wastewater to enhance the production of high-value metabolites. Ng and Pt performed best in terms of biomass yield (i.e., 5-6 g of dry weight, DW.L^-1) and efficient cleaning of the RAS wastewater from nitrite, nitrate, and phosphate (i.e., 100% removal). Csp produced about 3 g L^-1 of DW and reduced efficiently only nitrate, and phosphate (i.e., about 76% and 100% removal, respectively). The biomass of all strains was rich in protein (30-40 % of DW) containing all the essential AA except Methionine. The biomass of all three species was also rich in PUFAs. Finally, all tested species are excellent sources of antioxidant carotenoids, including fucoxanthin (Pt), lutein (Ng and Csp) and β-carotene (Csp). All tested species in our novel two-phase cultivation strategy thus showed great potential to treat marine RAS wastewater and provide sustainable alternatives to animal and plant proteins with extra added values.

Keywords: Carotenoids; Chlorella; Nannochloropsis; PUFA; Phaedactylum tricornum; RAS wastewater; proteins; two-phase cultivation.

Figure 1

Overview of two-phase microalgae cultivation for cleaning of RAS wastewater and production of high-value biomass. Ng, Nannochloropsis granulata, Pt, Phaeodactylum tricornutum, and Csp, Chlorella sp.

Figure 2

Growth conditions (A–C) and growth curve of (D) Nannochloropsis granulata (Ng, dark green line), (E) Phaeodactylum tricornutum (Pt, brown line), and (F) Chlorella sp (Csp, light green line) in a two-phase system in Multicultivator. (G) Biomass concentration obtained at the end of phase I and phase II cultivation in Ng (dark green bar), Pt (brown bar), and Csp (light green bar). Data shown in (D–G) are the means ± standard deviation of four biological replicates for Ng and Pt, and two biological replicates for Csp. Different letters indicate significant differences among the species (p< 0.05).

Figure 3

Protein content (A) and amino acid profile (B) of Nannochloropsis granulata (Ng, dark green bar), Phaeodactylum tricornutum (Pt, brown bar), and Chlorella sp (Csp, light green bar) grown in two-phase system in Multicultivator. Data shown are the means ± standard deviation of four biological replicates for Ng and Pt, and two biological replicates for Csp. Different letters indicate significant differences among the species (p< 0.05).

Figure 4

Fatty acid content (A), Saturated, monounsaturated and polyunsaturated fatty acid (B) and fatty acid profile (C) of Nannochloropsis granulata (Ng, dark green bar), Phaeodactylum tricornutum (Pt, brown bar), and Chlorella sp (Csp, light green bar) grown in a two-phase cultivation in Multicultivator. Data showed are the means ± standard deviation of biological replicates for Ng and Pt, and two biological replicates for Csp. Different letters indicate significant differences among the species (p< 0.05).

Figure 5

Pigment profile of Nannochloropsis granulata (Ng, dark green bar), Phaeodactylum tricornutum (Pt, brown bar), and Chlorella sp (Csp, light green bar) grown in a two-phase system in Multicultivator. Data shown are the means ± standard deviation of four biological replicates for Ng and Pt, and two biological replicates for Csp. Different letters indicate significant differences among the species (p<0.05).