Pollutant removal and resource recovery of co-cultivated microalgae Chlorella sp. and Phaeodactylum tricornutum for marine aquaculture wastewater

The microalgae treatment of mariculture wastewater can effectively reduce pollution of the coastal marine environment while utilizing the nitrogen and phosphorus nutrients of wastewater to accomplish resource recovery under carbon-neutral demand. Microalgae co-culture systems have attracted extensive interest due to their improved system stability and environmental adaptability compared to monocultures. This study aims to construct a microalgae co-culture system for actual mariculture wastewater treatment, with a thorough evaluation of species screening, condition optimization, synergistic mechanism analysis, and outdoor validation. The Chlorella sp. and Phaeodactylum tricornutum were screened to the co-culture system using simulated marine aquaculture wastewater. The optimal conditions were an inoculation ratio of 1:8 (Chlorella sp.: P. tricornutum, in cell density), a nitrogen-phosphorus (N/P) ratio of 12, and an aeration rate of 0.2 L/min. The removal rates of NH4+-N, TN, and TP were 100 %, 93.26 %, and 96.12 %, respectively; The contents of protein, polysaccharides, and lipid were 37.11 %, 33.99 %, and 10.21 %, respectively. Nitrogen and phosphorus were primarily removed by biological immobilization and assimilation. In the co-culture system with a 1:8 inoculum ratio, P. tricornutum showed inhibition to Chlorella sp. and the release of polysaccharides was stimulated in EPS. The soluble algae products of P. tricornutum could promote photosynthetic activities of Chlorella sp., and enhance the growth of Chlorella sp. as a carbon source under dark heterotrophic environments. The co-culture process increased the GS activity and decreased the GDH activity, while NiR and NR activities peaked earlier, improving microalgae nitrogen removal. The microalgal co-culture system was proven to treat actual wastewater under outdoor conditions, resulting in met water quality standards and a considerable increase in lipid content above the laboratory. The recycled biomass for the integrated production of bait and other by-products is a promising strategy. This study preliminary reveals the feasibility and resource potential of co-cultured microalgae for mariculture wastewater treatment.