Sequential growth of different microalgae species in incrementally increasing saline water for efficient utilization of nutrients and recycling of the growth media

To eliminate freshwater dependency in brackish- and marine-based algal biomass production, this study evaluates a sequential ‘crop rotation’ of microalgae species designed to handle the evaporative salinity buildup inherent to open cultivation systems. Four species, Limnospira sp., Coelastrella sp., Chroococcidiopsis sp., and Chlorocystis sp., were selected based on their distinct salinity tolerances and proven robustness at an outdoor pilot-scale open cultivation. Following indoor salinity-gradient growth experiments for these four species and their subsequent sequential growth experiment, the process was implemented in 1000-L outdoor open-raceway ponds, with daily evaporative losses compensated entirely with brackish or saline water. The results showed that the multi-strain sequence cultivation maintains continuous production and near-complete nitrogen recovery while culture salinity increases eighteenfold from 4 to 72 ppt. The biomass obtained from outdoor sequential cultivation contained protein and lipid that could potentially be utilized in a variety of applications, including sustainable aquafeed and biofuel production. An energy model shows that by decreasing freshwater demand and recycling nitrogen-rich medium, this technique can save up to 4.74 GJ of energy per tonne of biomass, providing a scalable, low-eutrophication path for saline agriculture.