Optimization of stocking density for Isochrysis galbana, Nannochlorum sp., and Tetraselmis tetrahele in the bioremediation of aquaculture wastewater

Microalgal density is one of the factors affecting bioremediation efficiency, but literature on the density that efficiently removes nitrogen and phosphorus from aquaculture wastewater is still very limited. Thus, this study was conducted to optimize the density of Isochrysis galbana, Nannochlorum sp., and Tetraselmis tetrahele to efficiently remove inorganic N (NH3-N, NO2-N) and P. Three densities such as low, medium, and high with an optical density at 680 nm of 0.1, 0.2, and 0.3, respectively, were evaluated for 12 days. Results showed that medium density was good for Isochrysis galbana and Nannochlorum sp. in the removal of both N and P with rates reaching 97.93% and 84.07%, 94.25% and 93.39%, and 59.39% and 71.65% for NH3-N, NO2-N, and P, respectively. For Tetraselmis tetrahele, the low density efficiently removed N, while the high density effectively removed P with the highest removal of 99.16%, 97.5%, and 51.55% for NH3-N, NO2-N, and P, respectively. Specific growth rates of Isochrysis galbana and Tetraselmis tetrahele were affected by density; the lower the densities, the higher their growth rates. The three densities did not differ significantly in Nannochlorum sp. Isochrysis galbana and Nannochlorum sp. biomass productivity were high in medium and high densities, while low and medium densities for Tetraselmis tetrahele. Overall, the optimization of microalgal density is beneficial in the efficient removal of nitrogen and phosphorus from aquaculture wastewater.