Bioremediation Potential of the Green Algae Ulva sp. Cultivated in Wastewater from Marine Recirculating Aquaculture Systems (RAS)

Ulva is a widespread green algal genus with promising potential regarding uptake of nutrients from Recirculating Aquaculture Systems (RAS) wastewater because of its high tolerance to various temperatures, water qualities, nutrient levels and salinities. The uptake of ammonium (NH4 +), nitrite (NO2 -), nitrate (NO3 -) and phosphate (PO4 3-) from wastewater, originating from RAS farming of Atlantic Salmon (Salmo salar) smolt and post smolt, were studied in Ulva sp. collected from the coast of Central Norway in April and September 2021. Two uptake experiments were conducted, both determining the initial short-term nitrogen and phosphorus uptake kinetics, by following the depletion of substrate concentrations over an incubation period of 8 and 10 hours. The uptake was either measured for nutrient concentrations in a gradient from 25 % to 100 % RAS-water, or in a biomass density gradient ranging from 0.25 g wet weight (WW) to 4 g WW per 250 mL with same RAS-water concentrations. Ammonium uptake rates, related to dry weight biomass (DW), revealed a linear relationship with RAS-water concentration. The maximum measured ammonium uptake rate was 387 ± 18 μg gDW -1 hour-1 for the 100 % RAS-water treatment (~ 160 μM NH4 +). Preferred nitrogen source in Ulva sp. was discovered to strongly be affected by the ratio of available nitrate and ammonium in the wastewater from RAS. At a nitrate:ammonium ratio of 12:1, ammonium was found the be the favored nitrogen source regarding uptake, and consequently inhibited the uptake of nitrate. However, at a higher nitrate:ammonium ratio (152:1), an uptake in nitrate in addition to ammonium uptake was discovered within the first 80 minutes of the experimental period, with a maximum measured nitrate uptake of 54 748 ± 7 366 μg gDW -1 h-1. The lowest density tested for in this study (0.25 gWW per 250 mL) were found to have the highest uptake rate of ammonium (82 ± 3 μg gDW -1 h-1) and nitrate (54 748 ± 7 366 μg gDW -1 h-1) related to biomass, and the uptake rate decreased negative exponentially with an increase in density. This can be explained by more nutrients being available per gram of Ulva sp. in the lower densities. However, the highest density (4 gWW per 250 mL) depleted the nutrients from the RAS medium more rapid compared to the lower ones. Internal tissue concentrations of carbon, nitrogen and phosphorus were revealed to not be significantly different after exposure to high nutrient medium compared to the initial concentration, indicating that tissue concentrations were a bad indicator for nutrient uptake in this current study. However, initial carbon:nitrogen ratios in the tissue varied among Ulva sp. harvested in April (C:N ratio of ~ 7) and September (C:N ratio of ~ 13), possibly explained by the experiments being conducted pre and post phytoplankton spring-bloom. From the results of the current study, the opportunistic macroalgae Ulva sp. was recognized as a potential organism to clean wastewater and to bioremediate nutrients from low saline (~ 15 ppt) RAS water as it holds the potential to remove ammonium and nitrate. However, the water released from the biofilter should have a high nitrate:ammonium ratio to optimize the removal rate of nitrate, which is the most abundant nitrogen source in such medium after water treatment in the biofilter.