Plasma-Activated Water as Nitrogen Source for Algal Growth: A Microcosm Study

Nitrogen is one of the most critical nutrients affecting algal cell biomass growth and thereby plays a crucial role in algal growth economics. Conventionally, all man-made nitrogen (N) sources are based on hydrogen derived from fossil fuels. This investigation examines the effect of a new N source, i.e., plasma-activated water (PAW), on yields of algal biomass. The N uptake rates of the algae for PAW were determined and it was compared with the conventional N sources. Four treatments of N source in duplicates were run to evaluate the potential of algae cultivation in batch microcosms (5 L). Mixotrophic algal cultures from a nearby eutrophied lake were incubated for 1 week with nitrate-N (NaNO₃), nitrite-N (NaNO₂), nitrate+nitrite (NaNO₃+ NaNO₂), and PAW as sole N source in basal Bold's media. Even though the cultures were not axenic, steady increase in chlorophyll and biomass was observed for all the cases studied. The highest biomass yield was observed in nitrate-N fed cultures, at an average of 32.5 mg/(L·day), followed by a combination of nitrate+nitrite [27.1 mg/(L·day)], PAW-N [26.6 mg/(L·day)], and nitrite-N [22.5 mg/(L·day)] fed cultures. Nitrogen uptake rates were similar in all cultures studied. The highest rates observed were in nitrite-N [4.05 mg/(L·day)], closely followed by PAW [3.56 mg/(L·day)]. The results suggest that PAW can be utilized as a N source with pre-exposed cultures. The algal biomass yields of PAW were comparable to conventional sources although cultures with PAW showed signs of better uptake of N.