N2O emissions during microalgae outdoor cultivation in 50 L column photobioreactors

This study investigated the potential environmental significance of nitrous oxide (N2O) emissions from outdoor microalgal cultivation. Chlorella vulgaris, Neochloris sp., and Arthrospira platensis were cultivated in 50 L pilot scale photobioreactors (PBRs) operated in fed batch mode and fed with either nitrate (NO3-) or ammonium (NH4+) as the nitrogen (N) source. When NO3- was supplied as N-source, Neochloris sp., and C. vulgaris cultures emitted 50.0-14,200 nmol u N2O.m(-2).h(-1) (36 days of cultivation, n= 136, median = 2220 nmol.N2O.m(-2).h(-1)) and 9.60-38,000 nmol.N2O.m(-2).h(-1) (90 days of cultivation, n= 332, median = 4050 nmol.N2O.m(-2).h(-1)), respectively. Based on the emissions representing 25-75% of the data recorded from C. vulgaris fed NO3 (1500-8000 nmol.N2O.m(-2).h(-1)), N2O emissions factors were estimated as 0.1-0.4% of the N input load of 25 g.N.d(-1). Further monitoring of C. vulgaris cultures showed that N2O emissions were positively correlated to biomass concentration (R-2 = 0.77) and light intensity (R-2 = 0.57). Nitrous oxide emissions were not detected when C. vulgaris was cultivated using ammonium as N-source (32 days of cultivation, n = 84), or when A. platensis cultures were cultivated with NO3- as N-source (36 days of cultivation, n=90). The use of ammonium as N-source or the selection of appropriate algae species could therefore provide simple N2O mitigation strategies.