N2O emissions during microalgae outdoor cultivation in 50 L column photobioreactors

Abstract This study investigated the potential environmental significance of nitrous oxide (N 2 O) 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 (NO 3 − ) or ammonium (NH 4 + ) as the nitrogen (N) source. When NO 3 − was supplied as N-source, Neochloris sp., and C . vulgaris cultures emitted 50.0–14,200 nmol·N 2 O·m − 2 ·h − 1 (36 days of cultivation, n  = 136, median = 2220 nmol·N 2 O·m − 2 ·h − 1 ) and 9.60–38,000 nmol·N 2 O·m − 2 ·h − 1 (90 days of cultivation, n  = 332, median = 4050 nmol·N 2 O·m − 2 ·h − 1 ), respectively. Based on the emissions representing 25–75% of the data recorded from C . vulgaris fed NO 3 − (1500–8000 nmol·N 2 O·m − 2 ·h − 1 ), N 2 O 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 N 2 O 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 NO 3 − 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 N 2 O mitigation strategies.