N2O emissions during microalgae outdoor cultivation in 50 L column photobioreactors

Abstract

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 50L 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 50.0–14,200nmol·N2O·m−2·h−1 (36days of cultivation, n=136, median=2220nmol·N2O·m−2·h−1) and 9.60–38,000nmol·N2O·m−2·h−1 (90days of cultivation, n=332, median=4050nmol·N2O·m−2·h−1), respectively. Based on the emissions representing 25–75% of the data recorded from C. vulgaris fed NO3− (1500–8000nmol·N2O·m−2·h−1), N2O emissions factors were estimated as 0.1–0.4% of the N input load of 25g·N·d−1. Further monitoring of C. vulgaris cultures showed that N2O emissions were positively correlated to biomass concentration (R2=0.77) and light intensity (R2=0.57). Nitrous oxide emissions were not detected when C. vulgaris was cultivated using ammonium as N-source (32days of cultivation, n=84), or when A. platensis cultures were cultivated with NO3− as N-source (36days 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.