Quantitative Measurement of Direct Nitrous Oxide Emissions from Microalgae Cultivation

Although numerous lifecycle assessments (LCA) of microalgae-based biofuels have suggested net reductions of greenhouse gas emissions, limited experimental data exist on direct emissions from microalgae cultivation systems. For example, 80 nitrous oxide (N2O) is a potent greenhouse gas that has been detected from microalgae cultivation. However, little quantitative experimental data exist on direct N2O emissions from microalgae cultivation, which has inhibited LCA performed to date. In this study, microalgae species Nannochloropsis sauna was cultivated with diurnal light dark cycling using a nitrate nitrogen source. Gaseous N2O emissions were quantitatively measured using Fourier transform infrared spectrometry. Under a nitrogen headspace (photobioreactor simulation), the reactors exhibited elevated N2O emissions during dark periods, and reduced N2O emissions during light periods. Under air headspace conditions (open pond simulation), N2O emissions were negligible during both light and dark periods. Results show that N2O production was induced by anoxic conditions when nitrate was present, suggesting that N2O was produced by denitrifying bacteria within the culture. The presence of denitrifying bacteria was verified through PCR-based detection of norB genes and antibiotic treatments, the latter of which substantially reduced N2O emissions. Application of these results to LCA and strategies for growth management to reduce N2O emissions are discussed.