Measurement of direct nitrous oxide emissions from microalgae cultivation under oxic and anoxic conditions
Date
2011
Authors
Fagerstone, Kelly Dawn, author
Marchese, Anthony John, 1967-, advisor
Bradley, Thomas H., advisor
De Long, Susan K., committee member
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Abstract
Lifecycle assessments (LCA) of microalgae-based biofuels have demonstrated net greenhouse gas (GHG) emissions reductions, but limited data exist on direct emissions of GHG's from microalgae cultivation systems such as open raceway ponds (ORP) or photobioreactors (PBR). For example, nitrous oxide (N2O) is a potent GHG that has been detected from microalgae cultivation. However, N2O emissions have not been experimentally quantified to determine their impact on overall lifecycle assessment of the microalgae-to-biofuels process. Theoretical calculations using the Intergovernmental Panel on Climate Change standards for terrestrial crops (1% of available nitrogen applied as fertilizer is converted to N2O) suggest the potential for significant levels of N2O from microalgae cultivation. In this study, microalgae species Nannochloropsis salina was cultivated with nitrate under conditions representative of PBR and ORP growth conditions with diurnal light-dark cycling. To examine the effect of dissolved oxygen on N2O emissions, experiments were conducted with an air headspace and nitrogen headspace, respectively. During these experiments N2O emissions were quantified utilizing Fourier Transform Infrared spectrometry. Under a nitrogen headspace, N2O emissions were elevated during dark periods and minimal during light periods. Under an air headspace, N2O emissions were negligible for both the light and dark periods. The experimental results show that N2O production was induced by anoxic conditions with nitrate present in the growth media, suggesting that N2O was produced by denitrifying bacteria within the microalgal growth media. The presence of denitrifying bacteria was verified through PCR-based detection of norB genes, which encode bacterial enzymes that produce N2O. Furthermore, antibiotic treatments inhibited N2O emissions. Application of these results to LCA and potential strategies for management of growth systems to reduce N2O emissions are discussed.
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Subject
nitrous oxide
biofuel
greenhouse gas
lifecycle analysis
microalgae