Abstract
Abstract. Using antibiotic assays and genomic analysis, this study demonstrates nitrous oxide (N2O) is generated from axenic Chlorella vulgaris cultures. In batch assays, this production is magnified under conditions favouring intracellular nitrite accumulation, but repressed when nitrate reductase (NR) activity is inhibited. These observations suggest N2O formation in C. vulgaris might proceed via NR-mediated nitrite reduction into nitric oxide (NO) acting as N2O precursor via a pathway similar to N2O formation in bacterial denitrifiers, although NO reduction to N2O under oxia remains unproven in plant cells. Alternatively, NR may reduce nitrite to nitroxyl (HNO), the latter being known to dimerize to N2O under oxia. Regardless of the precursor considered, an NR-mediated nitrite reduction pathway provides a unifying explanation for correlations reported between N2O emissions from algae-based ecosystems and NR activity, nitrate concentration, nitrite concentration, and photosynthesis repression. Moreover, these results indicate microalgae-mediated N2O formation might significantly contribute to N2O emissions in algae-based ecosystems (e.g. 1.38–10.1 kg N2O-N ha−1 yr−1 in a 0.25 m deep raceway pond operated under Mediterranean climatic conditions). These findings have profound implications for the life cycle analysis of algae biotechnologies and our understanding of the global biogeochemical nitrogen cycle.
Highlights
Nitrous oxide (N2O) is a major ozone-depleting atmospheric pollutant and greenhouse gas (Ravishankara et al, 2009; EPA, 2010)
To our knowledge, only Fagerstone et al (2011) have hitherto evidenced a bacterial origin to N2O emissions in algal cultures by showing N2O production stopped when Nannochloropsis salina was supplied with bacterial antibiotics and by detecting bacterial genes encoding for nitric oxide reductase (NOR) in the culture
Production was biologically-mediated and magnified when the algae were incubated in darkness and supplied with nitrite as sole exogenous N-source (Fig. 1; C. vulgaris does not assimilate N2)
Summary
Nitrous oxide (N2O) is a major ozone-depleting atmospheric pollutant and greenhouse gas (Ravishankara et al, 2009; EPA, 2010) The production of this compound from microalgal and cyanobacterial cultures ( referred to as “algae” for simplicity) was demonstrated more than 25 yr ago (Weathers, 1984; Weathers and Niedzielski, 1986) and has been suspected to cause measurable N2O emissions in various aquatic environments (Twining et al, 2007; Mengis et al, 1997; Wang et al, 2006; Oudot et al, 1990; FlorezLeiva et al, 2010). Algae have unequal abilities to synthesize N2O (Weathers, 1984) so findings on Published by Copernicus Publications on behalf of the European Geosciences Union
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