Nitrous oxide (N<sub>2</sub>O) production in axenic <i>Chlorella vulgaris</i> cultures: evidence, putative pathways, and potential environmental impacts

Abstract

Using antibiotic assays and genomic analysis, this study demonstrates nitrous oxide (N₂O) is generated from axenic <i>C. vulgaris</i> cultures. In batch assays, this production is magnified under conditions favoring intracellular nitrite accumulation, but repressed when nitrate reductase (NR) activity is inhibited. These observations suggest N₂O formation in <i>C. vulgaris</i> might proceed via NR-mediated nitrite reduction into nitric oxide (NO) acting as N_2O 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. These findings have profound implications for the life cycle analysis of algae biotechnologies and our understanding of the global biogeochemical nitrogen cycle

Response of the Ubiquitous Pelagic Diatom Thalassiosira weissflogii to Darkness and Anoxia

“…In higher plants, N 2 O is emitted from leaves by plant NO 3 - assimilation, strictly speaking during photoassimilation of NO 2 - in the chloroplast [ 49 ]. Recently, N 2 O production was also found in axenic, illuminated cultures of the green algae Chlorella vulgaris [ 50 ]. A release of N 2 O by phototrophic eukaryotes under darkness and anoxia has to our knowledge not been documented so far.…”

An overview of the potential environmental impacts of large-scale microalgae cultivation

“…In agreement with [93], it is very interesting that pioneering work confirming the potential release of N 2 O from axenic cultures of green algae by [94] and [95], has been forgotten for decades. Indeed, the evidence reported by [93] using Chlorella vulgaris to study the mechanisms controlling microalgae-mediated N 2 O production strongly suggests that nitrite intracellular accumulation and its reduction by Nitrate reductase trigger N 2 O emissions, which correlates with nitrite and nitrate concentrations and photosynthesis repression. These results also indicate the significant contribution that largescale microalgae cultivation can make to GHG emissions (e.g.…”

Biological Approaches Integrating Algae and Bacteria for the Degradation of Wastewater Contaminants—A Review

“…As the majority of N is being assimilated instead of being converted into oxides of nitrogen, the microalgae mediated treatment process releases lesser greenhouse gas than the conventional treatment. There is a negligible emission of Nitrogen Oxides using microalgal association in the wastewater treatment process (Fagerstone et al, 2011;Guieysse et al, 2013). Based on the analysis of Alcántara et al (2015), a microalgae wastewater treatment process is estimated to have an emission factor of 0.0047% g N 2 O-N g −1 N-input.…”