Over the last decades, several studies have reported emissions of nitrous oxide (N O) from microalgal cultures and aquatic ecosystems characterized by a high level of algal activity (e.g. eutrophic lakes). As N O is a potent greenhouse gas and an ozone-depleting pollutant, these findings suggest that large-scale cultivation of microalgae (and possibly, natural eutrophic ecosystems) could have a significant environmental impact. Using the model unicellular microalga Chlamydomonas reinhardtii, this study was conducted to investigate the molecular basis of microalgal N O synthesis. We report that C. reinhardtii supplied with nitrite (NO ) under aerobic conditions can reduce NO into nitric oxide (NO) using either a mitochondrial cytochrome c oxidase (COX) or a dual enzymatic system of nitrate reductase (NR) and amidoxime-reducing component, and that NO is subsequently reduced into N O by the enzyme NO reductase (NOR). Based on experimental evidence and published literature, we hypothesize that when nitrate (NO ) is the main Nitrogen source and the intracellular concentration of NO is low (i.e. under physiological conditions), microalgal N O synthesis involves the reduction of NO to NO by NR followed by the reduction of NO to NO by the dual system involving NR. This microalgal N O pathway has broad implications for environmental science and algal biology because the pathway of NO assimilation is conserved among microalgae, and because its regulation may involve NO.
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.
Phosphorus (P) assimilation and polyphosphate (polyP) synthesis were investigated in Chlamydomonas reinhardtii by supplying phosphate (PO43−; 10 mg P·L−1) to P‐depleted cultures of wildtypes, mutants with defects in genes involved in the vacuolar transporter chaperone (VTC) complex, and VTC‐complemented strains. Wildtype C. reinhardtii assimilated PO43− and stored polyP within minutes of adding PO43− to cultures that were P‐deprived, demonstrating that these cells were metabolically primed to assimilate and store PO43−. In contrast, vtc1 and vtc4 mutant lines assayed under the same conditions never accumulated polyP, and PO43− assimilation was considerably decreased in comparison with the wildtypes. In addition, to confirm the bioinformatics inferences and previous experimental work that the VTC complex of C. reinhardtii has a polyP polymerase function, these results evidence the influence of polyP synthesis on PO43− assimilation in C. reinhardtii. RNA‐sequencing was carried out on C. reinhardtii cells that were either P‐depleted (control) or supplied with PO43− following P depletion (treatment) in order to identify changes in the levels of mRNAs correlated with the P status of the cells. This analysis showed that the levels of VTC1 and VTC4 transcripts were strongly reduced at 5 and 24 h after the addition of PO43− to the cells, although polyP granules were continuously synthesized during this 24 h period. These results suggest that the VTC complex remains active for at least 24 h after supplying the cells with PO43−. Further bioassays and sequence analyses suggest that inositol phosphates may control polyP synthesis via binding to the VTC SPX domain.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.