Teuma, Laura2022-09-142022-09-142022http://hdl.handle.net/10179/17541Figure 1 is re-used with the publisher's permission.For decades, high emissions of the greenhouse gas and ozone-depleting pollutant nitrous oxide (N₂O) have been repeatedly reported from eutrophic environments. Acknowledging this fact, the Intergovernmental Panel on Climate Change (IPCC) recently increased the emission factor (EF) used to compute indirect N₂O emissions from wastewater discharge into eutrophic and nutrient-impacted aquatic environments from 0.005 to 0.019 kg N₂O-N emitted per kg of N received. However, because the IPCC still considers that bacterial nitrification and denitrification are the only significant N₂O biological mechanisms, it computes N₂O emissions by assuming a linear relationship between the amount of N₂O emitted from an aquatic environment and the amount of nitrogen (N) reaching this environment. This bacteria-centric assumption may be challenged by the ability of microalgae to synthesize significant amount of N₂O. Indeed, as microalgae blooms can be triggered by N and/or phosphorus (P) pollution, the ability of these organisms to produce N₂O may mean that N₂O emissions from eutrophic environments cannot only be correlated to N inputs. Thus, we hypothesize that microalgae significantly contribute to N₂O emissions in eutrophic aquatic environments and that P inputs should also be considered to accurately estimate N₂O emissions from eutrophic aquatic environments. This thesis initially sought to determine if algae-rich eutrophic ecosystems indeed generate significant N₂O emissions and if microalgae indeed contribute to these emissions. To reduce the scope of the research, emphasis was given to the study of eutrophic lakes as a ‘worse-case environment’. Our methodology was based around field monitoring and laboratory assays testing artificial and natural microalgae-based microcosms. Preliminary data gathered from the eutrophic Lake Horowhenua (Levin, New Zealand) show that the lake is a source of N₂O (0.4 – 8.7 g N-N₂O·ha⁻¹ ·yr⁻¹ , n = 29 sampling events). However, no relationship between algal biomass concentration and N₂O production could be evidenced. In parallel, laboratory batch assays were performed to investigate physiological conditions influencing microalgal N₂O synthesis and the potential enzymes involved. Preliminary data showed that wild type Chlamydomonas reinhardtii produced N₂O up to 31.5 ± 10.7 µmole N₂O·g DW⁻¹ over 24 hours in autotrophic conditions and when supplied 10 mM NO₂ˉ . Inhibiting the electron flow coming from PSII in the same conditions inhibited N₂O production, suggesting that the electron transport chain is involved in N₂O synthesis. These results informed us on the N₂O synthesis pathways that should be investigated during future field work and allowed us to identify candidate strains to create a mutant unable to synthesize N₂O for future microcosms study. Because accurate data is paramount to robust policy, this research is the first step to establish the significance of a new N₂O source that is not necessarily linked to N pollution and could trigger a paradigm shift in how N₂O emissions are estimated in greenhouse gas inventories. This may in turn affect how N₂O sources are prioritized for mitigation strategies and, if deemed necessary, will also provide the methodologies and knowledge needed for efficient monitoring of eutrophic aquatic bodies, including marine environments.enThe AuthorThesis