Nitrogen fixation

Nitrogen fixation is a chemical process by which molecular nitrogen (N2), which has a strong triple covalent bond, is converted into ammonia (NH3) or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. The nitrogen in air is molecular dinitrogen, a relatively nonreactive molecule that is metabolically useless to all but a few microorganisms. Biological nitrogen fixation or diazotrophy is an important microbe-mediated process that converts dinitrogen (N2) gas to ammonia (NH3) using the nitrogenase protein complex (Nif). Nitrogen fixation is essential to life because fixed inorganic nitrogen compounds are required for the biosynthesis of all nitrogen-containing organic compounds, such as amino acids and proteins, nucleoside triphosphates and nucleic acids. As part of the nitrogen cycle, it is essential for agriculture and the manufacture of fertilizer. It is also, indirectly, relevant to the manufacture of all nitrogen chemical compounds, which incl

Co-culture of microalgae and bacteria for wastewater treatment

Thierry Mounir

To address the problem of insucient wastewater treatment and eutrophication of lakes and rivers, the symbiotic association of microalgae and bacteria has shown great potential and numerous advantages. First, co-cultures can exhibit ecient treatment of domestic wastewater, with the combination of high nutrients (nitrogen and phosphorus) removal abilities of the microalgae and ecient degradation of organic matter by bacteria. Second, the symbiosis between microalgae and bacteria exchanging CO2 and O2 makes aeration not necessary, thus signicantly reducing the operating costs of the wastewater treatment plant. Third, recovered biomass of algae could be used as highpro t products like biofuels and food source. Fourth, the xation of the atmospheric CO2 by the microalgae through photosynthesis would contribute to greenhouse gas reduction in the atmosphere. Finally, the decayed microalgal biomass could serve as organic carbon source for denitrication by bacteria. However, this technology is not well-known yet, and further research on the in uence of the operating parameters have to be conducted. Furthermore, its main drawback is the harvesting of the microalgae. Due to their small size, microalgae demonstrate a low settling rate of 3.6 x 10􀀀3 m/h. A solution is to enhance aggregation of the biomass through occulation, followed by granulation to reach fully-developed microalgal-bacterial consortia. These granules exhibit high treatment performances and high settleability. The rst objective of this project was to identify the optimal microalgae:bacteria ratio for COD and nutrients removal. The second objective was to form fully-developed granules, starting from a co-culture, and assess the in uence of dierent agitation speeds on the granulation. The photobioreactor with a microalgae:bacteria ratio of 75:25 expressed the most promising results in COD removal eciency (95% after 8 days), nitrogen removal eciency (94% after 9 days), phosphorus removal eciency (93% after 7 days), and biomass productivity (144 mg/L/d). In this photobioreactor, nitrogen was removed by a combination of assimilation into microalgal biomass and nitrication/denitrication by the bacteria. The COD was removed by degradation by bacteria and mixotrophic microalgae that used the organic matter as carbon source. Finally, phosphorus was removed by assimilation into microalgal biomass. Although obvious aggregation between microalgae and bacteria could be observed, no fully-developed granules were formed after 89 batches of 24 hours. Because of low COD and nitrogen removals, no starvation period was experienced by the co-cultures. Therefore, extracellular polymeric substances were not released in sucient amounts to enhance the granulation. Furthermore, excessive stirring (200 rpm) may have lead to microalgal cells damage, thus promoting bacterial growth over microalgal growth. On the other hand, slower agitation speeds (80 and 120 rpm) seemed to enhance microalgal growth. Overall, the agitation speed appeared to have an indirect impact on the treatment performances and the settleability of the co-cultures, by signicantly in uencing the microalgal and bacterial growths.

2020

Soil aggregation and associated nutrient content along a distance-from-tree gradient in a low input cropping system. Comparing the effect for two tree species: Mangifera indica (mango, Anacardiaceae) vs. Faidherbia albida (faidherbia, Fabaceae)

Pauline Chrobot

Trees can play a major role in improving soil quality and thus have the potential to enhance sustainability of agroecosystems. Although this has been well established, knowledge on the effect of individual trees on aggregate stability and associated-nutrient content remains scarce. This study aims to quantify the spatial extent and magnitude of influence of single-standing Mangifera indica (mango, Anacardiaceae) and Faidherbia albida (faidherbia, Fabaceae) trees on soil aggregate stability and aggregate-associated nutrient content (total carbon, nitrogen and phosphorus) in a lowinput maize cropping system. Mango and faidherbia differ fundamentally in their properties (e.g., phenology and N-fixing capacity) providing an interesting study system. We hypothesize that 1) the extent of tree influence on soil will be proportional to the tree crown, 2) soil aggregation and aggregate-associated nutrient content will decrease with distance from tree for both species due to greater organic matter inputs from leaves and roots closer to the tree, 3) the magnitude of tree influence will be greater for faidherbia relative to mango due to its N-fixing property. To test these hypotheses, soil samples, collected along four transects, laid out around each single tree and taken at four different distances (1, 4, 10 and 15 m) from the tree trunk, were fractionated and total carbon, nitrogen and phosphorus content within the different fractions was determined. Our results show that both species had an impact on soil fertility, but only the spatial influence of mango was proportional to its crown. We could identify no change in the measured indicators with distance around faidherbia suggesting the possibility that we were likely still in its impact zone. These results support the hypothesis that faidherbia has an extensive positive impact on the surrounding soil, and that mango crown also improves subcanopy soil fertility via a better aggregation and an increase in associated-nutrient content.

2017

Co-culture of microalgae and bacteria for wastewater treatment

Thierry Mounir

2020

Pauline Chrobot

2017

Gene stacking applied for plant nitrogen use efficiency

Co-culture of microalgae and bacteria for wastewater treatment

Soil aggregation and associated nutrient content along a distance-from-tree gradient in a low input cropping system. Comparing the effect for two tree species: Mangifera indica (mango, Anacardiaceae) vs. Faidherbia albida (faidherbia, Fabaceae)

Co-culture of microalgae and bacteria for wastewater treatment

Soil aggregation and associated nutrient content along a distance-from-tree gradient in a low input cropping system. Comparing the effect for two tree species: Mangifera indica (mango, Anacardiaceae) vs. Faidherbia albida (faidherbia, Fabaceae)

Gene stacking applied for plant nitrogen use efficiency