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.

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An algal-bacterial consortium formed by Chlorella sorokiniana and a phenanthrene-degrading Pseudomonas migulae strain was able to biodegrade 200-500 mg/l of phenanthrene dissolved in silicone oil or tetradecane under photosynthetic conditions and without any external supply of oxygen. Phenanthrene was only removed when provided in organic solvent, which confirms the potential of two-phase systems for toxicity reduction. Phenanthrene was degraded at highest rates when provided in silicone oil rather than in tetradecane since this solvent probably sequestered the PAH, reducing its mass transfer to the aqueous phase. The influence of phenanthrene concentration, amount of inoculum and light intensity on pollutant removal was also investigated and, under the best conditions, phenanthrene was degraded at 24.2 g m(-3).h(-1). In addition to being cost-effective and mitigating the release of greenhouse gases into the atmosphere, photosynthetic oxygenation was especially beneficial to the use of two-phase partitioning bioreactors since it prevented solvent emulsification and/or volatilization and evidence was found that the microalgae release biosurfactants that could further enhance phenanthrene degradation.

11The degradation of the antibiotic tetracycline, supplied at 100 µg L -1 in domestic 12 wastewater, was studied in an outdoor, pilot scale, high rate algal pond (HRAP). Effective 13 operation was demonstrated with the biomass concentration and the chemical oxygen 14 demand removal efficiency averaging 1.2 ± 0.1 gTSS L -1 and 80 ± 4%, respectively, across 15 all operational periods. Tetracycline removal exceeded 93% and 99% when the HRAP was 16 operated at hydraulic retention times of 4 and 7 days, respectively. Batch tests and pulse 17 testing during HRAP operation repeatedly evidenced the significance of photodegradation 18 as a removal mechanism. Sorption dominated tetracycline removal during the night, but 19 accounted for less than 6% of the total pollutant removal based on sorbed tetracycline 20 extracted from biomass. Overall, these results provide the first demonstration of efficient 21 antibiotic removal, occurring mainly via indirect photodegradation, during relevant HRAP 22 operation (low pollutant concentration, domestic wastewater and natural sunlight). 23 24

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