Reevaluation of the global warming impacts of algae-derived biofuels to account for possible contributions of nitrous oxide

The environmental impacts of algae biofuels have been evaluated by life-cycle assessment (LCA); however, these analyses have overlooked nitrous oxide (N₂O), a potent greenhouse gas. A literature analysis was performed to estimate algal N₂O emissions and assess the impacts of growth conditions on flux magnitudes. Nitrogen source and dissolved oxygen concentration were identified as possible key contributors; therefore, their individual and combined impacts were evaluated using bench-scale experiments. It was observed that maximum N₂O emissions (77.5 μg/g algae/day) occur under anoxic conditions with nitrite. Conversely, minimum emissions (6.25 μg/g algae/day) occur under oxic conditions with nitrate. Aggregated N₂O flux estimates were then incorporated into a LCA framework for algae biodiesel. Accounting for "low" N₂O emissions mediated no significant increase (<1%) compared to existing GWP estimates; however, "high" N₂O emissions mediate an increase of roughly 25%, potentially jeopardizing the anticipated economic and environmental performances of algae biofuels.