Nitrous oxide emission from Ulva lactuca incubated in batch cultures is stimulated by nitrite, nitrate and light

Biomass yields from some species of macroalgae exceed the yields in traditional terrestrial production systems. This renewable carbon source possesses a potential for energy purposes and thus reduction in fossil fuel carbon dioxide (CO2) emissions. Previous experiments have indicated that nitrous oxide (N2O) may be produced by green algae. We investigated the N2O emissions in the green alga Ulva lactuca. Significant N2O emissions, along with CO2 uptake, were demonstrated from vital U. lactuca material from different natural populations incubated in the laboratory with nitrite (NO2−) and nitrate (NO3−) and at a light intensity of 225μmolphotonsm−2s−1. No emission of N2O was observed in darkness. The N2O emission increased in a Michaelis–Menten characteristic manner with increasing concentrations of both NO3− and NO2−. The light dependency indicated that the N2O emission was related to algal photosynthesis, and not bacterial activity. As algal NO3− reductase (NR) converts NO3− to NO2− in light, and N2O emission was observed from both NO3− and NO2−, it is proposed that NO2− reductase (NiR) activity may have generated the observed N2O, however the mechanism needs further investigation. This apparent N2O production by algae emphasizes the need for experiments under natural conditions in order to evaluate potential greenhouse gas balances associated with large-scale productions for energy purposes.