The isotopic signatures of nitrous oxide produced by eukaryotic and prokaryotic phototrophs

Abstract. Prokaryotic and eukaryotic microscopic phototrophs ('microalgae') can synthesize the potent greenhouse gas and ozone depleting pollutant nitrous oxide (N₂O). However, we do not know how much microalgae contribute to aquatic N₂O emissions because these organisms co-occur with prolific N₂O producers like denitrifying and nitrifying bacteria. Here we demonstrate for the first time that microalgae produce distinct N₂O isotopic signatures that will enable us to fill this knowledge gap. The eukaryotes Chlamydomonas reinhardtii and Chlorella vulgaris, and the prokaryote Microcystis aeruginosa synthesized N₂O 265–755 nmol·g-DW^-1·h^-1 when in darkness and supplied with 10 mM nitrite (NO₂^-). The N₂O isotopic composition (δ¹⁵N, δ¹⁸O, and site preference, SP) of each species was determined using a modified off-axis integrated-cavity-output spectroscopy analyser with an offline sample purification and homogenisation system. The SP values differed between eukaryotic and prokaryotic algae (25.8 ± 0.3 ‰ and 24.1 ± 0.2 ‰ for C. reinhardtii and C. vulgaris, respectively vs 2.1 ± 3.0 ‰ for M. aeruginosa), as did bulk isotope values. Both values differ from SP produced by denitrifiers. This first characterization of the N₂O isotopic fingerprints of microscopic phototrophs suggests that SP-N₂O could be used to untangle algal, bacterial, and fungal N₂O production pathways. As the presence of microalgae could influence N₂O dynamics in aquatic ecosystems, field monitoring is also needed to establish the occurrence and significance of microalgal N₂O synthesis under relevant conditions.