Abiotic Nitrous Oxide Production in Natural and Artificial Seawater

Abstract:

The ocean contributes approximately one third of global sources of nitrous oxide (N₂O) to the atmosphere. While nitrification is thought to be the dominant pathway for marine N₂O production, mechanisms remain unresolved. Previous studies have carried the implicit assumption that marine N₂O originates directly from enzymatic sources. However, abiotic production of N₂O is possible via chemical reactions between nitrogenous intermediates and redox active trace metals in seawater. In this study, we investigated N₂O production and isotopic composition in treatments with and without added hydroxylamine (NH₂OH) and nitric oxide (NO), intermediates in microbial oxidation of ammonia to nitrite, and Fe(III). Addition of substrates to sterile artificial seawater was compared with filtered and unfiltered seawater from Sapelo Island, coastal Georgia, USA. N₂O production was observed immediately after addition of Fe(III) in the presence of NH₂OH at pH 8 in sterile artificial seawater. Highest N₂O production was observed in the presence of Fe(III), NO, and NH₂OH. The isotopomer site preference of abiotically produced N₂O was consistent with previous studies (31 ± 2 ‰). Higher abiotic N₂O production was observed in sterile artificial seawater (salinity: 35 ppt) than filtered Sapelo Island seawater (salinity: 25 ppt) whereas diluted sterile artificial seawater (18 ppt) showed lowest N₂O production, suggesting that higher salinity promotes enhanced abiotic N₂O production. Addition of Fe(III) to unfiltered Sapelo Island seawater stimulated N₂O production. The presence of ammonia-oxidizing archaea (AOA), which lack known N₂O producing enzymes, in Sapelo Island seawater was confirmed by successful amplification of the archaeal amoA gene, whereas ammonia-oxidizing bacteria (AOB), which contain N₂O-producing enzymes were undetected. Given the few Fe-containing proteins present in AOA, it is likely that Fe(III) addition promoted N₂O production via an abiotic vs. enzymatic N₂O mechanism. Overall, this study suggests that abiotic N₂O production may be occurring in marine environments where microbial ammonia oxidation occurs in the presence of elevated Fe and/or other redox-active metals, such as coastal areas, oxygen minimum zones, and near the sediment-water interface.