Nitrous oxide production and isotopomer composition by fungi isolated from salt marsh sediments

The emissions of nitrous oxide (N 2 O), a potent greenhouse gas and ozone-depleting agent, have been steadily increasing from coastal environments, such as salt marsh sediments, as a result of anthropogenic nutrient loading. Biotic processes, including nitrification and denitrification, are the largest sources of N 2 O from salt marsh sediments. While it is assumed that the bulk of N 2 O from salt marsh sediment is produced by nitrification and bacterial denitrification, recent reports suggest fungal denitrification may contribute significantly. In this study, four fungi capable of growth under sulfidic conditions were isolated from salt marsh sediments in North Inlet, South Carolina, USA. Fungal species included Purpureocillium lilacinum, Trichoderma harzianum, Trichoderma virens , and Rhodotorula glutinis , as determined by sequencing the18S and 28S rRNA genes. The isotopomer signatures of N 2 O produced by these fungi were measured using isotope ratio mass spectrometry, which can be used to estimate the contribution of different sources of N 2 O. Up to 22.8% of nitrite provided in growth media was converted to N 2 O by fungal strains isolated from salt marsh sediments. The site preference (SP) of N 2 O produced by salt marsh sediment fungi ranged from 7.5 ± 1.6‰ to 33.4 ± 1.2‰. These values are lower than the SP of N 2 O from the model fungal denitrifier Fusarium oxysporum (37.1 ± 2.5‰), which is the SP typically used as an endmember in isotope mass balance considerations. The N 2 O SP values we measured expand the range of N 2 O SP used for isotope mass balances calculations to determine the relative contribution of fungi to N 2 O production in salt marsh sediments.