Isotopic examination of nitrogen utilization by marine fungi

There has been a growing appreciation for the role that fungi may play in facilitating elemental cycling in marine ecosystems. While the role of fungi in degradation of organic matter and carbon cycling has received substantially more attention, the role of nitrogen (N) cycling by marine fungi may also be important in some systems. Specifically, previous work has shown that a subset of fungi appear to possess the ability to respire nitrite producing the greenhouse gas nitrous oxide (N₂O) in a process known as fungal denitrification. Studies of fungal denitrification have identified unique enzymatic pathways (P450 nitric oxide reductase) and production of isotopically distinct N₂O (with a distinctive intramolecular isotopomeric composition). These tools have been useful for targeted studies of fungal denitrification in terrestrial systems. To date, however, investigations of marine fungal nitrogen cycling remain lacking. Here, I present results from experiments examining N and oxygen (O) isotope dynamics of nitrate and nitrite utilization by fungi, including both known denitrifying fungi (Fusarium oxysporum, Trichoderma harzanium) as well as a marine isolate from sediments of the Arabian Sea oxygen deficient zone (Aspergillus terreus, An-4). Experiments included investigation of both aerobic and anaerobic utilization of nitrate and nitrite as well as production of N₂O in some cases. Kinetic N and O isotope effects for nitrate and nitrite utilization were measured across different experimental conditions. In general kinetic N and O isotope effects of nitrogen utilization were quite low relative to those observed for denitrifying bacteria. Ongoing characterization of product N₂O will be highlighted. Overall this work helps to further our understanding of the potential impact of fungi on nitrogen cycling marine systems.