Denitrification and methane production in sediment of Hamilton Harbour (Canada)

A spatial study of denitrification potential of sediments in Belfast and Strangford Loughs and its significance

The C2H2 inhibition technique was employed to study seasonal denitrification potential rates in sediment slurries from tidal and subtidal sites in Belfast and Strangford Loughs, Northern Ireland. A comparison of denitrification rates obtained from this method with those obtained from the 15N-gas flux method generally showed good agreement. Depth profiles measured up to 1 m showed that denitrification decreased with depth, with highest values in the 0-5-cm fraction. For the Belfast Lough tidal system a multiple regression model was developed which explained 83% of the variation in denitrification potential. The independent variables were water content, sediment temperature, total oxidizable N in porewater and total organic N. The highest rate of denitrification potential, 2100 micromol N m(-2) h(-1), was found in areas where there was a high anthropogenic input of nutrients. Denitrification in sediments in both loughs can play a potentially significant role in removal of NO3- from the overlying water. In Belfast Lough the overall denitrification potential rate matched the external NO3-N inputs, whilst in Strangford Lough it exceeded it by sixfold, which suggests a potential to remove future additional anthropogenic inputs to the Lough.

Hexadecane mineralization and denitrification in two diesel fuel-contaminated soils

The effect of nitrate, ammonium and urea on the mineralization of [(14)C]hexadecane (C(16)H(34)) and on denitrification was evaluated in two soils contaminated with diesel fuel. In soil A, addition of N fertilizers did not stimulate or inhibit background hexadecane mineralization (4.3 mg C(16)H(34) kg(-1) day(-1)). In soil B, only NaNO(3) stimulated hexadecane mineralization (0.91 mg C(16)H(34) kg(-1) day(-1)) compared to soil not supplemented with any nitrogen nutrient (0.17 mg C(16)H(34) kg(-1) day(-1)). Hexadecane mineralization was not stimulated in this soil by NH(4)NO(3) (0.13 mg C(16)H(34) kg(-1) day(-1)), but the addition of NH(4)Cl or urea suppressed hexadecane mineralization (0.015 mg C(16)H(34) kg(-1) day(-1)). Addition of 2 kPa C(2)H(2) did not inhibit the mineralization process in either soil. Denitrification occurred in both soils studied when supplemented with NaNO(3) and NH(4)NO(3), but was not detected with other N sources. Denitrification started after a longer lag in soil A (10 days) than in soil B (4 days). In soil A microcosms supplemented with NaNO(3) or NH(4)NO(3), rates of denitrification were 20.6 and 13.6 mg NO(3)(-) kg(-1) day(-1), respectively, and in soil B, they were 18.5 and 12.5 mg NO(3)(-) kg(-1) day(-1), respectively. We conclude that denitrification may lead to a substantial loss of nitrate, making it unavailable to the mineralizing bacterial population. Nitrous oxide was an important end-product accounting for 30-100% of total denitrification. These results indicate the need for preliminary treatability studies before implementing full-scale treatment processes incorporating commercial fertilizers.

The response of methane consumption by pure cultures of methanotrophic bacteria to oxygen

The rates of CH4 oxidation by strains of groups I and II methanotrophs in pure culture were studied at various O2 concentrations from 0 to 63 % v/v. In the presence of nonlimiting dissolved CH4 and inorganic nitrogen, O2 concentrations from 0.45 to 20% v/v supported maximum rates of CH4 oxidation. The critical dissolved O2 concentration under our conditions was about 5.7 μM, below which O2 was limiting for CH4 oxidation. Concentrations of O2 up to 63% v/v depressed the activity of CH4 oxidation by ≥ 23%. We conclude that methanotrophs are not microaerophilic under the conditions of our experiments and that they have a high affinity for O2.Key words: CH4 oxidation, O2 response, Methylosinus trichosporium, Methylobacter luteus.

Differential inhibition by allylsulfide of nitrification and methane oxidation in freshwater sediment

Addition of nitrapyrin, allylthiourea, C(inf2)H(inf2), and CH(inf3)F to freshwater sediment slurries inhibited CH(inf4) oxidation and nitrification to similar extents. Dicyandiamide and allylsulfide were less inhibitory for CH(inf4) oxidation than for nitrification. Allylsulfide was the most potent inhibitor of nitrification, and the estimated 50% inhibitory concentrations for this process and CH(inf4) oxidation were 0.2 and 121 (mu)M, respectively. At a concentration of 2 (mu)M allylsulfide, growth and CH(inf4) oxidation activity of Methylosinus trichosporium OB3b were not inhibited. Allylsulfide at 200 (mu)M inhibited the growth of M. trichosporium by approximately 50% but did not inhibit CH(inf4) oxidation activity. Nitrite production by cells of M. trichosporium was not significantly affected by allylsulfide, except at a concentration of 2 mM, when growth and CH(inf4) oxidation were also inhibited by about 50%. Methane monooxygenase activity present in soluble fractions of M. trichosporium was not inhibited significantly by allylsulfide at either 200 (mu)M or 2 mM. These results suggest that the partial inhibition of CH(inf4) oxidation in sediment slurries by high allylsulfide concentrations may be caused by an inhibition of the growth of methanotrophs rather than an inhibition of methane monooxygenase activity specifically. We conclude that allylsulfide is a promising tool for the study of interactions of methanotrophs and nitrifiers in N cycling and CH(inf4) turnover in natural systems.

Effects of Methane Metabolism on Nitrification and Nitrous Oxide Production in Polluted Freshwater Sediment

We report the effect of CH 4 and of CH 4 oxidation on nitrification in freshwater sediment from Hamilton Harbour, Ontario, Canada, a highly polluted ecosystem. Aerobic slurry experiments showed a high potential for aerobic N 2 O production in some sites. It was suppressed by C 2 H 2 , correlated to NO 3 - production, and stimulated by NH 4 + concentration, supporting the hypothesis of a nitrification-dependent source for this N 2 O production. Diluted sediment slurries supplemented with CH 4 (1 to 24 μM) showed earlier and enhanced nitrification and N 2 O production compared with unsupplemented slurries (≤1 μM CH 4 ). This suggests that nitrification by methanotrophs may be significant in freshwater sediment under certain conditions. Suppression of nitrification was observed at CH 4 concentrations of 84 μM and greater, possibly through competition for O 2 between methanotrophs and NH 4 + -oxidizing bacteria and/or competition for mineral N between these two groups of organisms. In Hamilton Harbour sediment, the very high CH 4 concentrations (1.02 to 6.83 mM) which exist would probably suppress nitrification and favor NH 4 + accumulation in the pore water. Indeed, NH 4 + concentrations in Hamilton Harbour sediment are higher than those found in other lakes. We conclude that the impact of CH 4 metabolism on N cycling processes in freshwater ecosystems should be given more attention.