Nitrous oxide production and methane oxidation by different ammonia-oxidizing bacteria

Abstract

Ammonia-oxidizing bacteria (AOB) are thought to contribute significantly to N2O production and methane oxidation in soils. Most of our knowledge derives from experiments with Nitrosomonas europaea, which appears to be of minor importance in most soils compared to Nitrosospira spp. We have conducted a comparative study of levels of aerobic N2O production in six phylogenetically different Nitrosospira strains newly isolated from soils and in two N. europaea and Nitrosospira multiformis type strains. The fraction of oxidized ammonium released as N2O during aerobic growth was remarkably constant (0.07 to 0.1%) for all the Nitrosospira strains, irrespective of the substrate supply (urea versus ammonium), the pH, or substrate limitation. N. europaea and Nitrosospira multiformis released similar fractions of N2O when they were supplied with ample amounts of substrates, but the fractions rose sharply (to 1 to 5%) when they were restricted by a low pH or substrate limitation. Phosphate buffer (versus HEPES) doubled the N2O release for all types of AOB. No detectable oxidation of atmospheric methane was detected. Calculations based on detection limits as well as data in the literature on CH4 oxidation by AOB bacteria prove that none of the tested strains contribute significantly to the oxidation of atmospheric CH4 in soils.

FIG. 1

Accumulation of N₂O (in micrograms of N per flask) and NO₂⁻ (in milligrams of N per flask) during growth with ammonium as the substrate in the HEPES-buffered medium (initial pH, 7.5).

FIG. 2

Accumulation of N₂O (in micrograms of N per flask) and NO₂⁻ (in milligrams of N per flask) during growth with urea as the substrate in the HEPES-buffered medium (initial pH, 7.5).

FIG. 3

Accumulated N₂O-N as percentages of accumulated NO₂⁻-N in cells grown in the HEPES-buffered medium with ammonium (A) and urea (B). N.eur., N. europaea; N.multif., Nitrosospira multiformis.

FIG. 4

Measured pH in the cultures growing in the phosphate-buffered medium. N.multif., Nitrosospira multiformis; N.eur., N. europaea.

FIG. 5

Accumulation of NO₂⁻ (in milligrams of N per flask) during growth in the phosphate-buffered medium. N.multif., Nitrosospira multiformis; N.eur., N. europaea.

FIG. 6

Accumulation of N₂O (in micrograms of N per flask) during growth in the phosphate-buffered medium. N.eur., N. europaea.

FIG. 7

Yields of N₂O-N as percentages of NO₂⁻-N in the phosphate-buffered medium, plotted against the measured pH. N.multif., Nitrosospira multiformis; N.eur., N. europaea.