Anaerobic benzene biodegradation linked to nitrate reduction

Abstract

Benzene oxidation to carbon dioxide linked to nitrate reduction was observed in enrichment cultures developed from soil and groundwater microcosms. Benzene biodegradation occurred concurrently with nitrate reduction at a constant ratio of 10 mol of nitrate consumed per mol of benzene degraded. Benzene biodegradation linked to nitrate reduction was associated with cell growth; however, the yield, 8.8 g (dry weight) of cells per mol of benzene, was less than 15% of the predicted yield for benzene biodegradation linked to nitrate reduction. In experiments performed with [14C]benzene, approximately 92 to 95% of the label was recovered in 14CO2, while the remaining 5 to 8% was incorporated into the nonvolatile fraction (presumably biomass), which is consistent with the low measured yield. In benzene-degrading cultures, nitrite accumulated stoichiometrically as nitrate was reduced and then was slowly reduced to nitrogen gas. When nitrate was depleted and only nitrite remained, the rate of benzene degradation decreased to almost zero. Based on electron balances, benzene biodegradation appears to be coupled more tightly to nitrate reduction to nitrite than to further reduction of nitrite to nitrogen gas.

FIG. 1

Cumulative amounts of benzene degraded (squares) and nitrate utilized (circles) in typical transfer cultures derived from microcosms prepared with soil from site A (dashed lines) and from site B (solid lines). The arrows indicate times when benzene (approximately 10 mg/liter) was added to the cultures. d, days.

FIG. 2

Plot of benzene concentration versus time for transfer cultures grown in the presence of nitrate (■) and in the absence of nitrate (□). The data are means ± standard deviations from triplicate cultures (without nitrate) and means ± ranges from duplicate cultures (with nitrate). d, days.

FIG. 3

Net nitrate utilization (◊) and nitrite production (■) during benzene biodegradation (▴) in transfer cultures. The data are means ± ranges from duplicate cultures. d, days.