The diplomonad fish parasite Spironucleus vortens causes major problems in aquaculture of ornamental fish, resulting in severe economic losses in the fish farming industry. The strain of S. vortens studied here was isolated from an angelfish and grown in Keister's modified TY-I-S33 medium. A membrane-inlet mass spectrometer was employed to monitor, in a closed system, O(2), CO(2), and H(2) When introduced into air-saturated buffer, S. vortens rapidly consumed O(2) at the average rate of 62+/-4 nmol/min/10(7) cells and CO(2) was produced at 75+/-11 nmol/min/10(7) cells. Hydrogen production began under microaerophilic conditions ([O(2)]=33.+/-15 microM) at a rate of 77+/-7 nmol/min/10(7) cells. Hydrogen production was inhibited by 62% immediately after adding 150 microM KCN to the reaction vessel, and by 50% at 0.24 microM CO, suggesting that an Fe-only hydrogenase is responsible for H(2) production. Metronidazole (1 mM) inhibited H(2) production by 50%, while CO(2) production was not affected. This suggests that metronidazole may be reduced by an enzyme of the H(2) pathway, thus competing for electrons with H(+).