TABLE 2.

Stoichiometry of selenate respiration by strains KM, S7, and pn1^a

Strain	Acetate utilized (mM)	Protein increase (mg/liter)b	Carbon conversion to cells (%)c	Electrons produced (mM)d	Selenate reduced (mM)	Selenite produced (mM)	Selenium produced (mM)	Electrons consumed (mM)e	Electron balance (%)f
pn1	3.7 ± 1	23.1 ± 8	26 ± 4	21.9 ± 5.6	8.7 ± 1.4	7.9 ± 1.8	0	17.3 ± 2.9	81 ± 11
S7	3.8 ± 0.2	35 ± 4.4	39 ± 5	18.3 ± 2.2	9.3 ± 0.5	9.6 ± 0.9	0	18.6 ± 1	102 ± 8
KM	5.4 ± 1.1	ND	32g	29.1 ± 5.7	7.3 ± 0.5	4.6 ± 0.5	2.7 ± 0.2h	25.2 ± 1	88 ± 15

^aAll values are means ± standard deviations.

^bThe increase in cell carbon was estimated to be equal to the increase in protein concentration (28). ND, not determined.

^cPercentage of acetate used to produce new cells (calculated from the measured protein concentration).

^dCorrected for carbon conversion to biomass, the remaining substrate is assumed to be oxidized to CO₂.

^eAmounts of electrons used up for reduction of selenate to selenite or selenium based on measured concentrations of electron acceptors.

^fCalculated on the basis of the following stoichiometric equations as percentages of electrons consumed/electrons produced: C₂H₄O₂ + 2H₂O → 2CO₂ + 8H⁺ + 8e⁻; SeO₄²⁻ + 2H⁺ + 2e⁻ → SeO₃²⁻ + 4H₂O; and SeO₄²⁻ + 8H⁺ + 6e⁻ → Se⁰ + 4H₂O.

^gProtein could not be estimated due to the formation of a biofilm with precipitated selenium firmly adhering to the bottom of the culture vial. Hence, we assumed 32% carbon conversion to the cell based on an average of values obtained with strain S7 and strain pn1. We obtained about 60% electron balance when we did not account for biomass produced.

^hCalculated as difference of total selenate and soluble selenium as measured. Further confirmation for elemental selenium formation was demonstrated by XANES analysis.