FIG 1.

Syntrophic acetate oxidation for operating days 738, 797, 911, 933, 964, and 982. Low-ammonia reactors (R1 and R2) (1.8 g N liter⁻¹ total ammonia) maintained activity at or near the baseline level of 5% syntrophic acetate oxidation (95% acetate cleavage to produce methane and CO₂ from acetate), while high-ammonium reactors (R3 and R4) (4.4 to 4.8 g N liter⁻¹ total ammonia) shifted to 20 to 25% syntrophic acetate oxidation (75 to 80% acetate cleavage) between days 809 and 914. Operating conditions within this time period remained constant. Error bars (smaller than the points themselves) represent the ranges from replicate instrument analyses.