Fig. 5. Consumption of oxygen and sulfide by intact gill tissue. Traces of oxygen partial pressure (black line) and sulfide concentration (red line) over time demonstrate a rapid decline in oxygen and a coincident rapid removal of sulfide following bolus injections of 100 µmol l^–1 Na₂S (arrows along abscissa). Once anoxia is reached at 70 s with no further oxygen consumption, consumption of sulfide injected at 100 s continues more slowly than under normoxic conditions. Following an anoxic bout, a subsequent addition of sulfide results in decreased rates of oxygen and sulfide consumption and a decreased S:O₂ ratio. Spontaneous sulfide oxidation under oxygenated conditions (open circles) or anoxic conditions (open squares) is much slower than biological sulfide consumption. Sulfide oxidation rates in chambers with heat-killed gills (microwave oven for 1 min, see Materials and methods; data not shown) are comparable to rates without gills, indicating that biological sulfide consumption is catalyzed enzymatically.