Sulfide oxidation in the anoxic Black Sea chemocline

Abstract

The depth distributions of O 2 and H 2S and of the activity of chemical or bacterial sulfide oxidation were studied in the chemocline of the central Black Sea. Relative to measurements from earlier studies, the sulfide zone had moved upwards by 20–50 m and was now (May 1988) situated at a depth of 81–99 m. Oxygen in the water column immediately overlying the sulfide zone was depleted to undetectable levels resulting in a 20–30-m deep intermediate layer of O 2 - and H 2S-free water. Radiotracer studies with 35S-labelled H 2S showed that high rates of sulfide oxidation, up to a few micromoles per liter per day, occurred in anoxic water at the top of the sulfide zone concurrent with the highest rates of dark CO 2 assimilation. The main soluble oxidized products of sulfide were thiosulfate (68–82%) and sulfate. Indirect evidence was presented for the formation of elemental sulfur which accumulated to a maximum of 200 nmol l −1 at the top of the sulfide zone. Sulfide oxidation was stimulated by particles suspended at the chemocline, probably by bacteria. Green phototrophic sulfur bacteria were abundant in the chemocline, suggesting that photosynthetic H 2S oxidation took place. Flux calculations showed that the measured H 2S oxidation rates were 4-fold higher than could be explained by the downward flux of organic carbon and too high to balance the availability of electron acceptors such as oxidized iron or manganese. A nitrate maximum at the lower boundary of the O 2 zone did not extend down to the sulfide zone.