A Marine Biologist Descends in a Submarine Through the Waters

Multiple Choice

A marine biologist descends in a submarine through the waters of the Black Sea. The top hundred meters or so are well oxygenated, but deeper waters are rich in hydrogen sulfide. What biological metabolisms can explain the distribution of H₂S in the Black Sea?

A) Chemoautotrophic bacteria oxidize H₂S in deep Black Sea waters.
B) Chemoautotrophic bacteria oxidize H₂S at the boundary between oxygen-rich surface waters and oxygen-poor deep waters.
C) Photosynthetic bacteria deplete H₂S in surface waters of the Black Sea.
D) Anaerobic respiration by sulfate-reducing bacteria continually generates H₂S in deep Black Sea waters.