Under these conditions the corals were killed after another 3 h. The role of hydrogen sulfide as an accelerator of coral damage was demonstrated in an experiment in which we first exposed corals to oxygen-free water and pH 7 for 12 h and then added hydrogen sulfide in moderate concentrations. In contrast, the combination of anoxia and pH 7 was fatal within 1 d. In addition, air-saturated oxic water with pH 7 did not harm corals for 4 d. Corals survived in conditions without oxygen at pH 8.2 for the tested 4 d. In these experiments we used different combinations of oxygen and sulfide concentrations and pH in water. We then tested which factor initially might have caused the death of the coral by conducting tank experiments without the use of sediment. Our data suggest that the organic enrichment of coastal sediments is a key process in the degradation of coral reefs exposed to terrestrial runoff. Subsequently, the hydrogen sulfide formed by bacterial decomposition of coral tissue and mucus diffuses to the neighboring tissues, accelerating the spread of colony mortality. First, increased microbial respiration results in reduced O 2 and pH, initiating tissue degradation. We suggest that sedimentation kills corals through microbial processes triggered by the organic matter in the sediments, namely respiration and presumably fermentation and desulfurylation of products from tissue degradation.
When hydrogen sulfide was added after 12 h of anoxia and low pH, colonies died after an additional 3 h. However, the combination of anoxia and low pH led to colony death within 24 h. Dark incubations of corals showed that separate exposures to darkness, anoxia, and low pH did not cause mortality within 4 d. In the harmful organic-rich sediment, hydrogen sulfide concentrations were low initially but increased progressively because of the degradation of coral mucus and dead tissue.
Organic-rich sediments caused tissue degradation within 1 d, whereas organic-poor sediments had no effect after 6 d. In organic-rich, but not in organic-poor sediment, pH and oxygen started to decrease as soon as the sediment accumulated on the coral. Microsensor measurements were conducted in mesocosm experiments and in naturally accumulated sediment on corals. We investigated the mechanisms leading to rapid death of corals when exposed to runoff and resuspended sediments, postulating that the killing was microbially mediated.