Exploitation of a chemosynthetic food resource by the polychaete Capitella sp.

Abstract

In organically enriched sediments of coastal areas, sulfate-reducing bacteria decom- pose organic matter anaerobically, producing high levels of hydrogen sulfide. Chemoautotrophic sul- fur-oxidizing bacteria proliferate at the sulfide/oxygen interface and use hydrogen sulfide as an elec- tron donor. A few species of small polychaete worms, including Capitella sp. I, often dominate the macrofaunal benthic communities in such sulfide-rich environments in the organically enriched sed- iments. In this study, we conducted 2 laboratory experiments to determine whether Capitella sp. I can benefit trophically through the exploitation of the organic matter chemosynthetically produced by sulfur-oxidizing bacteria. In the first experiment, we reared juveniles of Capitella sp. I with natural sediment of very low organic content, with no additional organic matter, under dark conditions, and exposed them to 3 different levels of sodium sulfide. The worms reared in the sulfide treatments showed better survival, enhanced growth and reproduction. They had lower δ 13 C values (-24.3‰, mean) than control worms (-20.1‰, mean) and the sediments in which the worms were cultured (-21.4‰, mean). The distinctive δ 13 C signature of the worms in the sulfide treatments indicates that they did not share the same carbon source as the control worms. The second experiment was done in the same manner as the first, but the carbonate in the water was replaced with 13 CO2 in order to trace the autotrophic fixation of carbon dioxide by chemosynthetic bacteria occurring within the sedi- ments. The results indicate that fixation of 13 CO2 was promoted in sediments with Na2S amendments, and further enhanced by the presence of Capitella sp. I. The worms in these sediments had extremely high δ 13 C values (+5218.2‰). The results of this study introduce the possibility of enhanced survival and growth of Capitella sp. I in sulfide-rich environments in the organically enriched sediments, facilitated by its utilization of a novel source of organic matter.