Exploitation of a chemosynthetic food resource by the polychaete Capitellasp I

In organically enriched sediments of coastal areas, sulfate-reducing bacter ia decompose organic matter anaerobically, producing high levels of hydroge n sulfide. Chemoautotrophic sulfur-oxidizing bacteria proliferate at the su lfide/oxygen interface and use hydrogen sulfide as an electron donor. A few species of small polychaete worms, including Capitella sp. 1, often domina te the macrofaunal benthic communities in such sulfide-rich environments in the organically enriched sediments. In this study, we conducted 2 laborato ry 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 treatm ents showed better survival, enhanced growth and reproduction. They had low er delta C-13 values (-24.3 parts per thousand, mean) than control worms (- 20.1 parts per thousand, mean) and the sediments in which the worms were cu ltured (-21.4 parts per thousand, mean). The distinctive delta C-13 signatu re 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 don e in the same manner as the first, but the carbonate in the water was repla ced with (CO2)-C-13 in order to trace the autotrophic fixation of carbon di oxide by chemosynthetic bacteria occurring within the sediments. The result s indicate that fixation of (CO2)-C-13 was promoted in sediments with Na2S amendments, and further enhanced by the presence of Capitella sp. 1. The wo rms in these sediments had extremely high delta C-13 values (+5218.2 parts per thousand). The results of this study introduce the possibility of enhan ced survival and growth of Capitella sp. I in sulfide-rich environments in the organically enriched sediments, facilitated by its utilization of a nov el source of organic matter.