Distribution, identity and activity of symbiotic bacteria in anoxic aggregates from the hindgut of the sea urchin Echinocardium cordatum

Complex, anoxic aggregates of refractory organic matter and bacterial consortia are permanently present in the intestinal caecum, of the spatangoid sea urchin Echinocardium cordatum. This study documents the presence of sulphate-reducing bacteria and sulphur cycling in the aggregates using microsensors and S-35-radiotracers. The aggregates were covered by protruding filamentous bacteria, identified by fluorescence in situ hybridisation to be sulphate-reducing bacteria belonging to the genus Desulfonema. The filamentous bacteria did not contain sulphur globules, but only lipid droplets. There were no filamentous bacteria present belonging to the gamma-proteobacteria. The majority of the filamentous bacteria inside the aggregates also belonged to the genus Desulfonema. The volume specific oxygen respiration of the aggregates was about 16 times higher than the respiration of coastal sediments. Sulphate reduction occurred exclusively in the aggregates, and gross sulphate reduction rate was up to 180 times higher than in coastal sediments. Large amounts of FeS were present in the aggregates and about 80% of the reduced S precipitated with Fe-2divided by in the anoxic part of the aggregates. Thus iron is important in buffering H2S in the gut system of the sea urchin.