Association of various types of epi-bacteria with Halicryptus spinulosus (Priapulida)

The priapulid worm Halicryptus spinulosus von Siebold, from a sulfide-rich braclush water habitat, is associated with various types of epibacteria. TEM observations reveal bacteria morphologically similar to Thiobacillus and Leucothrix/Thiothrix. The Thiobacillus-type is the dominant organism; it occurs in microcolonies throughout the bacterial layer and frequently contains carboxysomes, which are indicative of chemoautotrophic activity. Analyses of ribulose-1,5-bisphosphate carboxylase, X-ray microanalysis, and the ratio of stable 13C/"C and ' 5 ~/ ' 4 ~ isotopes do not indicate a participation of chemoautotrophic bacteria in the nutrition of the worm. It is concluded that the epibacteria might be able to detoxify the hydrogen sulflde to some degree, provided oxygen or nitrate is available, and that the worms must rely on their own metabolism of hydrogen sulfide detoxification whenever their environment becomes anoxic.