Metabolism of nitrogen and sulfur in ectosymbiotic bacteria of marine nematodes (Nematoda, Stilbonematinae)

Nematodes of the family Stilbonematinae are known for their highly specific association with ectosymbiotic bacteria. These worms are members of the me iofauna in marine, sulfide-rich sediments, where they migrate around the re dox boundary layer. In this study, bacterial ectosymbionts of 2 species of marine nematodes, Stilbonema sp. and Laxus oneistus, were shown to be capab le of the respiratory reduction of nitrate and nitrite (denitrification). T he use of these alternative electron accepters to oxygen by the bacteria al lows the animals to migrate into the deeper, anoxic sediments, where they c an exploit the sulfide-rich patches of the deeper sediment layers. The accu mulation of thiols (sulfide, thiosulfate, sulfate and glutathione) in body tissues of the worms was determined following incubation in the presence of various electron donors (sulfide, thiosulfate) and accepters (nitrate). In their chemoautotrophic metabolic potential, the ectosymbionts of the 2 nem atode species were found to resemble the phylogenetically related, intracel lular symbionts of macrofaunal hosts of deep-sea hydrothermal vents and oth er sulfide-rich habitats.