13C/12C ratios in marine invertebrates from reducing sediments: confirmation of nutritional importance of chemoautotrophic endosymbiotic bacteria

ABSTRACT: The carbon isotope composition (6l3C%0) of the tissues of benthic invertebrates usuallyranges from - 16 to -20. In contrast we report that several common bivalve molluscs belonging to thesuperfamily Lucinacea and several small species of Pogonophora show much greater depletions,ranging from -23 to -31 in the bivalves and from -35 to -46 in the pogonophores. These bivalvesand pogonophores live in reducing sediments where the concentration of dissolved sulphide is low,usually < 1 PM. The gills of the bivalves and the trophosome tissue of the pogonophores containintracellular or sub-cuticular bacteria. The bacteria are autotrophs, as shown by ribulosebisphosphatecarboxylase activity of extracts of the bacteria-containing tissues. Comparisons of the 613C values of thebacteria-containing regions and the rest of the tissues indicate substantial transfer of organic carbon,half or more of the nutritional needs of the hosts. The organic carbon is produced by fixation of CO, bythe autotrophic bacteria, which oxidize reduced inorganic compounds, notably sulphide, to obtainenergy for the process. Similar depletions of 13C were previouslt reported for other autotrophicsymbiont-containing bivalves that live in habitats where dissolved sulphide concentrations are 2 or 3orders of magnitude higher than in the sediments analysed here. This form of nutrition, involvingsymbiosis with autotrophic bacteria, is evidently not dependent on high levels of dissolved sulphide and appears to be widespread in calcareous reducing sediments of the shelf and continental slope.