C-13 C-12 OF ORGANISMS FROM JUAN-DE-FUCA RIDGE HYDROTHERMAL VENTS - AGUIDE TO CARBON AND FOOD SOURCES

Soft tissue deltaC-13 values were determined in vestimentiferan tube w orms, alvinellid polychaetes and molluscs from Axial Seamount and Midd le Valley, North-east Pacific. Inorganic carbon in mollusc shells and water samples was also analysed. In the vestimentiferan, Ridgeia pisce sae, which lives in symbiosis with sulphur-oxidizing chemolithoautotro phic bacteria, tissue samples from the Axial vents showed deltaC-13 va lues from -11 to -16 parts per thousand, whereas at Middle Valley, whe re venting occurs through sediments, the deltaC-13 ranged from -16 to -26 parts per thousand. The tissues of an associated polychaete, Paral vinella palmiformis, which feeds on free-living bacteria, had deltaC-1 3 values in the range -21 to -26 parts per thousand. The bivalve Calyp togena from Middle Valley was more depleted than Ridgeia and Paralvine lla, -37 parts per thousand, doser to the ratios found in chemolithoau totrophic symbioses in non-vent habitats. Considerable, but variable, depletion (-23 to -42 parts per thousand) was found in small gastropod s. Mollusc shells and diluted vent water differed little in deltaC-13 compared to inorganic carbon in ambient deep sea-water. The vestimenti feran results have no simple explanation. In Ridgeia from the hotter v ents there is a slight size effect, with lesser discrimination against C-13 in the larger animals, but this is very much less marked than si te differences. The site effect suggests that local environmental cond itions, including temperature, may control discrimination against the heavy isotope during transport and fixation of CO2 by both symbiotic a nd free-living chemolithoautotrophic bacteria. Such variation must be taken into account when tracing carbon sources and carbon flow in hydr othermal vent communities. The new data from the juan de Fuca Ridge sy stem agree with data from other vent systems in showing a difference f rom the ratios (-16 to -20 parts per thousand) found in non-vent benth ic animals dependent on photosynthetic food sources. Carbon-13 depleti on typical of methanotrophy was not found. The main source of organic carbon for the Juan de Fuca Ridge vent fauna appears to be bacterial c hemolithoautotrophy based on reduced sulphur.