9504257 Cavanaugh Studies of stable isotopes allow valuable insight into various aspects of carbon cycling. For example, such stable carbon isotope data have been used to lend support to the putative dependence of several hydrothermal vent animals on organic matter produced by chemosynthetic endosymbionts. Unfortunately the observed isotope fractionations in various ecosystems have not always been readily understood, leading to equivocal conclusions regarding tropic relations between species in a given ecosystem. Again, the hydrothermal vent ecosystems provide an example of this. del13C values measured for symbiotic bivalves (del13C = -27% to -35%) mimic those of free-living chemoautotrophs, indicating the symbionts' nutritional role. In contrast, the food sources are less clear for tubeworm-chemoautotroph symbioses since these tissues exhibit surprisingly heavy stable isotope values (del13Cs = -9 to -35%). While a number of theories have been presented, the heavier isotope values for the symbioses have never been adequately explained. The recognition of different Rubisco forms being expressed may have far reaching implications, beyond interpretations of carbon cycling at hydrothermal vent ecosystems. The recent discovery of a form II Rubisco in a dinoflagellate indicates that this enzyme is not confined to the bacterial autotrophs, and may well explain the isotopically heavy del13C values of organic matter recovered from the open ocean. Accurately resolving the effects of various forms of the carbon fixation enzyme, Rubisco, may have a tremendous impact on interpretation of del13C data in ecological and paleontological studies. ***
