Rubisco in chemoautotrophic symbioses: Implications for the interpretation of stable carbon isotope values

Invertebrate-chemoautotroph symbioses, in which the bacterial partner provides an internal source of autotrophically fixed carbon for its host, fall into two main groups based on previously reported stable carbon isotope ratios: those with delta C-13 = 27 to -35 parts per thousand (referred to as the -30 parts per thousand group) and those with delta C-13 = -9 to -16 parts per thousand (referred to as the -11 parts per thousand group). The delta C-13 values of the -30 parts per thousand group resemble those of free-living chemoautotrophic bacteria, suggesting the role of the symbionts in the nutrition of the bivalves; there is presently no satisfactory explanation for the delta 13C values of the -11 parts per thousand group. Immunological and enzyme inhibition data are presented demonstrating that the CO2-fixing enzyme expressed in representatives of the -11 parts per thousand group is a form II ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), while a form I Rubisco is expressed in the -30 parts per thousand symbioses. Previously reported in vitro determinations on purified Rubisco show that the form II enzyme from free-living bacteria has a smaller kinetic isotope effect wit respect to CO2 than does form I. Thus, the C-13-enriched stable carbon isotope signature observed in the intact -11 parts per thousand symbioses appears to be at least partially due to this diminished enzymatic discrimination.