clam-chemoautotroph symbiosis Solemya velum

Many nutritive symbioses between chemoautotrophic bacteria and invertebrates, such as Solemya velum , have d 13 C values of ~30 to 35‰, considerably more depleted than phytoplankton. Most of the chemoautotrophic symbionts fix carbon with a form IA ribulose 1,5-bisphosphate carboxylase (RubisCO). We hypothesized that this form of RubisCO discriminates against 13 CO 2 to a greater extent than other forms. Solemya velum symbiont RubisCO was cloned and expressed in Escherichia coli , purified and characterized. Enzyme from this recombinant system fixed carbon most rapidly at pH 7.5 and 20–25 ∞ C. Surprisingly, this RubisCO had an e -value (proportional to the degree to which the enzyme discriminates against 13 CO 2 ) of 24.4‰, similar to form IB RubisCOs, and higher than form II RubisCOs. Samples of interstitial water from S. velum ’s habitat were collected to determine whether the dissolved inorganic carbon (DIC) could contribute to the negative d 13 C values. Solemya velum habitat DIC was present at high concentrations (up to ~ 5 mM) and isotopically depleted, with d 13 C values as low as ~ 6‰. Thus environmental DIC, coupled with a high degree of isotopic fractionation by symbiont RubisCO likely contribute to the isotopically depleted d 13 C values of S. velum biomass, highlighting the necessity of considering factors at all levels (from environmental to enzymatic) in interpreting stable isotope ratios.