As CO2 travels from the environment into autotrophic cells,
a broad suite of influences affect the actual d13C value of the CO2
that reaches RubisCO. If the cells rely on diffusion of CO2
across their cell membranes, enriched biomass d13C values may reflect
diffusive limitation. In this scenario, discrimination against 13CO2 by RubisCO
during carbon fixation drives the intracellular pool to become more enriched
in 13CO2. If the rate of diffusion is not rapid enough for this intracellular pool
to come to isotopic equilibrium with extracellular inorganic carbon,
the d13C value of intracellular CO2 will have a steady state value
that is more positive than extracellular DIC, and result in
biomass organic carbon with more enriched values.

This is likely to be the case for the deep-sea hydrothermal vent chemoautotrophic symbiosis
Riftia pachyptila (depicted above), which has very enriched d13C values,
from -9 to -16‰. This giant worm gets its organic carbon from carbon fixation
by its chemoautotrophic bacterial symbionts. These symbionts are packed
into an internal organ called the trophosome, and are the "grape"-shaped round bodies
in the SEM image to the right . They live within tubeworm cells called bacteriocytes.
Due to the “close quarters” and their rapid rates of carbon fixation, the symbionts
drive the carbon dioxide to more enriched values, since their RubisCO
is fixing 12CO2 more rapidly than 13CO2.

Image generously provided by Colleen M. Cavanaugh.