If autotrophs rely to some degree on HCO3- uptake,
the d13C value of the pool of CO2 and HCO3-
delivered to the cytosol will reflect discrimination
during transport and might affect the d13C value
of the carbon fixed by RubisCO.

In addition, high affinity transport "recaptures" any CO2
that leaks from the cell before fixation. As a result,
these cells approach "closed systems";
nearly all CO2 and HCO3- transported into the cell is fixed
and little fractionation with respect to
the source CO2 and HCO3- occurs,
masking the isotopic fractionation by the RubisCO.