The reason why biomass d13C values are more negative than air CO2
and ocean inorganic carbon is largely because RubisCO fixes 12CO2 a little faster
than 13CO2.

Shown to the left is what happens when you seal RubisCO with CO2
and ribulose 1,5-bisphosphate. As the reaction progresses,
the carbon dioxide concentration falls, and the d13C value of the carbon dioxide
remaining behind is driven to more positive d13C values because
12CO2 is being removed more rapidly than 13CO2.

However, all RubisCOs do not discriminate against 13CO2
to the same degree. Isotopic discrimination by RubisCO
can be described as an epsilon value, which is proportional
to the extent of discrimination by the enzyme and is
roughly equal to the difference between the d13C
of the CO2 source from which the RubisCO is drawing,
and the d13C of the CO2 that it fixes.

Form IA and IB enzymes discriminate against 13C
to a greater extent than form II. IA and IB’s

have epsilon values of 22 - 29‰, while form II RubisCOs
have epsilon values from 18-22‰. Interestingly,
chemoautotrophic organisms that use form II RubisCO
have more 13C in their biomass, which follows
from carbon fixation being catalyzed by an enzyme
that discriminates less.

Epsilon values for form IC and ID RubisCOs have not been measured yet,
despite the importance of form IC and ID-bearing autotrophs such as
diatoms and coccolithophores to the ocean carbon cycle.
We are currently determining epsilon values for these enzymes.

The image of coccolithophore Emiliania huxleyi to the left
was kindly provided by Claudia Sprengel and Jeremy Young