The Calvin-Benson cycle is the most widespread
autotrophic pathway in the biosphere.
RubisCO is the CO2-fixing enzyme
of the Calvin-Benson Cycle, and is directly responsible
for converting roughly 10^11 tons of CO2 to biomass annually.
RubisCO catalyzes the carboxylation of the five-carbon sugar
ribulose 1,5-bisphosphate to form two molecules
of phosphoglyceric acid.

The ultimate net gain of this pathway is one molecule of PGA
for every three turns of the cycle, and all the
original substrate molecules are regenerated.

RubisCO is thought to have originated over two billion years ago.
Since then, in the many past organisms that have used it to fix carbon,
it has diverged considerably.

Here is a phylogenetic tree to show the remarkable diversity
of known RubisCO and RubisCO-like enzymes. Form I and II
enzymes are known to function primarily as carboxylases.
Form III is present in some Archaea, but its function
is currently being determined. Form IV is scattered
through the Proteobacteria and Archaea,
and does not have catalytic activity as a carboxylase.
Instead, it may function in methionine synthesis.

Autotrophic microorganisms in the ocean use a variety of forms
of RubisCO to fix carbon:

Form IA RubisCO
Prochlorococci
Some Synechococci
Many chemoautotrophic bacteria

Form IB RubisCO
Most cyanobacteria
Eukaryotic algae with green chloroplasts

Form IC RubisCO
(?) Some manganese oxidizing bacteria
Nitrifying bacteria

Form ID RubisCO
Diatoms
Coccolithophores
Many dinoflagellates

Form II RubisCO
Peredinin-containing dinoflagellates, incl. coral zooxanthellae
Some chemoautotrophic bacteria

Maximum likelihood tree constructed by Tara Harmer and Colleen Cavanaugh.