Calvin-Benson Cycle: Reduction of CO2

 

Requires NAD(P)H, ATP, and ribulose-bisphosphate carboxylase (RuBP carboxylase or RubisCO)

 

May be most common enzyme on planet!  Also used by non-phototrophic autotrophs  Algae, plants, photo- and chemosynthetic bacteria, archeaea, … 

 

Catalyzes formation of two two molecules of PGA (3-phosphoglyceric acid) from RuBP and CO2

 

 

There are also other (less common) pathways by which carbon is fixed

 

 



Photolithotrophic autotrophy

Gets energy from light

Gets electrons from reduced inorganic molecules

Gets carbon from CO2

 

Example 1:  Spirulina, a cyanobacterium, lives in lakes

-       Energy: Light reaction of photosynthesis.  Light energy is captured when light hits a photosynthetic reaction center chlorophyll, raising it to a higher energy state, leading to generation of NADH and proton motive force

-       Electrons come from water (this is an oxygenic photosynthetic organism).   Electron flow leads to formation of ATP via proton motive force (photophosphorylation)

-       Calvin-Benson cycle (dark reaction of photosynthesis) used to fix carbon from CO2

 

Second example:  green sulfur bacteria, anaerobic photosynthetic organisms, get energy from photosynthesis, electrons from sulfur compounds, and use reverse citric acid cycle to fix carbon.

 

 


Chemolithotrophic autotrophy

            Get energy from oxidation of organic or inorganic compounds

            Get electrons from reduced inorganic molecules

            Get carbon from CO2

 

Example:  Thiobacillus,

-       Energy:  oxidizes H2S.  Oxidation occurs in steps.  First oxidation step results in the formation of elemental sulfur.  When supply of H2S is depleted, oxidize sulfur to sulfate. 

-       Electrons from sulfur compounds enter electron transport chain, transported to O2, leading to formation of ATP via proton motive force: oxidative phosphorylation (aerobic respiration)

-       Calvin-Benson cycle (dark reaction of photosynthesis) to fix carbon from CO2

-       Fermentation may take place when organism breaks down nitrogen compounds, etc.

 

Chemolithotrophic mixotrophy

Example:  Beggiotoa, also oxidizes H2S

-       Lacks the enzymes of the Calvin-Benson cycle, so requires organic compounds as carbon sources (mixotrophy)

-        

 


Photoorganotrophic heterotrophy

            Get energy from light

            Get electrons from organic molecules

            Gets carbon from reduced organic molecules, or from both organic and inorganic carbon

 

            Example: Rhodobacter photosynthetic microbe living in polluted water

-       Energy:  Light reaction of photosynthesis turns out ATP and NADH (photophosphorylation) Anaerobic photosynthesis

-       Electrons:  organic matter.  (This organism ferments organic compounds, makes ATP via substrate-level fermentation as well as donating these electrons to photosynthesis

-       Carbon: also gets from fermenting organic compounds

 


Chemoorganotrophic heterotrophy:

            Get energy from oxidation of organic or inorganic compounds

            Get electrons from organic molecules

            Get carbon from reduced organic molecules

 

            Example:  Helicobacter, pathogenic microbe, causes ulcers

-       Energy:  organic compounds.  Glycolysis, etc.  (Fermentation) results in reducing equivalents to enter respiration, as well as some ATP from substrate-level fermentation.  In respiration ATP formed by oxidative phosphorylation. This organism is a microaerophile, so the final electron acceptor is O2.

-       Electrons:  organic matter

-       Carbon compounds: organic matter