Carbon in the form of
CO2, HCO3- and CO3-2,
are oxidized forms of C,
and tend to be the only
forms present where O2 is
plentiful.
In anoxic environments
methanogens (Archaea)
convert organic C and
CO2 into methane (CH4).
Methane is a gas and can
bubble out of the water or
it can be oxidized to CO2
by methylotrophic
bacteria.
Methanogens are not true bacteria, they belong to the Archaea
Most methanogens can grow on CO2 (or organic C)and H2 as their sole energy source:
Chemoautotrophs —chemical bond energy is their energy source
they utilize CO2 as their C source
http://faculty.plattsburgh.edu/jose.deondarza/images/Organisms/methanogen.jpg
C-transformations in aerobic and anaerobic environments
Oxidation
state
Where do we find methanogens?
-4
0
+4
•Under anaerobic conditions organic molecules break down to methane instead of CO2—This
process is facilitated by methanogens (Archaea), which are chemoautotrophic bacteria.
•They utilize the energy released from 2H2 + Organic C (CH2O)→CH4 + H20 to build their
biomass.
How to assign Oxidation numbers
We keep track of the e- transfer using Oxidation numbers (Ox#)
For each e- transferred the Ox# changes by 1
2H2 + O2
0
0
2H2O
+1 -2
Some rules for Oxidation numbers
1. In free elements Ox# =0
2. For ions with one atom Ox# = charge. eg H+ Ox# of H+ = 1
3. Ox# of O in most compounds is -2,
4. Ox# of H in most compounds is +1,
5. For a complex ion like SO4-2 , the net Ox# = charge (Thus S=+6)
Carbon and Oxygen play a major role in biological
energy transformations
Oxidation means giving up electrons, and reduction means taking on
electrons (─)
The oxidation state of C in CO2 is +4, in Carbohydrates is 0
How many electrons are taken up by each C during photosynthesis?
Chemical equation for the reduction of CO2 by H2
CO2  H 2  CH 4  H 2O  E(energy)
CO 2  4 H 2  CH 4  2 H 2O
( 4)
(0)
(-4)
-
( 1)
8 e are accepted per C
-
1 e is donated per H
Nutrition and Metabolic Diversity
Four nutritional categories
Carbon Source
Energy Source
CO2
Organic
Light
Photoautotroph
Photoheterotroph
Chemical
Chemoautotroph
Chemoheterotroph