IUBio

Carbon autotrophy

JRP jean.pelmont at wanadoo.fr
Sun Jan 21 05:06:46 EST 2001


Mike Dalbey <dalbey at biology.ucsc.edu> wrote (écrivait) :


> (quote)
> AUTOTROPHIC means that most carbon for biosynthesis is derived specifically
> from CO2 or bicarbonate ion in solution. Biochemically, autotrophy is based on
> a pathway called the Calvin Cyle (in plants). AUTOTROPHIC is the opposite of
> HETEROTROPHIC.

Carbon autotrophy is not necessarily based on the Calvin-Benson cycle.
For those who are not too familiar with this, the presence of Rubisco is
indicative of the occurrence of such a cycle in a given organism, but
other autotrophic pathways do exist. Acetogenic bacteria can assimilate
CO2 via the acetyl-CoA synthase (or CO dehydrogenase). The methanogens
do it also when growing on CO2 + H2. Green phototrophic bacteria such as
Chlorobium use a reductive cycle similar to the Krebs cycle operating in
reverse, but different. This reductive cycle operates at a very low
redox potential, and these bacteria are strict anaerobes.

In carbon autotrophy, all of the carbon is available from CO2. But there
are different cases. Some organisms can shift from autotrophy to
heterotrophy, and vice versa, according to conditions. Some bacteria,
such as purple non oxygenic species, require vitamins for growth.
Therefore some organic compounds should be there, marginally.
Somespecies are mixotrophs, that is to say they can assimilate CO2 and
organic compounds simultaneously. 

Some more tricky cases are met with methanotrophs (*) and methylotrophs,
bacteria that are able to thrive on methane or methanol, respectively
(some yeasts are methylotrophs too). They are aerobes. Methane is
converted to methanol, then formaldehyde. Some of the latter enters the
metabolism through several cycles and is used as the carbon source. Some
of the formaldehyde is converted to CO2 as as source of energy. There is
some discussion to know if these organisms should be considered as
autotrophs or not. I do not know. Such a situation exists also in the
case of formate oxidizers. After all, this is just semantic, Nature
ignores our simple definitions, and offers a wide scale of solutions
that can be used at the same time :-)

Autotrophy is endergonic (requires energy). Photo-autotrophs as green
plants, cyanobacteria, purple and green bacteria (and some halobacteria
I believe) obtain energy from light. Chemolitho-autotrophs are all
procaryotes, obtaining energy from the chemical oxidation of a simple
inorganic compound : H2, ferrous iron, sulfide, tetrathionate, ammonia,
nitrite.... The oxidant is commonly dioxygen, but can be nitrate
(Thiobacillus denitrificans) or sulfate. In methanogens, energy can be
driven from the oxidation of H2 by CO2, or from the splitting of acetate
to methane and CO2. Thus CO2 is both a carbon source and an electron
acceptor.

In anaerobic marine or coastal sediments, methanogens, that are
exclusively archaebacteria) can be outcompeted by sulfate-reducing
bacteria (proteobacteria), because the latters can thrive vigorously
from the energy-producing sulfate reduction and the blotting of all the
precious H2. Some are autotrophic, others are not. Desulfovibrio baarsi
can be autotrophic on formate, because the oxidation of the latter to
CO2 is coupled to sulfate reduction, then the CO2 is assimilated. Also
the sulfate-reducers can tolerate low amounts of oxygen, while the
methanogens do not. However the methanogens can reduce sulfur, producing
large amounts of sulfide, fighting back other species. What a tricky job
and interesting field !
> 
Regards
_________________________________
(*) Methanotrophs using CH4 never meet with methanogens, who make CH4.
There is a very important methane cycle in the environment.
Methanotrophs are precious organisms for us, because the enzyme, methane
monoxygenase, is able to oxidize a very wide variety of substrates, some
are pollutants (for instance trichloroethylene). 

-- 
Jean R. Pelmont






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