Heterotrophs and Carbon Fixation

[Elersong]

So, I was reviewing some old notes concerning microbial metabolism, and I came up on a problem. In some of my notes, I have it written that heterotrophs use pre-fixed carbon in the form of organics to meet their need of a carbon source because they cannot fix their own carbon. In other notes, however, I have it written that heterotrophs simply cannot use only carbon dioxode as their carbon source and therefore use organics as a supplement.

My core inquiry is this: It is rather obvious that heterotrophs use organics in order to obtain carbon, but is it also possible for them to make use of carbon dioxide in order to obtain carbon as a supplement to that obtained through organics?

[victor]

Yup, it's possible.
The example can be seen in industrial fermentation of citric acid by Aspergillus niger. In order to produce citric acid, the CO2 produced from pyruvate decarboxylation is re-used again by combining it with another pyruvate to produce oxaloacetate (by the action of pyruvate carboxylase and PEP-carboxykinase enzymes).
At this stage, we obtain 2 equivalent of oxaloacetate where both of them are combined with acetyl-CoA yielding 2 citrate where one of them are released out of the cell.
In this example, we can see that the CO2 is re-used again. But it's not obtained directly from air CO2 like the plant does.

Hope this helps,
Victor Apriel

[MrMistery]

That's kind of interesting.
But in that case is the organism still called a heterotroph? Isn't it a mixotroph?

[Elersong]

That is interesting.

So I guess a better way to ask my question is this: If a heterotroph can behave in a manner in which it obtains carbon from both organics and CO2, then what is the criteria that differentiates mixotrophs from heterotrophs?

[victor]

Well I can say it's still called as heterotrophs (chemoorgano heterotrophs), because in order to generate those CO2, they still need organic molecules as the sole electron donor.

And I can say that mixotrophs are organisms which do not belong to these 4 nutrition types:
- Photolyto autotrophs
- Photoorgano heterotrophs
- Chemolyto autotrophs
- Chemoorgano heterotrophs
So if you found a microorganism which in example use a redox chemistry as a basis of energy yielding (chemo), using an inorganic electron donor like Fe2+ (lyto), but fixing organic compund also from an organic precursor (heterotroph), then I guess it's classified into mixotroph.

[MrMistery]

mixotroph is also an organism that can fit into more than one category. The most famous example is Euglena who is both a photolytoautotroph and a heterotroph

[victor]

Yeah, that thing is Plantanimal

[saranyal]

Heterotrophs are just opposite to autotrophs.
The carbon fixing mechanism is quite differnt too!

-----------------
saranya.l
Blaze Infotech