What is meant by "complexity of carbon sources"?
Consider all the different types of carbon molecules one can find on earth.

Proteins.  Sugars like cellulose and starch.  Alkanes like methane and tar.  CO2.  Alcohols.  A boatload of variety.

Now think of how each one is used or can be used by an organism.  I will chow down on a potato.  And some alcohol. Termites eat cellulose.  Anaerobic bacteria use methane.  Plants eat up the CO2.

One carbon nothing eats is CaCO3 or limestone.  I am not sure why.  Oh - also diamond.  Not many things can metabolize a diamond.  Although maybe they don't get the opportunity to try...