Interesting result and discussion. I second Rick's comment that these TCA cycle have special roles in metabolism. Both plants and fungi excrete organic acids into their substrates. Malic, citric, succinic and fumaric acids have been detected in forest leaf litter in some studies.
For me the most curious effect of these acids is that discovered in the 1970's in the plant tissue culture setting. Oddly, NH4 is used rather poorly as a sole N source for plant cells in tissue culture. This can be boosted by adding one of the TCA organic acids to the medium; succinic and alpha-ketoglutaric are the most effective, but both citric and malic acids have an effect. Sorry I didn't copy the link to this but the reference is below.
Behrend,J. and Mateles, R.I. (1976) Plant Physiol 58: 510 - 512
Another intriguing study by the same group indicates that the use of nitrate as the sole N source can be inhibited by adding single amino acids to the medium, all that is except arginine (and possibly alanine) which also can counter the inhibitory effect of the others when added together.
Behrend,J. and Mateles, R.I. (1975) Plant Physiol 56: 584 - 589