Amino Acids with High Biosynthetic Energy Costs Are Conserved By Soil Communities.

Rhodococcus sp. (ATCC  49988) was grown on ¹⁴C-glucose in liquid culture.  Washed, heat-killed cells were added at a rate of 0.7 mg cells/g (35 kBq /g) to a Catlin soil.  Soils received 1000 µg/g cellulose or 100 µg/g NH₄-N to produce N or C-limited conditions.  N-limitation resulted in rapid immobilization of inorganic N pools.  Carbon limitation resulted in increased inorganic N species, presumably due to mineralization.  Total amino acids were recovered from soil by acid hydrolysis and free amino acids were recovered with sodium-Chelex 100.  Amino acids were measured by ion exchange chromatography and radioactivity detection.  ¹⁴C-labeled amino acid co-position of soil extracts was similar to that of added cells regardless of nutritional conditions.  In C-limited soil, the ratio of total to labeled amino acids widened over time.  The rate of label dilution was most rapid for asp, gly, ala, and ser, and slowest for phe and trp.  The amino acids thr, glu, gln, and ile became diluted at an intermediate rate.  Rate of label dilution was proportional to energetic costs of amino acids as estimated by number of high-energy molecules used during synthesis.  In N-limited soil, ¹⁴C-amino acid pools were diluted nearly to extinction within one week, with the exception of leu, ile, phe and trp, which were relatively undiluted compared to initial conditions.  Mineralization of ¹⁴C-amino acids was greater under N limitation.  Label dilution of free amino acids responded similarly to that in the hydrolysates, except that the free amino acid pool was initially more dilute and the components, leu, ile, phe and trp were reduced in concentration relative to hydrolysates.  Results suggest costly amino acids tend to be retained in original form and immobilized in protein, whereas less costly amino acids appear to be more quickly degraded, particularly under N-limited conditions (presumably recycled for N content).