Amino acid cycling in plankton and soil microbes studied with radioisotopes: measured amino acids in soil do not reflect bioavailability

In radioisotope studies in plankton, bacteria turn over the nanomolar ambient concentrations of dissolved amino acids within a few hours. Uptake follows Michaelis-Menten kinetics. In contrast, within minutes the very abundant bacteria and fungi in soil take up all labeled amino acids added at nanomolar to millimolar final concentrations; uptake kinetics accordingly cannot be measured. This rapid uptake agrees with earlier findings that soil microbes exist in a starving or low-activity state but are able to keep their metabolism poised to take up amino acids as they become available. How can this rapid uptake of added amino acids be reconciled with persistent soil concentrations of 10-500 mu M of total dissolved amino acids? Although respiration of added amino acid carbon has been used to deduce uptake kinetics, the data indicate that in both soil and in eutrophic natural waters constant percentages of individual amino acids are respired; this percentage varies from less than 10% of the amount taken up for basic amino acids to more than 50% for acidic amino acids. We conclude that relatively fixed internal metabolic processes control the percent of amino acid respired and that the mu M concentrations of amino acid measured in water extracts from soil are unavailable to microbes. Instead, these relatively high concentrations reflect amino acids in soils that are chemically protected, hidden in pores, or released from fine roots and microbes during sample preparation.