Limitations of atp estimates of microbial biomass

Serious limitations were found in the use of ATP measurements as indicators of Pseudomonas paucimobilis biomass and activity in sterilized soil. These limitations were related to effects of moisture stress, nutrient variation and protozoan predation on soil ATP levels and ATP per bacterium. Rewetting dry soil resulted in an increase (2.2-fold) in ATP per bacterium, as did additions of glucose (5.9-fold) and (NH₄)₂SO₄ (2.8-fold). ATP per bacterium also differed between grazed and ungrazed bacteria. Carbon dioxide evolution and ATP concentrations varied independently with time. Thus, except under low nutrient conditions, when neither bacterial numbers nor ATP measurements changed, ATP concentrations did not indicate bacterial population dynamics or respiration. Carbon and nitrogen effects on ATP concentrations were reduced by adding an excess of glucose a few hours before ATP measurements.