Growth efficiency, growth rate and the remineralization of organic substrate by bacterioplankton - revisiting the Pirt model

The growth yield of aquatic bacteria plays a critical role in the biogeochemical cycle of organic carbon in the sea. The Pirt model (1982) proposes a relationship of specific growth rate (mu, d(-1)), respiration rate (r, d(-1)) and growth yield (Y, unitless) of bacteria when the growth rate is limited by the concentration of the organic substrate and carbon stoichiometry is applied. Published data from natural populations of bacterioplankton within a temperature range of 10 to 25 degrees C follow the Pirt model, where the growth yield is approaching asymptotically a maximum value (epsilon) with increasing growth rate. The specific rate of the maintenance metabolism (a, d(-1)) is the shape factor defining the curve. From the model a relationship between the specific respiration rate and the growth rate {r= [mu(1/epsilon - 1) + a/epsilon]} can be derived. Based on the published data these model parameters were estimated: epsilon = 0.51 and a = 0.58. Our model results should help explain the wide range of growth efficiencies reported for natural bacterioplankton.