pH Tolerance in Freshwater Bacterioplankton: Trait-Variation of the Community measured by Leucine Incorporation.

pH is an important factor determining bacterial community composition in soil and water. We have directly determined the community tolerance (trait variation) to pH in communities from 22 lakes and streams ranging in pH from 4 to 9 using a growth based method not relying on distinguishing between individual populations. pH in the water samples was altered to up to 16 pH values, covering in situ pH ± 2.5 units, and the tolerance was assessed by measuring bacterial growth (Leu incorporation) instantaneously after pH adjustment. The resulting unimodal response curves, reflecting community tolerance to pH, were well modeled with a double logistic equation (mean R(2) = 0.97). Optimal pH for growth (pHopt) among the bacterial communities was closely correlated with in situ pH, with a slope (0.89 ± 0.099) close to unity. The pH interval, in which growth was ≥90 % of that at pHopt, was 1.1 to 3 pH units wide (mean 2.0 pH units). Tolerance response curves of communities originating from circum-neutral pH were symmetrical, while in high (pH 8.9) and especially low pH waters (pH<5.5) asymmetric tolerance curves were found. In low pH waters decreasing pH was more detrimental for bacterial growth than increasing pH, with a tendency for the opposite for high pH waters. A pH tolerance index, using the ratio of growth at only two pH values (pH 4 and 8), was closely related to pHopt (R(2) = 0.83), allowing for easy determination of pH tolerance during rapid changes in pH.