Impact of ionic aluminium on extracellular phosphatases in acidified lakes

We studied direct inhibiting effects of aluminium (Al) on extracellular pho sphatases produced by the plankton of acidified lakes in the Bohemian Fores t. In laboratory experiments we tested the effect of different Al concentra tions (0-1000 mug l(-1)) on kinetic parameters of acid phosphatases (pH opt imum similar to5.0) at pH between 4.5 and 5.2. We observed a significant re duction of an apparent substrate affinity at Al concentrations between 300 and 1000 mug l(-1) at pH 4.5 and 4.8 (but not at 5.2). In contrast, maximum acid phosphatase activity (AcPA) remained unchanged. Such behaviour of sat uration kinetics is compatible with the assumption that ionic Al acts as a competitive inhibitor of acid phosphatases. To decide whether the observed Al effects could be explained alternatively by complexation of Al with subs trate, we tested statistically the best fits of data with both possible mod els (competitive versus complexation). Experimental results supported the c ompetitive hypothesis rather than the complexation model suggested original ly by some authors. Furthermore, we tested the Al effect within a wide rang e of pH from 4.0 to 6.0. For pH values < 5.2, the results of an Al-pH matri x experiment gave a more detailed picture: the higher the Al concentration, the wider the pH range in which Al could negatively affect AcPA. The ecolo gical ramifications of this effect were evaluated in the context of field A cPA data on three strongly acidified lakes.