Abstract:We studied extracellular acid phosphatase activity (AcPA) of planktonic microorganisms, aluminium (Al) speciation, and phosphorus (P) cycling in three atmospherically acidified (pH of 4.5-5.1) mountain forest lakes: Čertovo jezero (CT), Prášilské jezero (PR), and Plešné jezero (PL) in the Bohemian Forest (Šumava, Böhmerwald). Microorganisms dominated pelagic food webs of the lakes and crustacean zooplankton were important only in PR, with the lowest Al concentrations (193 µg L −1 ) due to 3-4 times lower terrestrial input. The lakes differed substantially in Al speciation, i.e., in the proportion of ionic and particulate forms, with the highest proportion of ionic Al in the most acid CT (pH = 4.5). The P concentration in the inlet of PL (mean: 22.9 µg L −1 ) was about five times higher than in CT and PR (3.9 and 5.1 µg L −1 , respectively). Average total biomass of planktonic microorganisms in PL (593 µg C L −1 ) was, however, only ∼2-times higher than in CT and PR (235 and 272 µg C L −1 , respectively). Enormous AcPA (means: 2.17-6.82 µmol L −1 h −1 ) and high planktonic C : P ratios suggested severe P limitation of the plankton in all lakes. Comparing 1998 and 2003 seasons, we observed changes in water composition (pH and Al speciation) leading to a significant increase in phytoplankton biomass in the lakes. The increase in the seston C : P ratio during the same time, however, indicates a progressive P deficiency of the lakes. The terrestrial Al inputs, together with in-1ake processes controlling the formation of particulate Al, reduced P availability for planktonic microorganisms and were responsible for the differences in AcPA. At pH < 5, moreover, ionic Al forms caused inhibition of extracellular phosphatases. We postulate that both particulate and ionic Al forms affect P availability (i.e., inhibition of extracellular phosphatases and inactivation of P), specifically shape the plankton composition in the lakes and affect plankton recovery from the acid stress.

We studied direct inhibiting effects of aluminium (Al) on extracellular phosphatases produced by the plankton of acidified lakes in the Bohemian Forest. In laboratory experiments we tested the effect of different Al concentrations (0-1000 microg l(-1)) on kinetic parameters of acid phosphatases (pH optimum approximately 5.0) at pH between 4.5 and 5.2. We observed a significant reduction of an apparent substrate affinity at Al concentrations between 300 and 1000 microg 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 saturation 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 substrate, we tested statistically the best fits of data with both possible models (competitive versus complexation). Experimental results supported the competitive hypothesis rather than the complexation model suggested originally by some authors. Furthermore, we tested the Al effect within a wide range of pH from 4.0 to 6.0. For pH values < 5.2, the results of an Al-pH matrix experiment gave a more detailed picture: the higher the Al concentration, the wider the pH range in which Al could negatively affect AcPA. The ecological ramifications of this effect were evaluated in the context of field AcPA data on three strongly acidified lakes.

In aquatic microbial ecology, it has been considered that most extracellular enzymes except phosphatases are of bacterial origin. We tested this paradigm by evaluating the relationship between bacterial cell number and the activity of three glycolytic enzymes from 17 fresh waters and also from a laboratory experiment. Our large sets of pooled data do not seem to support such a simple explanation, because we found only a weak correlation of bacterial number with activity of α-glucosidase (r s = 0.63), -glucosidase (r s = 0.45), and -N-acetylhexosaminidase (r s = 0.44). We also tested relations of the enzymatic activities to potential sources of natural substrates: dissolved organic carbon (DOC) and phytoplankton (as chlorophyll a). Their correlations with the enzymatic activities tested were very weak or insignificant. On the other hand, we found evidence for distinct producers of extracellular enzymes by analysing enzyme kinetics. The kinetics usually did not follow the simple Michaelis-Menten model but a more complex one, indicating a mixture of two enzymes with distinct affinity to a substrate. In combination with size fractionation, we could sometimes even distinguish three or more different enzymes. During diatom blooms, the diatom biomass tightly correlated with -N-acetylhexosaminidase activity (> 4 µm fraction). We also documented very tight relationships between activity of both glucosidases and dry weight of Daphnia longispina (r s = 1.0 and 0.60 for α-and -glucosidases, respectively) in an alpine clear-water lake. Our data and evidence from other studies indicate that extracellular glycosidic activities in aquatic ecosystems cannot generally be assigned only to bacteria. Also invertebrate animals and other eukaryotes (fungi, diatoms, protozoa etc.) should be considered as potentially very important enzyme producers.

The microbial loop of a naturally acidic bog lake, GroSe Fuchskuhle (Northeastern Germany), that had been artificially divided into 4 basins, was investigated. In the northeast (NE) and southwest (SW) basins, which differ strongly in chemistry and primary production, we conducted intensive studies of the main carbon fluxes through microbial food webs. In the less acidic, NE basin, much higher phytoplankton as well as bacterial biomass and production were found in parallel with negligible numbers of larger zooplankters. Weakly top-down controlled populations of protists were characterized by an exceptionally low numerical proportion of heterotrophic nanoflagellates (HNF) to ciliates (-1.5-3.5). The ciliate community was dominated by a scuticociliate, Cyclidium sp. (>95% of total ciliates), with an estimated grazing rate equal to 4 6 4 0 % of heterotrophic bacterial production. In contrast, in the more humic, SW basin, both phyto-and bacterioplankton dynamics seemed to be top-down controlled by abundant populations of small fine-filter feeding cladocerans, Cerioduphnia quadrungula and Diuphanosomu bruchyurum. Consequently, ciliates disappeared from the food web structure of the SW basin, HNF dropped to negligible numbers and bacteria showed very uniform morphology, dominated by small cocci or short rods. Our investigations have shown that the division of the lake into separate compartments can lead to very different microbial food web structures with extreme species compositions.