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Microbes can modulate ecosystem function since they harbor a vast genetic potential for biogeochemical cycling. The spatial and temporal dynamics of this genetic diversity should be acknowledged to establish a link between ecosystem function and community structure. In this study, we analyzed the genetic diversity of bacterial phosphorus utilization genes in two microbial assemblages, microbialites and bacterioplankton of Lake Alchichica, a semiclosed (i.e., endorheic) system with marked seasonality that varies in nutrient conditions, temperature, dissolved oxygen, and water column stability. We focused on dissolved organic phosphorus (DOP) utilization gene dynamics during contrasting mixing and stratification periods. Bacterial alkaline phosphatases (phoX and phoD) and alkaline beta-propeller phytases (bpp) were surveyed. DOP utilization genes showed different dynamics evidenced by a marked change within an intra-annual period and a differential circadian pattern of expression. Although Lake Alchichica is a semiclosed system, this dynamic turnover of phylotypes (from lake circulation to stratification) points to a different potential of DOP utilization by the microbial communities within periods. DOP utilization gene dynamics was different among genetic markers and among assemblages (microbialite vs. bacterioplankton). As estimated by the system's P mass balance, P inputs and outputs were similar in magnitude (difference was

The prokaryotic picoplankton (PPP) composition was studied in the tropical high-altitude saline Lake Alchichica, Mexico, using fluorescence in situ hybridization. Thetemporal dynamics of the PPP followed the annual hydrodynamics of this warm-monomictic lake. The abundance of both the autotrophic (APP) and heterotrophic(HPP) functional groups of PPP increased during the mixing period to reachmaximum values at the beginning of the stratification period, when the highesthybridization percentages for the domain Bacteria were also registered. The increasein PPP was most likely related to the availability of nutrients soon after the start ofthe mixing period. Throughout the rest of the stratification, APP and HPP decreasedsignificantly together with the hybridization percentages, mostly related to theexhaustion of nutrients in the mixing zone. In spite of being oligosaline, the prokar-yotic composition of Lake Alchichica was more typical of fresh than of saltier waterbodies. During the mixing period, the Pcy and Planctomycetes (PLA) formed the mainbacterioplankton fraction. Meanwhile throughout the stratification period PLA,Betaproteobacteria (BET) and Gammaproteobacteria (GAM) were the most abun-dant groups in the epilimnion and metalimnion. The domain Archaea (ARCH), BET,GAM, Paracoccus the principal denitrifier of the Alphaproteobacteria and sulphate-reducing bacteria were dominant in the oxycline and hypolimnion in associationwith dissolved oxygen depletion and the nutrient enrichment of the deeper strata.KEYWORDS: prokaryoplankton; warm-monomictic; oligosaline; tropical high-altitude

Summary We investigated net growth rates of distinct bacterioplankton groups and heterotrophic nanoflagellate (HNF) communities in relation to phosphorus availability by analysing eight in situ manipulation experiments, conducted between 1997 and 2003, in the canyon-shaped imov reservoir (Czech Republic). Water samples were size-fractionated and incubated in dialysis bags at the sampling site or transplanted into an area of the reservoir, which differed in phosphorus limitation (range of soluble reactive phosphorus concentrations ‐ SRP, 0.7‐96 m g l - 1 ). Using five different rRNA-targeted oligonucleotide probes, net growth rates of the probe-defined bacterial groups and HNF assemblages were estimated and related to SRP using Monod kinetics, yielding growth rate constants specific for each bacterial group. We found highly significant differences among their maximum growth rates while insignificant differences were detected in the saturation constants. However, the latter constants represent only tentative estimates S ˇ n ˇ r s ˇ C ˇ e ˇ C ˇ e ˇ R ˇ mainly due to insufficient sensitivity of the method used at low in situ SRP concentrations. Interestingly, in these same experiments HNF assemblages grew significantly faster than any bacterial group studied except for a small, but abundant cluster of Betaproteobacteria (targeted by the R-BT065 probe). Potential ecological implications of different growth capabilities for possible life strategies of different bacterial phylogenetic lineages are discussed.

Elimination of inoculated Vibrio cholerae (≥10 7 cells ml -1 ) within a brackish water bacteria assemblage (Mecoacan Lagoon, State of Tabasco, Mexico) was studied in laboratory microcosms with filtration-fractionated water. Feeding of a ciliate, Cyclidium glaucoma was evaluated using fluorescently labelled V. cholerae O1. Even though V. cholerae was not exploited as the major food source, ciliates were able to eliminate it efficiently. An addition of chitin directly supported the growth of bacteria, although not so much of V. cholerae, and indirectly the growth of the protistan assemblage. Generally, the changes in a bacterial assemblage structure were the most important in V. cholerae elimination.

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 (rs = 0.63), β-glucosidase (rs = 0.45), and β-N-acetylhexosaminidase (rs = 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 β-N-acetylhexosaminidase activity (>4 μm fraction). We also documented very tight relationships between activity of both glucosidases and dry weight of Daphnia longispina (rs = 1.0 and 0.60 for α- and β-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. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The microbial loop of a naturally acidic bog lake, Grosse 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 46-80% 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, Ceriodaphnia quadrangula and Diaphanosoma brachyurum. 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.

Extracellular hydrolysis of 4-methylumbelliferyl β-N-acetylglucosaminide was measured in the oligomesotrophic Piburger See and the eutrophic Rimov reservoir during spring and summer phytoplankton blooms, respectively. Total enzymatic activity (TEA) ranged between 0.2 and 19.1 nmol 1 -1 h -1 in the reservoir and between 0.8 and 12.4 nmol 1 -1 h -1 in the lake. High-affinity (K m 100 μmol 1 -1 ) enzymes were kinetically identifiable in most samples from both localities. The low-affinity enzyme activity (LEA) usually accounted for >60% (mean: 80%) of TEA. LEA and diatom biomass significantly correlated over time in the reservoir epilimnion (r s = 0.578) and in the lake metalimnion (r s = 0.862). As diatoms possess chitin and take up its monomer, N-acetylglucosamine, two explanations of the observed relationships are suggested: extracellular β-N-acetylglucosaminidase activity partly originates either from ectoenzymes of chitinolytic bacteria attached to diatom cells or from ectoenzymes of diatoms, enabling them to take up N-acetylglucosamine from ambient amino sugars instead of synthesizing it de novo. A significant positive correlation of LEA with crustacean abundance was found in the lake epilimnion (r s = 0.850), apparently reflecting the growing spring populations of frequently moulting juvenile crustaceans. A possible contribution of chitinolytic bacteria, accompanying the crustacean populations, to LEA is discussed.

Ciliate picoplanktivory was studied in two different freshwater systems with abundant auto- trophic picoplankton (APP): the eutrophic Rimov Reservoir (South Bohemia) during the late summer of 1993 and the oligo- to mesotrophic lake Piburger See (Tyrol) during May 1994. Picoplankton were sized by an image-analysis system and species-specific grazing rates of ciliates on bacteria and APP were determined using fluorescently labelled prey. According to the grazing data (as the mean uptake rate of picoplankton ind.~' h1), ciliates were split into three ecological groups in order of their decreas- ing picoplanktivory and increasing significance of nanophytoplankton in their diet, (i) Highly efficient fine suspension feeders consisting of vorticellids (4200 bacteria, 560 APP), four oligotrichs and Cyclidium sp. (380-1580 bacteria, 57-210 APP), with 2 u.m. (ii) Less efficient fine suspension feeders, coarse filter feeders and detritophages consisting of Cinetochilum margaritaceum, Pelagostrombidium fallax, Cyrtolophosis mucicola and Coleps spp. (60-173 bacteria, 2-27 APP), with 7-32% of individuals ingesting algae >2 n.m. (iii) Raptorial feeders consisting of four prostomatids belonging to the genera Urotricha and Balanion, with a negligible importance of picoplanktivory (8-61 bacteria, 0.2-14 APP). With the exception of one prostomatid. >58% of individuals ingested algae >2 (j.m. Grazing data for the six species from the first group were converted into organic carbon. Assuming a 35 % gross growth efficiency, the calculated potential doub- ling times were between 29 (Hatteria grandinella) and 43 h (an unidentified oligotrich), except for the mixotrophic Pelagohalteria viridis (118 h). The doubling times estimated from changes in ciliate abun- dance for four out of the six species were very close to the calculated values, indicating that this group of heterotrophic ciliates can meet all of its carbon requirements by feeding exclusively on picoplankton.

Participation of nitrilotriacetic acid degrading bacterial strain NTA-1 in the continuous-cultivated mixed culture was studied under different conditions including predation pressure of the ciliate Dexiostoma campyla (STOKES, 1886). From the viewpoint of dispersed/flocculated biomass distribution, significant relationships between NTA-1 and total bacteria ratio, and dispersed and total biomass ratio were proved in the systems without high concentrations of ciliates. The ciliate concentrations reaching 104 ml−1 stabilized flocculated biomass growth without directly affecting NTA-1 portion. Using fluorescently labelled NTA-1 bacteria, filter feeding rates of ciliates were evaluated (maximum individual uptake rate upon NTA-1 bacteria as a number of bacteria per ciliate per hour being 120 h−1 and 260 h−1 under ciliate division rate of 0.3 day−1 and 1 day−1, respectively). Biomass balance showed that dispersed NTA-1 bacteria should not serve as the sole feeding source for these free-swimming ciliates. The role of diversity of mixed bacterial diet in ciliate growth and the role of ciliate predation in stabilizing bacterial assemblage structure was proved.

Phagotrophic protists are diverse and abundant in aquatic and terrestrial environments, making them fundamental to the transfer of matter/energy within their respective food webs. Recognising their grazing impact is essential to evaluate the role of protists in ecosystems, and this includes appreciating prey selectivity. Efforts have been made by groups and individuals to understand selective grazing behaviour by protists: many approaches and perspectives have been pursued, not all of which are compatible. This article, which is not a review, is the product of our discourse on this subject at the SAME 10 meeting. It is the work of individuals, assembled for their breadth of backgrounds, approaches, views, and expertise. Firstly, to communicate ideas and approaches, we develop a framework for selective feeding processes and suggest 6 steps: searching, contact, capture, processing, ingestion, digestion. We then separate study approaches into 2 categories: (1) those examining whole organisms at the community, population, and individual levels, and (2) those examining physiology and molecular attributes. Finally, we explore general problems associated with the field of protistan selective feeding (e.g. linking food selection into food webs and modeling). We do not present all views on any one topic, nor do we cover all topics; instead, we offer opinions and suggest avenues for continued study. Overall, this paper should stimulate further discourse on the subject and provide a roadmap for the future.

Ingestion of fluorescent particles by natural protozoan assemblage was studied in the Řimov Reservoir (Southern Bohemia) from April to October, 1987. Attached and free-living bacterial abundance, proportion of active bacteria, density of suspended particles and biomass of cladocerans were also monitored. Heterotrophic nanoflagellates (HNF; 5–12.8 102ml−1) were the dominant bacterial micrograzers during the spring period and consumed 3 to 9% of the total bacteria per day. After the spring phytoplankton bloom maximum densities of suspended particles and attached bacteria (up to 28% of the total counts) were found. Development of cladocerans in May sharply decreased the proportion of attached bacteria and kept them below 5% of the total counts. All the studied components of plankton except Cladocera decreased during the clearwater phase. The most significant drop was observed in the numbers of protozoans, and they were negligible for bacterial elimination. Bacterial losses during that time apparently were due to cladoceran grazing. During the summer period, ciliates (15–142 ml−1) were mostly dominant micrograzers, and protozoan community grazing increased up to 21% of bacterial standing stock per day. The proportion of active bacteria was strongly correlated with protozoan grazing (r=0.83).

Relationship between bacteria and ciliates was investigated in continuous cultures with and without the recycling of flocculated biomass. Monocultures of ciliates of various ecological types were used together with mixed cultures of bacteria or bacteria and flagellates. The effect of ciliates on the distribution of flocculated and dispersed, biomass was tested with respect to characterizing an optimum mixed culture for activated-sludge treatment. Monoprotozoal populations were not successful in stabilizing the morphology of a bacterial community in a system with recycling of flocculated biomass. Stabilization was observed (with particular ecological types of ciliates) in the system with a high residence time without recycling and/or in the system with the recycling inoculated by both flagellates and ciliates.

We analysed early spring protozooplankton peaks following winter stratification in two different basins (northeast, NE; southwest, SW) of an artificially-divided bog lake Grosse Fuchskuhle (Brandenburg, Germany). The maximum ciliate biomass and numbers (660 and 990 μg l -1 of organic carbon, and 290 and 260 cells ml -1 in NE and SW, respectively) were reached in the surface layer during the ice melting and then continuously decreased. The surface layers were numerically dominated by species of the genus Urotricha, while in the ciliate biomass during the first part of the study, a mixotrophic oligotrich, Pelagostrombidium mirabile prevailed (C org up to 940 μg l -1 ). We observed a conspicuous ciliate peak that could not be related to the feeding activity of the dominant ciliate species upon picoplankton. Seemingly, the peak was related to the specific conditions resulting from the ice melting where these ciliates could be concentrated. Additionally, the mixotrophy of oligotrichous species allowed them to penetrate below the oxycline, although feeding on nanoplankton such as flagellates was also very likely. Our direct measurements of uptake rates of bacteria showed that the abundant strombiduds were not nutritionally dependent on bacterivory. However, the ciliate uptake rate (up to 670 bacteria ciliate -1 h -1 ) contributed significantly to the overall bacteria mortality rate. The maximum (community average) cell-specific ciliate feeding rate of over 200 bacteria ciliates 1 h -1 was reached, along with an increasing contribution of scuticociliates, paralleled by a drop in large oligotrichs.

The in situ growth rates of dominant ciliate species were studied during and shortly after phytoplankton peaks in two water bodies: the eutrophic Ri'mov Reservoir (South Bohemia, Czech Republic) and the oligo-mesotrophic Piburger See (Tyrol, Austria). Growth rate estimates based on changes in ciliate abundances in incubated pre-screened samples (EN) were compared with those derived from the ciliate cell volume and ambient temperatures (£T). The values of EN were always rather lower than those of £T. During the studies, the food supply limited the ciliate growth depending on the ciliate feeding mode. An ecological grouping into filter feeding versus raptorial feeding ('hunt- ing') species, on the one hand, and attached/crawling (browsing) versus free swimming species, on the other hand, clearly affected experimental estimation. Both fine filter feeders (namely attached) and browsers exhibited a calculated £N closer to the theoretical (maximum) £T than did hunters and coarse filter feeders. It was apparent, for example, comparing £N and £T (day1) of the following species: filter feeders Halieria grandinella (£N = 0.42; £T >1.4), Strobilidium hexakinetum (0.34; >1.9), Pelagohalte- ria viridis (0.27; >0.9), Vorticella aquadulcis complex (0.75; >1.0); raptorial Balamon planctonicum (0.65; >1.5), Urotricha furcata (in Ri'mov Reservoir 0.65; >2.1; in Piburger See 0.20; >1.5), Rhab- doaskenasia minima (0.22; >1.0), Askenasia acrostomia (0.12; >0.6); opportunistic Cyrtolophosis mucicola (0.42; >1.6) and Cinetochilum margaritaceum (0.86; >1.4). Predation by rotifers apparently affected measurements in several samples containing —400 rotifers I"1; however, it seemed to be of little importance in the water column.

A limnological survey of eight small, atmospherically acidified, forested glacial lakes in the Bohemian Forest (Sumava, Bohmerwald) was performed in September 2003. Water chemistry of the tributaries and surface layer of each lake was determined, as well as species composition and biomass of the plankton along the water column, and littoral macrozoobenthos to assess the present status of the lakes. The progress in chemical reversal and biological recovery from acid stress was evaluated by comparing the current status of the lakes with results of a survey four years ago (1999) and former acidification data since the early 1990s. Both the current chemical lake status and the pelagic food web structure reflected the acidity of the tributaries and their aluminium (Al) and phosphorus (P) concentrations. One mesotrophic (Plesne jezero) and three oligotrophic lakes (Cerne jezero, Certovo jezero, and Rachelsee) are still chronically acidified, while four other oligotrophic lakes (Kleiner Arbersee, Prasilske jezero, Grosser Arbersee, and Laka) have recovered their carbonate buffering system. Total plankton biomass was very low and largely dominated by filamentous bacteria in the acidified oligotrophic lakes, while the mesotrophic lake had a higher biomass and was dominated by phytoplankton, which apparently profited from the higher P input. In contrast, both phytoplankton and crustacean zooplankton accounted for the majority of plankton biomass in the recovering lakes. This study has shown further progress in the reversal of lake water chemistry as well as further evidence of biological recovery compared to the 1999 survey. While no changes occurred in species composition of phytoplankton, a new ciliate species was found in one lake. In several lakes, this survey documented a return of zooplankton (e.g., Cladocera: Ceriodaphnia quadrangula and Rotifera: three Keratella species) and macrozoobenthos species (e.g., Ephemeroptera and Plecoptera). The beginning of biological recovery has been delayed for ∼20 years after chemical reversal of the lakes.