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Nitrogen and phosphorous loading drives eutrophication of aquatic systems. Lakes and reservoirs are often effective N and P sinks, but information is needed on the variability of their biogeochemical dynamics, especially for tropical systems. A long-term N and P mass balance (2003-2018) in a small tropical eutrophic reservoir lake, Valle de Bravo (VB), Mexico, showed it is a net sink of N (-41.7 g N m-2 y-1), and P (-2.7 g P m-2 y-1), mainly through net sedimentation, equivalent to 181% and 68% of their respective loading (23.0 g N m-2 y-1 and 4.2 g P m-2 y-1). N mass balance showed that VB has a high net N atmospheric influx (31.6 g N m-2 y-1), which was 1.3 times the external load, and likely dominated by N2 fixation. During a period of high water level fluctuations (WLF), the net N atmospheric flux decreased by half compared to high level years. WLF can be a useful management tool to improve the trophic status of water bodies by decreasing anoxic conditions and net atmospheric fluxes, possibly through decreasing nitrogen fixation and/or promoting denitrification and other microbial processes that alleviate the N load.

The phytoplankton biomass (chlorophyll-a, Chl-a) is directly related to the total production of lakes. Chl-a in temperate lakes oscillates on an annual scale. However, Chl-a oscillations in tropical lakes have hardly been documented, particularly over multiple years. Here, we described the periodicity of the Chl-a by performing a continuous wavelet analysis of 21 years (1998–2018), monthly Chl-a data from tropical, saline Lake Alchichica, Mexico. Parallel wavelet analyses were made on environmental time series (i.e., euphotic zone, mixed layer, temperature, dissolved oxygen concentration, dissolved inorganic nitrogen, soluble reactive phosphorus, soluble reactive silica). Throughout the time series, the wavelet transforms identified a regular and predictable annual cycle of the Chl-a associated with the warm-monomictic thermal-mixing pattern, the variability of the annual Chl-a cycle, and the presence of other cyclicities, 2-year and ~4–5 years, associated with external forcing agents (e.g., North Pacific Oscillation). The water quality variables display a recurrent annual cycle. At the same time, the trophic variables (nutrient concentration) showed the same cyclicity as Chl-a (1-year, 2-year, and 4-year), suggesting the external forcing agents promote Chl-a augment through nutrient increase made available from stronger, deeper, mixing periods.

In this review, we include rotifers, copepods, and cladocerans, including other groups not usually deemed as zooplankters: i.e., protists, acari, and large branchiopods. The objectives of this study were to integrate the dispersed literature on the taxonomy and diversity of these freshwater zooplankton groups and to explain (1) how these contributions can be arranged in distinct historical periods and (2) how this knowledge has allowed the detection of exotic and threatened species. We divided the freshwater zooplankton studies in Mexico into three historical periods: the first one comprised the 1840s to the 1940s when foreign researchers carried out most studies during several expeditions. Spanish researchers promoted surveys on different zooplankton taxa at the end of this first period. The second period, from the early 1950s to the end of the 1990s of the XX century, showed a remarkably increased research activity in its last ten years only (that is, during the 1990s to 2000), represented by contributions of a new generation of Mexican zooplanktologists. This period yielded more complete zooplankton listings and detailed morphological descriptions of rotifers, cladocerans, copepods, and large branchiopods. The third period started from the year 2000 to date. During this time, listings and online faunistic baselines based on integrative taxonomy have been the primary trend. An account of exotic zooplankters and conservation issues of several native species are discussed. The results of this review show that the knowledge of the freshwater zooplankton of this country has increased significantly over the last 40 years, with at least 408 first records of species for Mexico. Currently, the knowledge of Mexican freshwater zooplankton is among the most complete in the world. However, it is estimated that only a small fraction of the true diversity has been documented.

The pelagic / anoxic hypolimnion population of Spirostomum teres was investigated as a part of the long-term ciliates’ monitoring (2003–2016) in an oligo- to mesotrophic monomictic hyposaline crater lake Alchichica (Puebla / Veracruz, Mexico), including an analysis of picoplankton (both heterotrophic, HPP and autotrophic, APP) and inorganic compounds of nitrogen (ammonium, nitrite, nitrate), phosphorus (dissolved reactive phosphorus, DRP) and silicon. Additionally, detailed studies of the ciliate vertical distribution and feeding activity measured upon fluorescently labelled APP (picocyanobacteria) were carried out. The results were compared with those from a neighbour freshwater crater lake La Preciosa and with a meromictic karstic lake La Cruz (Cuenca, Spain). The ciliate vertical distribution within the water column was very well defined: During the first decade, the benthic population was frequently found throughout a developing stratification of the lake. The established stratification of the lake turned the conditions favourable for the formation of an oxycline / hypolimnion population, typically, several meters below the deep chlorophyll maximum (formed basically by diatoms); the population preferred the layers without detectable dissolved oxygen. However, an observed gradient of light (PAR) could support both oxygenic and anoxygenic photosynthesis. Late stratification after deepening of the thermocline reduced the layers with S. teres population to a minimum apparently due to the drastic change in physicochemical conditions within a metalimnion, coupled with an oxycline, and limited to 1 to 2 meters; microstratification was found. Last years, the very bottom population disappeared or it was reduced and the late stratification S. teres peaks were smaller or did not appeared. Generally, S. teres oxycline / anoxic hypolimnion population was observed from June through November. Optimum picoplankton numbers in conditions that supported the ciliate growth were found: The ciliate was peaking at APP of 0.6 to 1 × 105 cells mL–1; the optimum of HPP was observed round 1.4 × 106 cells mL–1. S. teres was efficiently feeding upon picocyanobacteria in numbers of 105 cells mL–1 reaching the clearance rate of 2000 nL cell–1h–1, which represented in average 130 to 210 cells cell–1h–1 ingested. Feeding upon purple sulphur bacteria was observed but only during the end of the lake stable stratification when the ciliate population was already dropping. On the other hand, the volume specific clearance of S. teres upon picocyanobacteria (103 h–1) did not support the hypothesis that they could serve as a sole prey. Feeding upon eukaryote phytoplankton (chlorophytes Monoraphidium minutum, diatoms Cyclotella choc tawhatcheeana) could be of higher importance that previously supposed. Additionally, a use of ingested and retained photosynthetic prokaryotes is hypothesized.

Nitrogen and phosphorous loading drives eutrophication of aquatic systems. Lakes and reservoirs are often effective N and P sinks, but the variability of their biogeochemical dynamics is still poorly documented, particularly in tropical systems. To contribute to the extending of information on tropical reservoirs and to increase the insight on the factors affecting N and P cycling in aquatic ecosystems, we here report on a long-term N and P mass balance (2003–2018) in Valle de Bravo, Mexico, which showed that this tropical eutrophic reservoir lake acts as a net sink of N (−41.7 g N m y) and P (−2.7 g P m y), mainly occurring through net sedimentation, equivalent to 181% and 68% of their respective loading (23.0 g N m y and 4.2 g P m y). The N mass balance also showed that the Valle de Bravo reservoir has a high net N atmospheric influx (31.6 g N m y), which was 1.3 times the external load and likely dominated by N fixation. P flux was driven mainly by external load, while in the case of N, net fixation also contributed. During a period of high water level fluctuations, the net N atmospheric flux decreased by 50% compared to high level years. Our results outlining water regulation can be used as a useful management tool of water bodies, by decreasing anoxic conditions and net atmospheric fluxes, either through decreasing nitrogen fixation and/or promoting denitrification and other microbial processes that alleviate the N load. These findings also sustain the usefulness of long-term mass balances to assess biogeochemical dynamics and its variability., This research was funded by UNAM, PAPIIT-IN207702 and CONACYT-SEMARNAT, C01-1125 projects to M.M-I

The response of soil microbiota to hydrocarbon contamination has been studied normally several months after the event. However, as those studies represent a “long-term” context for microbial processes, since protozoa succession can be achieved after 28 days, we wonder how fast does trophic structure of ciliates and flagellates recover from a strong pulse of petroleum contamination under the influence of Medicago sativa. We hypothesized that the root effect of M. sativa would promote faster recovery of the protozoan trophic structure, in comparison with the level reached in unplanted microcosms. The abundances of individuals, species, and trophic groups of ciliates and flagellates were determined at 1, 7, 14, and 30 days after a single pulse of 50,000 ppm of light petroleum on soil microcosms unplanted and planted with M. sativa, and their respective controls. Protozoan diversity and trophic groups were strongly reduced immediately after the pulse of contamination. Ciliates and flagellate trophic diversity increased steadily in all microcosms after 7 days. However, unplanted contaminated and planted contaminated microcosms remained the poorest communities and reached full recovery of trophic groups after 30 days. Also, the protozoan communities were segregated into 2 groups: the first from petroleum and the second formed by non-polluted microcosms. These results suggest that petroleum is a strong selection factor leading to an alternative protozoan community composition and the root effect of M. sativa promotes faster recovery of ciliate and flagellate communities after a devastation produced by a strong pulse of petroleum contamination.

The nitrogen (N) cycle is highly dependent on microbial processes. Distribution of these communities is one of the most important factors in the variation of the N cycling in warm‐monomictic lakes. However, the effects of the alternation of water stratification and mixing periods on the ecology of microbial communities involved in these processes are rarely studied in this type of aquatic ecosystem. We explored vertical and seasonal patterns of picoplankton and the genetic potential for ammonium oxidation (amoA gene for bacteria and archaea), denitrification (nirS and nirK), anammox (hzsA), and DNRA (nrfA) and their relationships with the main limnological variables in Lake Alchichica (Central Mexican Plateau) to provide insight into the distribution and importance of these planktonic communities in warm‐monomictic tropical lakes. Ten depths were sampled during late stratification (November 2015) and mixing (February 2016) periods, covering the epilimnion, metalimnion (oxycline), and hypolimnion layers in the first case. We showed that temperature and oxygen stratification shaped the distribution of picoplankton and functional N genes in this lake. These communities also varied in relation to nutrient availability and underwent temporal changes throughout the water column. The amoA genes, along with autotrophic picoplankton, were more abundant during the stratification, indicating that nitrification could be potentially more important during this period, mainly at the oxycline layer. Denitrifying genes showed strong variations during the stratification period, with highest gene copy numbers at the oxycline and hypolimnion layers. Anoxic conditions were characterised by a relative increase in the abundance of the nrfA gene with depth, which was positively correlated with NH₄ ⁺ concentration. On the other hand, the hzsA gene was not detected in any sample. Our findings highlight the importance of thermal stratification as one of the main factors influencing the genetic potential for N transformations within the water column in warm‐monomictic tropical lakes.

The response of planktonic (phytoplankton, ciliates, rotifers and crustaceans) and littoral (Ephemeroptera, Plecoptera, Trichoptera and Heteroptera: Nepomorpha) assemblages to chemical recovery was studied over a twelve-year period (1999–2011) in eight glacial lakes in the Bohemian Forest (central Europe). The region suffered from high atmospheric pollution from the 1950s to the late 1980s, but has since been recovering from acidification due to 86% and 44% decrease in sulphur and nitrogen deposition, respectively, during the 1990s–2000s. Despite the rapid improvement in water chemistry of all the eight studied lakes, only four have partly recovered so far (low-aluminium lakes), while the other four lakes still remain strongly acidic (high-aluminium lakes). Although biotic responses (especially in the low-Al lakes) showed important signs of recovery, such as reappearance of some indigenous or acid-sensitive species, decline in eurytopic acid-tolerant species and colonisation by vagile species, the assemblages of all the lakes still suffer from acid stress. Our results also indicate an increasing role of biotic interactions between colonisers and residents leading to the reconstruction of aquatic food webs in the low-Al lakes.

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

The impact of feeding on autotrophic picoplankton (APP) on the ciliate composition of the assemblage was surveyed monthly along a depth gradient in the maar crater, athalassohaline, warm monomictic Lake Alchichica (Puebla, Mexico) from June 2003 to December 2005. Numbers of APP were evaluated from their autofluorescence. DAPI staining and the Fluorescently Labeled Bacteria technique were employed to count ciliates and estimate their feeding rates. A total of 38 taxa of ciliates have been identified using Quantitative Protargol Staining. Peritrichs followed by minute spirotrichs (particularly Halteria grandinella) often numerically dominated the ciliate assemblage and emerged as the most efficient APP feeders. A maximum of 54 ciliate cells ml(-1) was observed in the surface layer at the end of the mixing period, during the development of diatoms (Cyclotella alchichicana), the cyanobacterial bloom (Nodularia sp.) and its decay. Vorticellids (Pelagovorticella natans, Vorticella sp.) had the highest APP uptake (median 130 APP cil(-1) h(-1)). Mixotrophic Euplotes cf. daidaleos were important APP grazers near the oxycline. Scuticociliates (Cyclidium glaucoma, Uronema nigricans and an anaerobic cf. Isocyclidium globossum), were numerically dominant within the hypolimnetic assemblages and did not ingest APP. Generally, APP were not an important food source for the majority of the ciliate assemblage, being positively selected by a few species during the APP decay in aerobic and microaerobic conditions.

The recent interest in the plankton structures and dynamics in tropical and subtropical lakes has revealed important trends that set these lakes apart from temperate lakes, and one of the main differences is the enhanced importance of the microbial food web with respect to net plankton. Ciliates are a key component of subtropical and tropical microbial webs because of their role as dominant picoplankton grazers and their ability to channel picoplankton production to the uppermost trophic levels. Plankton ciliates have been found to play a crucial role in the survival of fish larvae in lakes that share several features with Lake Catemaco, a eutrophic tropical Mexican lake. Therefore, the plankton ciliate composition, abundance, and biomass of Lake Catemaco were studied to assess their role in the microbial food web. The data were obtained from surface and bottom water samples collected at eleven points during three surveys in 2011 and an additional survey in 2013, with the surveys covering the local climatic seasons. The most abundant components of the plankton ciliate assemblages were small prostomatids (Urotricha spp.), choreotrichs (Rimostrombidium spp.), cyclotrichs (Mesodinium and Askenasia), and scuticociliates (Cyclidium, Cinetochilum, Pleuronema, and Uronema). Other important ciliates in terms of abundance and/or biomass were haptorids (Actinobolina, Belonophrya, Monodinium, Paradileptus, and Laginophrya), Halteria, oligotrichs (Limnostrombidium and Pelagostrombidium), Linostomella, Bursaridium, Cyrtolophosis, and Litonotus. The ciliate abundance averaged 57 cells mL-1 and ranged from 14 to 113 cells mL-1. The mean ciliate biomass was 71 µg C L-1 and ranged from 10 to 202 µg C L-1. Differences were not detected in ciliate abundance or biomass between the sampling points or sampling depths (surface to bottom); however, significant differences were observed between seasons for both variables. Nano-sized filamentous cyanobacteria were the most abundant component of the plankton, and their abundance was assessed through epifluorescence microscopy counts. The autotrophic and heterotrophic picoplankton abundance was measured through epifluorescence, and their abundance and biomass were higher at the study site relative to other shallow freshwater ecosystems. The total ciliate biomass distribution patterns were similar to those of filamentous cyanobacteria and autotrophic or heterotrophic picoplankton, although the nanociliate biomasses peaked when the picoplankton and filamentous cyanobacteria were least abundant. The consequences of this increased importance of ciliates on the structure of the plankton at Lake Catemaco will be discussed along with the probable causes.

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.

Seasonal changes in the structure of ciliate assemblages in eleven oligotrophic mountain lakes at different altitudes, covering a wide pH range from 4.9 to 6.9, were studied. Seven mountain lakes lay above the timberline (0vre NeÅdalsvatn and Stavsvatn, Norway; Lochnagar, Scotland; Starolesnianske pleso and Nizne Terianske pleso, Slovak Republic; Chuna ozero, Russia; Lago Paione Superiore, Italy) and four acidified lakes (Cerne, Certovo, Plesne and Prasilske jezero, Sumava Mountains, Czech Republic) in the mountain forest. Additionally, thr tropical high latitude Lago de Alchichica (Mexico) was analysed. Ciliate taxons were identified using the quantitative protargol staining approach, and feeding patterns were tentatively detected by using fluorescence microscopy methods. Nano- to microphytoplankton hunters and/ or mixotrophic ciliates prevailed in all acidic lakes. Numbers of ciliates were very low (seasonal lake mean below 200 cells/I) except in two Tatra lakes (to 21000 cells/I). Prostomes of the genera Urotricha, Holophrya and Prorodon dominated in most of the samples, both numerically as well as in biomass (seasonal lake mean from 13.6 to 100% and 4.6 to 99.7%, respectively), particularly in acidified water lakes (Starolesnianske, Certovo and Prasilske jezero). Among mixotrophs, oligotrichs of genera Pelagostrombidium and Limnostrombidium were the most prominent (up to 49.9 and 64.6%, respectively). The picoplankton-feeders (minute oligotrichs, peritrichs and scuticociliates) were found to be an important component even in the oligotrophic environment but this ecological type never dominated within the water column (up to 22.2 and 30.3 %, respectively). Major food sources of gymnostomes, prevailing only in Lochnagar (82.5 and 87.5 %, respectively; Mesodinium sp.), remained unclear; large Askenasia spp. were apparently mixotrophic. Using a cluster analysis, the lakes were grouped according to the total numbers of ciliates and contributions of distinct ecological groups with different feeding patterns. Within low acidic lakes, Lochnagar differed from all others. The rest of the lakes was divided into two subgroups: remote pristine lakes of the Northern transect (the Norwegian lakes and Chuna ozero), and the others, geographically affiliated to the south transect (Pyreneans, Alps, Tatra Mts.). Such a differentiation is in good agreement with the clustering based on chemical parameters reported for the lakes. No marked differences in the ciliate distribution were detected when the above timberline- and forest surrounded mountain lakes were compared.

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)

International audience; Since ciliates rarely possess structures that easily fossilize, we are limited in our ability to use paleontological studies to reconstruct the early evolution of this large and ecologically important clade of protists. Tintinnids, a group of loricate (house-forming) planktonic ciliates, are the only group that has a significant fossil record. Putative tintinnid fossils from rocks older than Jurassic, however, possess few to no characters that can be found in extant ciliates; these fossils are best described as `incertae sedis eukaryotes'. Here, we review the Devonian fossil Nassacysta reticulata and propose that it is likewise another `incertae sedis eukaryote due to the lack of any unambiguous ciliate characters. Future tintinnid fossil descriptions would be most helpful if: (i) neutral terminology is used in the descriptions but ciliate-specific terminology in the interpretations; (ii) the current ciliate classification is used, although fossil data may expand or modify classifications based on modem forms; (iii) close collaboration with specialists studying extant ciliates is done; and (iv) editors include an expert of extant ciliates in the review process. (C) 2015 Elsevier B.V. All rights reserved.

Dissolved organic phosphorus utilization by different members of natural communities has been closely linked to microbial alkaline phosphatases whose affiliation and diversity is largely unknown. Here we assessed genetic diversity of bacterial alkaline phosphatases phoX and phoD, using highly diverse microbial consortia (microbialites and bacterioplankton) as study models. These microbial consortia are found in an oligo-mesotrophic soda lake with a particular geochemistry, exhibiting a low calcium concentration and a high Mg : Ca ratio relative to seawater. In spite of the relative low calcium concentration in the studied system, our results highlight the diversity of calcium-based metallophosphatases phoX and phoD-like in heterotrophic bacteria of microbialites and bacterioplankton, where phoX was the most abundant alkaline phosphatase found. phoX and phoD-like phylotypes were more numerous in microbialites than in bacterioplankton. A larger potential community for DOP utilization in microbialites was consistent with the TN : TP ratio, suggesting P limitation within these assemblages. A cross-system comparison indicated that diversity of phoX in Lake Alchichica was similar to that of other aquatic systems with a naturally contrasting ionic composition and trophic state, although no phylotypes were shared among systems.

(2000). Pelagic food webs and microbial loop in low-alkalinity mountain lakes. SIL Proceedings, 1922-2010: Vol. 27, No. 1, pp. 513-516.

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.

Autotrophic picoplankton (APP) was monitored regularly in Lake Alchichica (Puebla/Veracruz; Mexico) for nearly 10 years. In the present study, the control of APP development by viruses is analysed. The maximum cyanobacteria abundance was localised within the water column using WET-Labs ECO Triplet fluorometer and oxygen production by 2µm fraction was measured at four depths using clear & dark bottles. Autofluorescence and DAPI staining combined with an image analysis (Image J) were applied to evaluate APP and bacterial biomass, respectively. The Sherrs´ fluorescently labelled bacteria-protocol was employed to evaluate a role of ciliates. Viruses were estimated microscopically as "virus like particles" according to SYBR Gold protocol, taking CCD camera images. APP were peaking (over 106 cells/mL) during transition between mixing- and stratification period, being responsible for important oxygen production only within the epilimnion; Synechococcus strains were molecularly identified in the isolates. Viruses (107 to 108 VLP/mL) peaked in the surface layer during mixing or in the metalimnion, being coupled apparently more tightly with APP than with bacteria numbers. Both elevated ciliates' APP elimination during mixing period, and the maximum APP elimination around the oxycline in the late stratification roughly coincided with VLP maxima.

The most updated picture of the 16S rRNA phylogenetic tree of picocyanobacteria, including marine and freshwater strains, confirms the indication that Synechococcus 5.3 is not included in Synechococcus 5.1 but is its sister clade. The non-marine picocyanobacteria are considered as closely related to Cyanobium strains and proposed as monophyletic "Cyanobium-like" lineage. Nevertheless the relationships of these lineages to one another remain ambiguous. From this comes the need to enlarge the database with new sequences of isolated Synechococcus strains from a wider range of locations and environments to obtain a more realistic view of Synechococcus genus dispersal and evolution. In this study we aimed to resolve better the Synechococcus phylogenetic tree enlarging the sequence data set of non-marine picocyanobacteria including strains isolated from "extreme" lakes. Therefore we selected four high altitude athalassohaline maar crater-lakes in Mexico as a source for "non-marine halotolerant" Synechococcus, five glacial ultraoligotrophic North Patagonian lakes as extreme ecosystems, and six Italian lakes of glacial, volcanic and morenic origin, with different trophic conditions. Our objective was to create a new phylogenetic tree of non-marine Synechococcus upgraded adding 43 new sequences derived from isolated picocyanobacteria and to refine the taxonomic relationships between non-marine Synechococcus clades and Synechococcus 5.2 and 5.3. The new sequences fall into a number of both novel and previously described clades within the phylogenetic tree of 16S rRNA gene. The new cluster of Lake Nahuel Huapi (North Patagonia) forms a sister clade to the subalpine cluster II and the marine Synechococcus sub-cluster 5.2. Our finding of the novel clade of "Halotolerants" close to the marine sub-cluster 5.3 (Synechococcus RCC307) constitutes an important demonstration that euryhaline and marine strains affiliate closely. The intriguing results obtained shed new light on the importance of the non-marine halotolerants in the phylogenesis of picocyanobacteria.

In late summer 1993 an intensive study was carried out on protozoan grazing in the epilimnion and metalimnion of the eutrophic Rimov Reservoir in south Bohemia. On average, - 70% of bacterial production was consumed by heterotrophic flagellates and -20% by ciliates. Ciliate numbers increased from 5 to 70 cells ml l over the 5-week study period. Ciliates ~30 pm in size were numerically dominant in both layers and included Halteria grandinella and Strobilidium hexakinetum (Oligotrichida), Cyrtolophosis mucicola (Cyrtolophosida), Cinetochilum margaritaceum (Scuticociliatida), Urotricha spp., and Coleps sp. (Prostomatida). Ciliate species-specific grazing rates on bacteria and picocyanobacteria were determined. The highest individual cell grazing rates, 4,200 bacteria and 560 picocyanobacteria cell-’ h-l, were observed in Vorticella aquadulcis-complex. Oligotrichs ingested on average 360-2,130 bacteria and 76-2 10 picocyanobacteria cell ’ h - I, with H. grandinella (1,560 bacteria cell I h-l), due to its high abundance, as the most important ciliate bacterivore within the system. C. mucicola ingested on average 173 bacteria and 27 cyanobacteria cell- ’ h- l; C. margaritaceum, 57 bacteria and 7 picocyanobacteria cell-’ h--l; and prostomatids, 23-100 bacteria and 2-14 picocyanobacteria cell-’ h- I. Although there was a tight relationship between grazing rates on bacteria and picocyanobacteria (rs = 0.89, n = 12, P < 0.00 I), most of the ciliate species preferred larger picoplankton (i.e. picocyanobacteria), as indicated by their clearance rates. According to our data, several oligotrichous ciliate species and Cyclidium sp. can grow in pelagic conditions and exclusively on picoplankton food at rates of one doubling every 24-75 h.

Swelling over the face in infancy may be caused by parotitis, abscess glandulae parotis, osteomyelitis, purulent lymphonode, bone tumor or by infantile cortical hyperostosis (the Caffey-Silverman syndrome). The case report shows the typical clinical, laboratory and radiographic changes seen in infantile cortical hyperostosis. It is a benign condition, often disappearing spontaneously but also cases of recurrence are known. The knowledge of the typical signs and laboratory findings enables the determination of a quick and reliable diagnosis.

To evaluate trophic relationships between ciliates and bacterioplankton during the stratification period in a lake, samples from three different layers [the upper part of the metalimnion (UM), the base of the metalimnion (BM) and the hypolimnion] were studied. The autotrophic picoplankton numbers, phylogenetic composition of heterotrophic picoplankton (HPP), and HPP composition in ciliates’ food vacuoles were analyzed. Additionally, in situ incubations in dialysis bags were performed at the same selected depths to assess potential changes in picoplankton composition related to ciliates’ feeding activity. Among the in situ HPP assemblage, no phylogenetic group dominated in the selected layers within the course of the study. The ciliate assemblage was dominated by scuticociliates, haptorids, and hypotrichs. Comparing ciliates’ food vacuole content and in situ HPP composition, a high preference for A lphaproteobacteria was found at all three depths. P lanctomycetes , and D elta - and G ammaproteobacteria were selected at BM and hypolimnion, respectively. However, selection of a given phylogenetic group in the time course of this study was observed only for A lphaproteobacteria in the UM. Similar trends were found in the incubations, but no relationship was found between the vacuole content of the ciliates and changes in HPP composition.

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.

The distribution of the ciliate assemblage was surveyed along a depth gradient in the maar crater, athalassohaline, warm monomictic Lake Alchichica (Puebla, Mexico) from June 2003 to October 2006 (monthly). DAPI staining was employed to count ciliates while the Quantitative Protargol Staining was used for their identification. Peritrichs often numerically dominated the ciliate assemblage; a maximum of 54 cells ml #1471;¹ (Rhabdostyla sp.) was observed in the surface layer at the end of the mixing period, during the development of diatoms (Cyclotella alchichicana), cyanobacterial (Nodularia sp.) bloom and its decay. Minute spirotrichs (particularly Halteria grandinella) and a haptorid, Belonophrya pelagica occasionally dominated the epilimnion while mixotrophic Euplotes cf. daidaleos and Pelagothrix sp. were important round the oxycline along with haptorids, particularly Phialina sp. Scuticociliates Cyclidium glaucoma, Uronema nigricans, and anaerobic ciliates Isocyclidium globosum and Caenomorpha sp. dominated within the hypolimnetic assemblages.

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).

In situ elimination of fluorescently labelled Vibrio cholerae (FLB) was measured in two saline water bodies in Mexico: in a brackish water lagoon Mecoaca´n (Gulf of Mexico; State of Tabasco) and an athalassohaline lake Alchichica (State of Puebla). Disappearance rates of fluorescently labelled V. cholera O1 showed that they were eliminated from the environment at an average rate of 32% and 63 %/day respectively (based on the bacterial standing stocks). The indirect immunofluorescence method confirmed the presence of V. cholerae O1 in the lagoon. However the elimination of FLB was not directly related either to the presence or absence of the bacterium in the water body or to the phytoplankton concentration. (authors)