Search

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 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.

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.

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.

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.

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.

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)

Population dynamics of polyphosphate-accumulating bacteria (PP bacteria) was studied in a laboratory sequencing batch reactor simulating anaerobic-oxic sludge system. The competition between PP bacteria and another microorganism (“G bacteria”) for anaerobic-oxic utilization of acetate as the sole source of organic carbon was observed. The competition was found to be seriously influenced by protozoan and metazoan grazing: Predation-resistant “G bacteria” forming large compact flocs outcompeted PP bacteria. Several breakdowns of enhanced biological phosphorus removal were observed. The first one was related to the development of an euglenid flagellate Entosiphon sulcatus and attached ciliates Vorticella microstoma and V. campanula. The second system collapse was connected with a rapid proliferation of rotifers. An alternative-prey predation was thought to be a mechanism of PP bacteria elimination.