Virus and microbial loop dynamics over an annual cycle in three contrasting Antarctic lakes

1. Viral and microbial loop dynamics were investigated over an annual cycle in three contrasting saline Antarctic lakes – Highway Lake (salinity 4‰), Pendant Lake (salinity 19‰) and Ace Lake, a meromictic system (with a mixolimnion salinity of 18‰) in order to assess the importance of viruses in extreme, microbially dominated systems. 2. Virus like particles (VLP) showed no clear seasonal pattern, with high concentrations occurring in both winter and summer (range 0.89 × 10⁷ ± 0.038 to 12.017 × 10⁷ ± 1.28 mL^?1). VLP abundances reflected lake productivity based on chlorophyll a concentrations. Bacterial abundances and biomass did not correlate with VLP numbers except in Pendant Lake, the most productive of the three lakes studied. 3. Pendant Lake supported the highest bacterial biomass (range Highway: 18.44 ± 1.35 to 59.43 ± 2.80 ng C mL^?1; Ace: 14.42 ± 2.69 to 68.39 ± 2.95 ng C mL^?1; Pendant: 31.36 ± 3.94 to 115.95 ± 4.49 ng C mL^?1) so that virus to bacteria ratios (VBR) (range 30.48 ± 7.96 to 96.67 ± 8.21) were higher in Ace Lake (range 30.58 ± 3.98 to 80.037 ± 1.60) and Highway Lake (range 18.63 ± 3.12 to 126.74 ± 6.50). 4. Negative correlations occurred between VLP and cryptophytes (dominant phototrophic nanoflagellates), suggesting that they were not hosts to lytic viruses. Among the other protists only the heterotrophic nanoflagellates of Highway Lake (dominated by the marine choanoflagellate Diaphanoeca grandis) showed a positive correlation with VLP. 5. The VLP was negatively correlated with photosynthetically active radiation (PAR) and temperature, both of which increased with ice thinning and breakout, increasing viral decay. In winter VLP probably persisted in cold, dark water. 6. High VLP concentrations and high VBR (values at the upper end of those reported for marine and lacustrine systems) indicated that viruses, most of which were probably bacteriophage, are a major element within the microbial communities in extreme, saline lakes.     

Viral dynamics and patterns of lysogeny in saline Antarctic lakes

Antarctic lakes are extreme ecosystems with microbially dominated food webs, in which viruses may be important in controlling community dynamics. A year long investigation of two Antarctic saline lakes (Ace and Pendant Lakes) revealed high concentrations of virus like particles (VLP) (0.20–1.26 × 10⁸ ml⁻¹), high VLP: bacteria ratios (maximum 70.6) and a seasonal pattern of lysogeny differing from that seen at lower latitudes. Highest rates of lysogeny (up to 32% in Pendant Lake and 71% in Ace Lake) occurred in winter and spring, with low or no lysogeny in summer. Rates of virus production (range 0.176–0.823 × 10⁶ viruses ml⁻¹ h⁻¹) were comparable to lower latitude freshwater lakes. In Ace Lake VLP did not correlate with bacterial cell concentration or bacterial production but correlated positively with primary production, while in Pendant Lake VLP abundance correlated positively with both bacterial cell numbers and bacterial production but not with primary production. In terms of virus and bacterial dynamics the two saline Antarctic lakes studied appear distinct from other aquatic ecosystems investigated so far, in having very high viral to bacterial ratios (VBR) and a very high occurrence of lysogeny in winter.     

Light climate and phytoplankton photosynthesis in maritime Antarctic lakes

The responses of phytoplankton populations to seasonal changes in radiation flux in two Antarctic lakes with extensive winter ice-cover are described. A phytoplankton capable of photosynthesis was found throughout the year in both systems. During winter, low incident radiation combined with thick layers of snow and ice prevented in situ photosynthesis becoming detectable. The beginning of spring was marked by a reduction in snow cover which resulted in a considerable increase in surface penetrating radiation. Planktonic algae rapidly adapted to utilise these increased levels efficiently, though they still showed characteristics of strong shade adaptation.Loss of ice cover at the start of the short open water period further increased the radiation levels and a summer population developed which was much less shade adapted. Saturation and photoinhibition effects were widespread during this period as the algae proved unable to utilise high radiation levels efficiently. They were however effective at the radiation fluxes prevalent in the lower part of the rapidly circulating water columns.     

Water quality and plankton periodicity in two contrasting mine lakes in Jos,Nigeria

Water quality and plankton periodicity was studied in two mine lakes near Jos in the younger granite area of Plateau State, Nigeria. The investigation was carried out for 8 months. Transparency, pH and NO₃-N were significantly higher in Lake II while DOM, alkalinity, PO ₄ ^3– -P, BOD and chloride were significantly higher in Lake I. The order of dominance in Lake I was Bacillariophyceae, Cyanophyceae, Chlorophyceae and Dinophyceae while for Lake II; Chlorophyceae, Cyanophyceae, Bacillariophyceae and Dinophyceae. In Lake I the zooplankton in order of dominance were Cladocera, Rotifera, Copepoda, and nauplii and in Lake II Cladocera, Copepoda, Rotifera and nauplii. Although both lakes would seem unproductive based on the PO ₄ ^3– -P and NO ₃ ^– -N levels, Lake I appears more productive than Lake II.     

Microbial diversity and function in Antarctic freshwater ecosystems

Freshwater lakes occur through much of Antarctica and are characterized by short food chains dominated by microbes. Comparatively few studies have been made of continental freshwater lakes until recently, with the main emphasis being on the less extreme maritime Antarctic lakes. The wide range of trophic status seen at the northern extremes of the maritime Antarctic reduces markedly further south, but a wide range of micro-organisms occur throughout the latitudinal range. Information on seasonal and spatial patterns of microbial activity for freshwater lakes demonstrate rapid changes in community composition at certain times of year despite constant low temperatures. Benthic communities of cyanobacteria and bacteria are a feature of most lakes and are involved in a wide range of geochemical cycling. There is a need for more detailed taxonomic information on most groups and considerable potential for molecular studies.     

Characteristics of oligotrophic hardwater lakes in a postglacial land-rise area in mid-Sweden

1. We describe some remarkable ephemeral, oligotrophic hardwater lakes formed because of land rise in the coastal areas of the Baltic Sea, that are unique in Sweden and probably also worldwide. Two younger, coastal lakes were studied by regular sampling for 1 year and compared with an older (i.e. greater altitude) lake, that passed through the oligotrophic hardwater stage some 3–4000 years ago. 2. Despite some differences in composition of the catchment, the two younger lakes were similar with regard to water chemistry and plankton community composition. The concentration of phosphorus was low while nitrogen was high, resulting in very high N/P quotients (101 and 131). Although water colour was moderate, the concentration of organic carbon was extremely high (average values of ≥ 20 μg TOC L^–1), consisting mainly of dissolved compounds (DOC). 3. While the plankton was poorly developed, sediments in both lakes were covered by a layer of photosynthesising micro‐organisms. This substantial `microbial mat', which has not been described in detail before, was up to 15 cm thick and dominated by cyanobacteria and purple sulphur bacteria. The concentration of sediment phosphorus was extremely low (352 μg g^–1 dw) in one of the lakes and dominated by organic‐bound (residual) phosphorus. 4. Deep sediments in the older lake, representing its oligotrophic hardwater period, differed in phosphorus composition from the currently oligotrophic hardwater lakes by having a strong dominance of HCl‐extractable (Ca‐bound) phosphorus. This indicates that phosphorus, initially organic‐bound within the microbial mat, is subsequently bound to calcium. We hypothesise that this is promoted by the environmental conditions created by the benthic photosynthetic activity, in combination with the prevailing hardwater conditions. 5. The rich and flourishing microbial community on the sediments may also explain the high concentration of DOC in the lake.     

Comparative analysis of bacterioplankton assemblages from maritime Antarctic freshwater lakes with contrasting trophic status

The bacterioplankton assemblages of eight maritime Antarctic lakes with a wide range of trophic status and geographic span (six lakes from Hope Bay, Antarctic Peninsula and two from Potter Peninsula, King George Island) were described using denaturing gradient gel electrophoresis and band sequencing during two consecutive austral summers (2003–2004). Analyses of the gels identified a total of 230 bands spread across 57 different positions. Among those bands, 14 were shared between lakes from Hope Bay and Potter Peninsula, 17 were observed only in particular lakes, and 17 were registered both years in the same lake. We successfully reamplified and sequenced 43 bands located in 36 different positions belonging to Bacteroidetes, Actinobacteria, Betaproteobacteria and Cyanobacteria. The closest matches for 63% of the sequenced bands were from Antarctic or from other cold environment clones and sequences already in the databases, suggesting the widespread dominance of microbial communities adapted to cold habitats. The results of the multivariate analyses (Cluster Analysis and CCA) indicated that the nutrient status of the lake influences the bacterioplankton assemblages. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Dynamics and trophic roles of heterotrophic protists in the plankton of a freshwater tidal estuary

Freshwater tidal estuaries comprise the most upstream reaches of estuaries and are often characterised by the presence of dense bacterial and algal populations which provide a large food source for bacterivorous and algivorous protists. In 1996, the protistan community in the freshwater tidal reaches of the Schelde estuary was monitored to evaluate whether these high food levels are reflected in a similarly high heterotrophic protistan biomass. Protistan distribution patterns were compared to those of metazoan zooplankton to evaluate the possible role of top-down regulation of protists by metazoans. Apart from the algivorous sarcodine Asterocaelum, which reached high densities in summer, heterotrophic protistan biomass was dominated by ciliates and, second in importance, heterotrophic nanoflagellates (HNAN). HNAN abundance was low (annual average 2490 cells ml^–1) and did not display large seasonal variation. It is hypothesised that HNAN were top-down controlled by oligotrich ciliates throughout the year and by rotifers in summer. Ciliate abundance was generally relatively high (annual average 65 cells ml^–1) and peaked in winter (maximum 450 cells ml^–1). The decline of ciliate populations in summer was ascribed to grazing by rotifers, which developed dense populations in that season. In winter, ciliate populations were probably regulated `internally' by carnivorous ciliates (haptorids and Suctoria). Our observations suggest that, in this type of productive ecosystems, the microbial food web is mainly top-down controlled rather than regulated by food availability.     

Diversity and pharmaceutical screening of fungi from benthic mats of Antarctic lakes

During the MICROMAT project, the fungal diversity of microbial mats growing in the benthic environment of Antarctic lakes was accessed for the discovery of novel antibiotics and anticancers. In all, 160 filamentous fungi belonging to fifteen different genera and 171 yeasts were isolated from 11 lakes, classified and cultivated in different media and at different temperatures. Filamentous fungi were then screened to discover novel antimicrobial and cytotoxic compounds. A total of 1422 extracts were prepared by solid phase extraction of the culture broths or by biomass solvent extraction. 47 (29%) filamentous fungi showed antimicrobial activity; most of them inhibited the growth of gram-positive Staphyloccus aureus (14%), gram-negative E. coli (10%), and of yeasts Candida albicans (11%) and Cryptococcus neoformans (8%). Less activity was detected against representatives of enterobacteria and filamentous fungi. The most productive in terms of bioactivities were cold-tolerant cosmopolitan hyphomycetes such as Penicillium, Aspergillus, Beauveria and Cladosporium. Two bioactive bis-anthraquinones (rugulosin and skyrin) were identified by LC–MS as the main products in a strain of Penicillium chrysogenum isolated from a saline lake in the Vestfold Hills. LC–MS fractionation of extracts from two diverse species of Aspergillus, that exhibited relatively potent antimicrobial activities, evidenced a chemical novelty that was further investigated. To our knowledge, this is the first report of new antibiotics produced by fungi from benthic microbial mats from Antarctic lakes. It can be concluded that these microbial assemblages represent an extremely rich source for the isolation of new strains producing novel bioactive metabolites with the potential to be developed as drugs.     

Cyanobacteria from benthic mats of Antarctic lakes as a source of new bioactivities

Aims:  To exploit the cyanobacterial diversity of microbial mats growing in the benthic environment of Antarctic lakes for the discovery of novel antibiotic and antitumour activities.
Methods and results:  In all, 51 Antarctic cyanobacteria isolated from benthic mats were cultivated in the laboratory by optimizing temperature, irradiance and mixing. Productivity was generally very low (≤60 mg l⁻¹ d⁻¹) with growth rates ( μ ) in the range of 0·02–0·44 d⁻¹. Growth rates were limited by photosensitivity, sensitivity to air bubbling, polysaccharide production or cell aggregation. Despite this, 126 extracts were prepared from 48 strains and screened for antimicrobial and cytotoxic activities. Seventeen cyanobacteria showed antimicrobial activity (against the Gram-positive Staphylococcus aureus , the filamentous fungus Aspergillus fumigatus or the yeast Cryptococcus neoformans ), and 25 were cytotoxic. The bioactivities were not in accordance with the phylogenetic grouping, but rather strain-specific. One active strain was cultivated in a 10-l photobioreactor.
Conclusions:  Isolation and mass cultivation of Antarctic cyanobacteria and LC-MS (liquid chromatography/mass spectrometry) fractionation of extracts from a subset of those strains (hits) that exhibited relatively potent antibacterial and/or antifungal activities, evidenced a chemical novelty worthy of further investigation.
Significance and impact of the study:  Development of isolation, cultivation and screening methods for Antarctic cyanobacteria has led to the discovery of strains endowed with interesting antimicrobial and antitumour activities.     

The biodiversity and ecology of Antarctic lakes: models for evolution

Antarctic lakes are characterised by simplified, truncated food webs. The lakes range from freshwater to hypersaline with a continuum of physical and chemical conditions that offer a natural laboratory in which to study evolution. Molecular studies on Antarctic lake communities are still in their infancy, but there is clear evidence from some taxonomic groups, for example the Cyanobacteria, that there is endemicity. Moreover, many of the bacteria have considerable potential as sources of novel biochemicals such as low temperature enzymes and anti-freeze proteins. Among the eukaryotic organisms survival strategies have evolved, among which dependence on mixotrophy in phytoflagellates and some ciliates is common. There is also some evidence of evolution of new species of flagellate in the marine derived saline lakes of the Vestfold Hills. Recent work on viruses in polar lakes demonstrates high abundance and high rates of infection, implying that they may play an important role in genetic exchange in these extreme environments.     

Biogeographical distribution and ecological ranges of benthic cyanobacteria in East Antarctic lakes

For the first time, the cyanobacterial diversity from microbial mats in lakes of Eastern Antarctica was investigated using microscopic and molecular approaches. The present study assessed the biogeographical distribution of cyanobacteria in Antarctica. Five samples were taken from four lakes spanning a range of different ecological environments in Larsemann Hills, Vestfold Hills and Rauer Islands to evaluate the influence of lake characteristics on the cyanobacterial diversity. Seventeen morphospecies and 28 16S rRNA gene-based operational taxonomic units belonging to the Oscillatoriales, Nostocales and Chroococcales were identified. The internal transcribed spacer was evaluated to complement the 16S rRNA gene data and showed similar but more clear-cut tendencies. The molecular approach suggested that potential Antarctic endemic species, including a previously undiscovered diversity, are more abundant than has been estimated by morphological methods. Moreover, operational taxonomic units, also found outside Antarctica, were more widespread over the continent than potential endemics. The cyanobacterial diversity of the most saline lakes was found to differ from the others, and correlations between the sampling depth and the cyanobacterial communities can also be drawn. Comparison with database sequences illustrated the ubiquity of several cyanobacterial operational taxonomic units and their remarkable range of tolerance to harsh environmental conditions.     

Influence of environmental conditions, bacterial activity and viability on the viral component in 10 Antarctic lakes

The influence of biotic and environmental variables on the abundance of virus-like particles (VLP) and lysogeny was investigated by examining 10 Antarctic lakes in the Vestfold Hills, Antarctica, in the Austral Spring. Abundances of viruses and bacteria and bacterial metabolic activity were estimated using SYBR Gold (Molecular Probes), Baclight (Molecular Probes) and 6-carboxy fluorescein diacetate (6CFDA). Total bacterial abundances among the lakes ranged between 0.12 and 0.47 x 10(9) cells L(-1). The proportion of intact bacteria (SYTO 9-stained cells) ranged from 13.5% to 83.5% of the total while active (6CFDA-stained) bacteria ranged from 33% to 116%. Lysogeny, as determined with Mitomycin C, was only detected in one of the lakes surveyed, indicating that viral replication was occurring predominantly via the lytic cycle. Principal component analysis and confirmatory correlation analysis of individual variables showed that high abundances of VLP occurred in lakes of high conductivity with high concentrations of soluble reactive phosphorus and dissolved organic carbon. These lakes supported high concentrations of chlorophyll a, intact bacteria, rates of bacterial production and virus to bacteria ratios. Thus, it was suggested that viral abundance in the Antarctic lakes was determined by the trophic status of the lake and the resultant abundance of intact bacterial hosts.     

Bacterial diversity from benthic mats of Antarctic lakes as a source of new bioactive metabolites

During the MICROMAT project, the bacterial diversity of microbial mats growing in the benthic environment of Antarctic lakes was accessed for the discovery of novel antibiotics. In all, 723 Antarctic heterotrophic bacteria belonging to novel and/or endemic taxa in the α-, β- and γ-subclasses of the Proteobacteria, the Bacteroidetes branch, and of the high and low percentage G+C Gram-positives, were isolated, cultivated in different media and at different temperatures, and then screened for the production of antimicrobial activities. A total of 6348 extracts were prepared by solid phase extraction of the culture broths or by biomass solvent extraction. 122 bacteria showed antibacterial activity against the Gram-positives Staphylococcus aureus and to a lower extent Enterococcus faecium, and versus the Gram-negative Escherichia coli. Few of these strains showed also some antifungal activity against Cryptococcus neoformans, Aspergillus fumigatus and to a lower extent Candida albicans. LC–MS fractionation of extracts from a subset of strains (hits) that exhibited relatively potent antibacterial activities evidenced a chemical novelty that was further investigated. Two strains of Arthrobacter agilis produced potent antibacterial compounds with activity against Gram-positives and possibly related to novel cyclic thiazolyl peptides. To our knowledge, this is the first report of new antibiotics produced by bacteria from benthic microbial mats from Antarctic lakes. With no doubts these microbial assemblages represent an extremely rich source for the isolation of new strains producing novel bioactive metabolites with the potential to be developed as antibiotic compounds.     

Microbial ecology of planktonic filamentous phototrophic bacteria in holomictic freshwater lakes

The microbial ecology of the filamentous phototrophic bacterium, Chloronema giganteum, has been studied in the water column of three central European lakes (Schlein, Buchen and Vechten). In these lakes an anoxic layer, termed the transition zone, was located between the oxycline and the redoxcline. The migration capacity of Chloronema through this zone appears to be responsible for the natural preponderance of either straight or spiral forms. When the transition zone is less than 1 m thick the straight form is dominant, but when this transition zone is wider than 2 m the spiral form is enriched. The intermediate situations favour both filamentous forms.     

Observations on the plankton of some Costa Rican lakes

We sampled 30 lakes in Costa Rica in the wet season (July–August) of 1991 for phytoplankton (with integrated and whole water samples), and 17 for zooplankton (with net tows). Taxa of plankton and community richness were poorly related to geography, morphology, chemistry, and other biota. Neither the zooplankton nor the phytoplankton appeared to influence the composition of the other, and neither were apparently influenced by the presence of fish.Phytoplankton richness reflected primarily sampling method, but also tended to decrease with elevation and with Secchi disk depth, and tended to increase with pH and alkalinity. Chlorophytes were the most abundant division in 14 lakes; these lakes tended to be unstratified, turbid, and located at higher elevation. Diatoms were common in 4 of the 7 lakes with elevated silica (over 30 ppm). Each lake showed at least a 3 : 1 dominance by copepods, cladocera, or insect larvae. Copepods dominated 7 of the 17 lakes, most of which were shallow, turbid, and had low alkalinity. Cladocera dominated 7 lakes that were typically deeper and located at low-to mid-elevations. Insect larvae dominated two small, turbid lakes.     

Diversity and genomics of Antarctic marine micro-organisms

Marine bacterioplanktons are thought to play a vital role in Southern Ocean ecology and ecosystem function, as they do in other ocean systems. However, our understanding of phylogenetic diversity, genome-enabled capabilities and specific adaptations to this persistently cold environment is limited. Bacterioplankton community composition shifts significantly over the annual cycle as sea ice melts and phytoplankton bloom. Microbial diversity in sea ice is better known than that of the plankton, where culture collections do not appear to represent organisms detected with molecular surveys. Broad phylogenetic groupings of Antarctic bacterioplankton such as the marine group I Crenarchaeota, alpha-Proteobacteria (Roseobacter-related and SAR-11 clusters), gamma-Proteobacteria (both cultivated and uncultivated groups) and Bacteriodetes-affiliated organisms in Southern Ocean waters are in common with other ocean systems. Antarctic SSU rRNA gene phylotypes are typically affiliated with other polar sequences. Some species such as Polaribacter irgensii and currently uncultivated gamma-Proteobacteria (Ant4D3 and Ant10A4) may flourish in Antarctic waters, though further studies are needed to address diversity on a larger scale. Insights from initial genomics studies on both cultivated organisms and genomes accessed through shotgun cloning of environmental samples suggest that there are many unique features of these organisms that facilitate survival in high-latitude, persistently cold environments.     

Aquatic macrophytes in saline lakes of the Canadian prairies

Vascular macrophyte species richness decreases with increasing salinity. Only three species of submerged plants (Potamogeton pectinatus, Ruppia maritima, R. occidentalis) tolerate hypersaline waters (>50 g l^-1, total of ionic constituents). Eight emergent species occur in more saline habitats but only five (Scirpus maritimus var. paludosus, Distichlisstricta, Puccinellia nuttalliana, Scirpus americanus, Triglochin maritima) occur commonly over a range of saline lakes into the hypersaline category. Usually, species tolerant of high salinities are found over the entire saline spectrum and even extend into subsaline waters (<3 g l^-1) and thrive there. A major increase in the number of species occurs below 5 g l^-1. As the water recedes plants such as Salicornia rubra, Suaeda calceoliformes, Hordeum jubatum and Sonchus arvensis invade.Submerged angiosperm distribution is controlled by total ion concentration and substrate texture plays no apparent role. Although angiosperms normally grow in all kinds of substrates, they occupy coarse substrates in Wakaw lake because suitable fine substrates are densely colonized by charophytes. In this lake light limited growth occurs to a depth of 5% of surface light. Light was not limiting in Redberry Lake but angiosperm growth was limited to the upper 8 m (10% or more of surface light). Thermal stratification and depth (pressure) were probably limiting istead. In meromictic Waldsea Lake the depth of the chemocline (6 m, 5% surface light) delimits angiosperm growth.     

Viruses in the plankton of the Rybinsk Reservoir

The role of autochthonous viruses in the regulation of bacterioplantkon abundance and production was studied in the Rybinsk Reservoir. During the ice-free period, the number of virus-like particles varied within the range of (11.0–57.4) × 10⁶ particles/ml. The virus to bacterioplankton abundance ratio ranged within 3.0–9.4. From 4 to 25% of bacterioplankton was infected by phages. A single infected cell contained up to 80 mature virus particles. The phage-induced bacterioplankton mortality in different parts of the reservoir constituted 3.7–41.8% (22.5% on average) of bacterioplankton daily production. Heterotrophic flagellates grazed from 7.6 to 68.8% (27.5% on average) of the daily bacterial production. Thus, along with flagellates, viruses are an important factor controlling bacterioplankton development in the reservoir.