Protozoan growth rates in Antarctic lakes

The growth rates of heterotrophic nanoflagellates (HNAN), mixotrophic cryptophytes, dinoflagellates and ciliates in field assemblages from Ace Lake in the Vestfold Hills (eastern Antarctica) and Lakes Fryxell and Hoare (McMurdo Dry Valleys, western Antarctica), were determined during the austral summers of 1996/1997 and 1997/1998. The response of the nanoflagellates to temperature differed between lakes in eastern and western Antarctica. In Ace Lake the available bacterial food resources had little impact on growth rate, while temperature imposed an impact, whereas in Lake Hoare increased bacterial food resources elicited an increase in growth rate. However, the incorporation of published data from across Antarctica showed that temperature had the greater effect, but that growth is probably controlled by a suite of factors not solely related to bacterial food resources and temperature. Dinoflagellates had relatively high specific growth rates (0.0057–0.384 h⁻¹), which were comparable to Antarctic lake ciliates and to dinoflagellates from warmer, lower latitude locations. Temperature did not appear to impose any significant impact on growth rates. Mixotrophic cryptophytes in Lake Hoare had lower specific growth rates than HNAN (0.0029–0.0059 h⁻¹ and 0.0056–0.0127 h⁻¹, respectively). They showed a marked seasonal variation in growth rate, which was probably related to photosynthetically active radiation under the ice at different depths in the water column. Ciliates' growth rates showed no relationship between food supply and mean cell volume, but did show a response to temperature. Specific growth rates ranged between 0.0033 and 0.150 h⁻¹ for heterotrophic ciliates, 0.0143 h⁻¹ for a mixotrophic Plagiocampa species and 0.0075 h⁻¹ for the entirely autotrophic ciliate, Mesodinium rubrum. The data indicated that the scope for growth among planktonic Protozoa living in oligotrophic, cold extreme lake ecosystems is limited. These organisms are likely to suffer prolonged physiological stress, which may account for the highly variable growth rates seen within and between Antarctic lakes. Accepted: 7 December 1999     

The distribution of microplankton in the McMurdo Dry Valley Lakes,Antarctica: response to ecosystem legacy or present-day climatic controls?

Plankton abundance and biomass were investigated in five lakes of the McMurdo Dry Valleys, Antarctica: Lakes Bonney, Fryxell, Joyce, Hoare and Miers. Despite plankton communities being dominated by organisms <100 m in length, there were striking differences between the lakes, including large variations in plankton vertical distribution and differences in total plankton biomass. Bacterial biomass was highest in the anoxic monimolimnia of the meromictic lakes, reaching 191 g C l^–1 in Lake Fryxell. Photosynthetic nanoflagellates dominated phytoplankton in the five lakes studied. Highest chlorophyll a concentrations were recorded at the chemocline of Lake Fryxell (21 g chl a l^–1). Heterotrophic nanoflagellate concentrations were low, ranging from 2 cells ml^–1 in Hoare to 237 cells ml^–1 in Bonney. By Antarctic standards, ciliates were relatively successful in terms of biomass and diversity in Lakes Fryxell and Hoare. In contrast, Lake Miers possessed extremely low ciliate abundance (<0.04 cells ml^–1). On both sampling occasions, copepod nauplii were observed in Lake Joyce. This is the first recording of crustacean zooplankton within the McMurdo Dry Valley Lakes. Because the foodwebs of these lakes are structured by bottom-up forces, differences in plankton distributions could be related to the physicochemical characteristics of each lake. The effect of lake evolution (legacy) and present-day climate change on planktonic dynamics is discussed.     

The microfauna of algal mats and artificial substrates in Southern Victoria Land lakes of Antarctica

The first microfaunal colonization study of continental lakes in Antarctica is reported. In addition to the general taxonomic composition of six Antarctic lakes, detailed community composition analyses of two adjacent lakes, Lake Fryxell and Lake Hoare, using polyurethane foam artificial substrates suspended in the open water and placed directly over benthic algal mats are also presented. Protozoans, rotifers, tardigrades and nematodes constituted the microfaunal community in all lakes. Higher metazoans do not occur in lakes that we studied. Protozoans were the most diverse of the four groups, and the first dinoflagellates reported from southern Victoria Land, Gymnodinium and Glenodinium, were collected from Lake Fryxell. The greatest number of protozoan taxa (55) were associated with the algal mats in the more productive Lake Fryxell. Both the physical and chemical properties and taxonomic diversity showed marked variability between all lakes that were studied, especially the adjacent lakes, Fryxell and Hoare. Ecological variables which regulate taxonomic diversity in other lakes of the world also appear to exert similar influences in remote Antarctic lakes.     

Particulate organic and dissolved inorganic carbon stable isotopic compositions in Taylor Valley lakes,Antarctica: the effect of legacy

In perennially ice-covered lakes of Taylor Valley, Antarctica, “legacy”, a carryover of past ecosystem events, has primarily been discussed in terms of nutrient and salinity concentrations and its effect on the current ecology of the lakes. In this study, we determine how residual pools of ancient carbon affect the modern carbon abundance and character in the water columns of Lakes Fryxell, Hoare, and Bonney. We measure the stable carbon isotopic compositions and concentrations of particulate organic carbon (POC) and dissolved inorganic carbon (DIC) in the water column of these lakes over four seasons (1999–2002). These data are presented and compared with all the previously published Taylor Valley lacustrine carbon stable isotopic data. Our results show that the carbon concentrations and isotopic compositions of the upper water columns of those lakes are controlled by modern processes, while the lower water columns are controlled to varying degrees by inherited carbon pools. The water column of the west lobe of Lake Bonney is dominated by exceptionally high concentrations of DIC (55,000–75,000 μmol l⁻¹) reflecting the long period of ice-cover on this lake. The east lobe of Lake Bonney has highly enriched δ¹³C_DIC values resulting from paleo-brine evaporation effects in its bottom waters, while its high DIC concentrations provide geochemical evidence that its middle depth waters are derived from West Lake Bonney during a hydrologically connected past. Although ancient carbon is present in both Lake Hoare and Lake Fryxell, the δ¹³C_DIC values in bottom waters suggest dominance by modern primary productivity-related processes. Anaerobic methanogenesis and methanotrophy are also taking place in the lower water column of Lake Fryxell with enough methane, oxidized anaerobically, to contribute to the DIC pool. We also show how stream proximity and high flood years are only a minor influence on the carbon isotopic values of both POC and DIC. The Taylor Valley lake system is remarkably stable in both inter-lake and intra-lake carbon dynamics. Handling editor: K. Martens     

Pelagic ciliated protozoa in two monomictic, southern temperate lakes of contrasting trophic state: seasonal distribution and abundance

Two lakes of contrasting trophic state in the central NorthIsland of New Zealand were sampled monthly for protozoan ciliatesand potential food resources. Oligotrichs dominated numbersin both lakes. Subdominants in oligotrophic Lake Taupo includedAskenasia, Pscudobalanion and Urotri-cha, and in eutrophic LakeOkaro Prorodon, Coleps, Urocentrum, Stentor and Spirostomumwere important. Biomass was dominated by large predatory ciliatesand Stentor in Lake Taupo, and Prorodon and Stentor in LakeOkaro. The importance of Prorodon and Stentor to ciliate biomassis unusual and has not been reported for northern hemispherelakes. Small ciliates (<20 µm) capable of consumingparticles <2 µm were a major component of the ciliatecommunity in Lake Taupo. Peaks in ciliate abundance occurredat the same time in both lakes: in autumn, at the beginningof mixis and in spring. Ciliates were vertically stratifiedduring mixis and stratification in both lakes. The effect wasmore pronounced during deoxygenation of the hypolimnion in LakeOkaro which excluded oligotrichs and introduced benthic ciliates.Ciliates were less abundant (mean 40001^–1 in Lake Okaroand 9001^–1 in Lake Taupo) than in comparable northerntemperate lakes. There was no correlation between the seasonaldistribution of ciliates and chlorophyll a, primarily causedby a winter peak in chlorophyll a dominated by large speciesof phytoplankton in Lake Taupo, at a time when ciliate numberswere low. The only consistent, significant correlations weretotal ciliate numbers and individual species of ciliates withbacterial concentrations in both lakes and with picophytoplanktonin Lake Taupo.     

Intestinal ciliate composition found in the feces of the Turk rahvan horse Equus caballus, Linnaeus 1758

Species composition and distribution of large intestinal ciliates were investigated in the feces from 15 Turk rahvan horses, living in the vicinity of Izmir, Turkey. Twenty-two ciliate genera consisting of 36 species were identified. This is the first report on intestinal ciliates in Turk rahvan horses and no previously unknown species were observed. The mean number of ciliates was 14.2 ± 13.9 × 10⁴ cells ml⁻¹ of feces and the mean number of ciliate species per host was 9.9 ± 7.1. No ciliates were observed in 2 horses. Bundleia and Blepharocorys were considered to be the major genera since these ciliates were constantly found in high proportions. In contrast, Paraisotricha, Didesmis and Gassovskiella were only observed at low frequencies. The ciliates found in this survey had almost the same characteristics as those described in previous reports, suggesting that there was no significant geographic variation in the intestinal ciliate fauna of equids.     

Phytoplankton nutrient deficiency in lakes of the McMurdo dry valleys, Antarctica

1. The influence of inorganic nitrogen and phosphorus enrichment on phytoplankton photosynthesis was investigated in Lakes Bonney (east and west lobes), Hoare, Fryxell and Vanda, which lie in the ablation valleys adjacent to McMurdo Sound, Antarctica. Bioassay experiments were conducted during the austral summer on phytoplankton populations just beneath the permanent ice cover in all lakes and on populations forming deep-chlorophyll maxima in the east and west lobes of Lake Bonney. 2. Phytoplankton photosynthesis in surface and mid-depth (13 m) samples from both lobes of Lake Bonney were stimulated significantly (P < 0.01) by phosphorus enrichment (2 μM) with further stimulation by simultaneous phosphorus plus NH₄⁺ (20 μM) enrichment. Similar trends were observed in deeper waters (18 m) from the east lobe of Lake Bonney, although they were not statistically significant at P < 0.05. Photosynthesis in this lake was never enhanced by the addition of 20 μM NH₄⁺ alone. Simultaneous addition of phosphorus plus nitrogen stimulated photosynthesis significantly (P < 0.01) in both Lake Hoare and Lake Fryxell. No nutrient response occurred in Lake Vanda, where activity in nutrient-enriched samples was below unamended controls; results from Lake Vanda are suspect owing to excessively long sample storage in the field resulting from logistic constraints. 3. Ambient dissolved inorganic nitrogen (DIN) (NH4⁺+ NO₂^?+ NO₃^?): soluble reactive phosphorus (SRP) ratios partially support results from bioassay experiments indicating strong phosphorus deficiency in Lake Bonney and nitrogen deficiency in Lakes Hoare and Fryxell. DIN : SRP ratios also imply phosphorus deficiency in Lake Vanda, although not as strong as in Lake Bonney. Particulate carbon (PC): particulate nitrogen (PN) ratios all exceed published ratios for balanced phytoplankton growth, indicative of nitrogen deficiency. 4. Vertical nutrient profiles in concert with low advective flux, indicate that new (sensu Dugdale & Goering, 1967) phytoplankton production in these lakes is supported by upward diffusion of nutrients from deep nutrient pools. This contention was tested by computing upward DIN : SRP flux ratios across horizontal planes located immediately beneath each chlorophyll maximum and about 2 m beneath the ice (to examine flux to the phytoplankton immediately below the ice cover). These flux ratios further corroborated nutrient bioassay results and bulk DIN : SRP ratios indicating phosphorus deficiency in Lakes Bonney and Vanda and potential nitrogen deficiency in Lakes Hoare and Fryxell. 5. Neither biochemical reactions nor physical processes appear to be responsible for differences in nutrient deficiency among the study lakes. The differences may instead be related to conditions which existed before or during the evolution of the lakes.     

Temporal and spatial distributions of dissolved organic carbon and nitrogen in two small lakes on the Southwestern China Plateau

Temporal and spatial distributions of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), chlorophyll-a and inorganic nitrogen were investigated in two small mountainous lakes (Lake Hongfeng and Baihua), on the Southwestern China Plateau, based on almost 2 years’ field observation. DOC concentrations ranged from 163 μM to 248 μM in Lake Hongfeng and from 143 μM to 308 μM in Lake Baihua, respectively, during the study period. DON concentrations ranged from 7 μM to 26 μM in Lake Hongfeng and from 14 μM to 47 μM in Lake Baihua. DOC showed vertical heterogeneity with higher concentrations in the epilimnion than in the hypolimnion during the stratification period. The DON concentration profiles appeared to be more variable than the DOC profiles. Apparent DON maxima occurred in the upper layer of water. In Lake Hongfeng, DOC concentration in the surface water was highest at the end of spring and early summer. DON concentration was 2–5 μM higher in May 2003 and in June 2004 than in adjacent months. DOC and chlorophyll-a concentrations were significantly correlated (r = 0.79, P < 0.05). The period of highest concentrations of DOC in Lake Hongfeng was also the season of concentrated rainfall. Algae activity and allochthonous input might result in an increase of DOC and DON concentrations together. In Lake Baihua, the maximum concentrations of DOC and DON in the surface water occurred simultaneously in May 2003 and February 2004. DOC concentrations were significantly correlated with DON (r = 0.90, P < 0.01), indicating the common sources. Allochthonous input, biological processes, stratification and mixing were the most important factors controlling the distributions and cycling of dissolved organic matter (DOM) and inorganic nitrogen in these two lakes. Inference from the corresponding vertical distributions of DOM and inorganic nitrogen indicated that DOM played potential roles in the internal loading of nitrogen and metabolism in the water body in these small lakes. The carbon/nitrogen (C/N) ratio showed a potential significance for tracing the source and biogeochemical processes of DOM in the lakes. These results are of significance in the further understanding of biogeochemical cycling and environmental effects of DOM and nitrogen in lake ecosystems.     

Inorganic nitrogen uptake and regeneration in perennially icecovered Lakes Fryxell and Vanda, Antarctica

The isotope ¹⁵N was used to examine nitrogen dynamics in LakesFryxell and Vanda, two permanently ice-covered Antarctic lakes.Half-saturation constants for NH₄⁺. uptake in the shallow watersof both lakes were <10 µg N l^–1; uptake kineticexperiments on populations forming the deep-chlorophyll layersof these lakes showed zero-order kinetics and could not be fittedwith the Michaelis-Menten equation. Elevated uptake within thefirst few minutes following pulses of NH₄⁺. and NO₃^– occurredin both lakes. NH₄⁺ regeneration, determined from isotope dilutionexperiments, exceeded uptake at 4.6 m in Lake Fryxell, was lessthan uptake at 9 m in Lake Fryxell and was equal to uptake at10 m in Lake Vanda under the experimental conditions. NO₃^–uptake was suppressed by NH₄⁺ levels as low as 2 µg NH₄⁺-N l^–1 in Lake Fryxell; the suppression was strongestin the near-surface populations. Substrate-saturated C:N uptakeratios (g:g) in Lake Fryxell decreased from 8.4 near the surfaceto 1.8 at the bottom of the trophogenic zone. Overall, the nitrogendynamics in these lakes are similar to other lakes and the openocean in that biological productivity during the austral summeris supported by regenerated nutrients.     

Seasonality of planktonic ciliated protozoa in 20 subtropical Florida lakes of varying trophic state

The planktonic ciliate populations of 20 Florida lakes ranging from oligotrophic to hypereutrophic were examined monthly for one year. Oligotrophic lakes displayed abundance peaks during fall mixis and biomass peaks in late winter and fall. Mesotrophic systems exhibited a spring-fall bimodality in ciliate abundance with a biomass maxima occurring during fall. Eutrophic/hypereutrophic lakes had pronounced abundance and biomass maxima during summer, with the large ciliates Plagiopyla nasuta and Paramecium trichium often contributing heavily to the midsummer biomass peak. Members of the Oligotrichida numerically dominated abundance and biomass peaks in oligotrophic lakes while the Scuticociliatida dominated the communities of higher trophic states. Total ciliate abundance and biomass were strongly correlated with chlorophyll a concentrations as were various ciliate taxonomic groups. The relationship between ciliate seasonal distribution in these subtropical lakes with lake thermal regimes and trophic state is discussed.     

The balance between photosynthesis and grazing in Antarctic mixotrophic cryptophytes during summer

SUMMARY 1. Grazing and photosynthetic contributions to the carbon balance of planktonic, mixotrophic cryptophytes in Lakes Fryxell and Hoare in the Taylor Valley, Antarctica were measured during November and December 2000.
2. The cryptophytes never became entirely photosynthetic, although carbon derived from grazing decreased in December. Individual grazing rates ranged between 5.28 and 10.08 bacteria cell⁻¹ day⁻¹ in Lake Fryxell and 0.36–11.76 bacteria cell⁻¹ day⁻¹ in Lake Hoare. Grazing rates varied temporally and with depth in the water column. In Lake Fryxell, which is a meromictic lake, highest grazing occurred just above the chemocline. Individual photosynthetic rates ranged from 0.23 to 1.35 pg C cell⁻¹ h⁻¹ in Lake Fryxell and 0.074 to 1.08 pg C cell⁻¹ h⁻¹ in Lake Hoare.
3. Carbon acquisition by the cryptophyte community gained through grazing ranged between 8 and 31% during November in Lake Fryxell, dropping to between 2 and 24% in December. In Lake Hoare grazing contributed 12–21% of the community carbon budget in November and 1–28% in December. Around 4% of the carbon acquired from grazing and photosynthesis was remineralised through respiration.
4. Mixotrophy is probably a major survival strategy for cryptophytes in the extreme lakes of the Dry Valleys, because perennial ice-cover severely limits light penetration to the water column, whereas these phytoflagellates are not normally mixotrophic in lower latitude lakes. The evidence suggests that mixotrophy may be a mechanism for supplementing the carbon budget, as well as a means of acquiring nutrients for growth.     

Changes in bacterial and ciliate densities with trophic status in Mediterranean shallow lakes

Ciliate and bacterial densities and their link with eutrophication were studied in fourteen shallow lakes in northwest Spain. Total phosphorus (TP) in these lakes varied between 30 μg l⁻¹ and 925 μg l⁻¹ and chlorophyll a concentration (chla) between 0.5 μg l⁻¹ and 107 μg l⁻¹. Bacterial abundance ranged from 1 × 10⁶ to 14 × 10⁶ cells ml⁻¹, while ciliate abundance ranged from 0.6 cells ml⁻¹ to 229 cells ml⁻¹. Lakes were classified into three trophic types from their TP and chla concentrations. Bacterial abundance was significantly correlated with trophic type, as well as with TP and with chla separately, whereas ciliate abundance was only correlated with chla. No significant relationship could be established between bacterial and ciliate abundance across the trophic gradient. A general pattern was observed in the ratios of bacterial abundance to TP and chla concentrations, of decreasing ratios with increases in the nutrient loading. This pattern was not found for ciliates. The dominant zooplankton group in 13 of the 14 lakes studied was Rotifera, which accounted for a mean of 71% of total zooplankton abundance (41% of zooplankton biomass). The positive correlation between bacteria and ciliates with this group, and the absence of any relationship with Cladocera suggest that top down control by cladocerans was weaker in our lakes than previously shown in northern European shallow lakes. Rotifers could be important predators of bacteria in the high-nutrient lakes of our study. Higher slopes of regressions on bacterial abundance towards the hypertrophic range indicate that top-down control was weaker in our lakes than in northern European shallow lakes.     

Overwintering populations of Mesodinium rubrum (Ciliophora: Haptorida) in lakes of the Vestfold Hills, East Antarctica

 The autotrophic ciliate Mesodinium rubrum Lohmann was observed during winter and spring in saline lakes ranging in salinity from 2 to 78‰ in the Vestfold Hills, Antarctica. The ciliate remained active during winter, and contained chlorophyll even though the level of light available for photosynthesis was minimal. No evidence of encystment as a means of survival during winter was observed. A seasonal study in one of the lakes, Ace Lake, revealed that M. rubrum was present throughout the year at abundances ranging from 1×10⁴ to 3.5×10⁵ cells l^-1. During the winter period, when little light penetrated the lake’s ice cover, cells were most common immediately under the ice at 2 m, where cell numbers were typically 8×10⁴ cells l^-1. Received: 3 January 1996/Accepted: 21 April 1996     

Phylogeny of Intestinal Ciliates,Including Charonina ventriculi,and Comparison of Microscopy and 18S rRNA Gene Pyrosequencing for Rumen Ciliate Community Structure Analysis

The development of high-throughput methods, such as the construction of 18S rRNA gene clone or pyrosequencing libraries, has allowed evaluation of ciliate community composition in hundreds of samples from the rumen and other intestinal habitats. However, several genera of mammalian intestinal ciliates have been described based only on morphological features and, to date, have not been identified using molecular methods. Here, we isolated single cells of one of the smallest but widely distributed intestinal ciliates, Charonina ventriculi, and sequenced its 18S rRNA gene. We verified the sequence in a full-cycle rRNA approach using fluorescence in situ hybridization and thereby assigned an 18S rRNA gene sequence to this species previously known only by its morphology. Based on its full-length 18S rRNA gene sequence, Charonina ventriculi was positioned within the phylogeny of intestinal ciliates in the subclass Trichostomatia. The taxonomic framework derived from this phylogeny was used for taxonomic assignment of trichostome ciliate 18S rRNA gene sequence data stemming from high-throughput amplicon pyrosequencing of rumen-derived DNA samples. The 18S rRNA gene-based ciliate community structure was compared to that obtained from microscopic counts using the same samples. Both methods allowed identification of dominant members of the ciliate communities and classification of the rumen ciliate community into one of the types first described by Eadie in 1962. Notably, each method is associated with advantages and disadvantages. Microscopy is a highly accurate method for evaluation of total numbers or relative abundances of different ciliate genera in a sample, while 18S rRNA gene pyrosequencing represents a valuable alternative for comparison of ciliate community structure in a large number of samples from different animals or treatment groups.     

Diel patterns of photosynthate biosynthesis by phytoplankton in permanently ice-covered Antarctic lakes under continuous sunlight

Diel patterns of photosynthate biosynthesis by Antarctic freshwaterphytoplankton growing under the variable but continuous sunlightof summer were found to be similar in many respects to thosereported from other aquatic environments where light/dark periodsalternate. Lipid synthesis by freshwater phytoplankton in LakesVanda and Fryxell predominated during periods when solar radiationand photosynthesis were most intense; the inverse was generallytrue of the protein and polysaccharide fractions. The majorphotosynthetic end-products in both lakes were protein and polysaccharide,which together accounted for 60–81% of the total cellularcarbon incorporation. Less than 4% of the carbon was incorporatedinto lipid in Lake Vanda; >12% appeared in the lipid fractionin Lake Fryxell. The Lake Fryxell populations showed evidenceof photoinhibition of complete photosynthesis during ‘midday’when irradiance was most intense. I_k values, computed from thephotosynthesis irradiance relationships in Lake Fryxell, corroborateother studies suggesting that the phytoplankton populationsin permanently ice-capped Antarctic lakes are among the mostshade-adapted yet reported.     

First report of folliculinid ciliates affecting Caribbean scleractinian corals

This is the first report of a ciliate of the genus Halofolliculina infecting hard coral species of six families (Acroporidae, Agaricidae, Astrocoeniidae, Faviidae, Meandrinidae and Poritidae) and milleporids in the Caribbean. Surveys conducted during 2004–2005 in Venezuela, Panama and México confirmed that this ciliate affects up to 25 scleractinian species. The prevalence of this ciliate at the coral community level was variable across sites, being most commonly found at Los Roques, Venezuela, and at Bocas del Toro, Panama (prevalence 0.2–2.5%), but rarely observed in the Mexican Caribbean. Ciliates were more prevalent within populations of acroporids (Acropora palmata, Acropora cervicornis and Acropora prolifera) in Los Roques. Recent observations also corroborate the presence of these ciliates in Curacao and Puerto Rico. Our observations indicate that ciliates affecting corals have a wider distribution than previously thought, and are no longer exclusively found in the Indo-Pacific and Red Sea.     

Fungal diversity and functionality are driven by soil texture in Taylor Valley,Antarctica

The McMurdo Dry Valleys surface is mainly constituted from unconsolidated permafrost. Despite the combination of cold and dry conditions, transiently wetted soils close to lake edges are hotspots of intense biological activity, that can support the surrounding soil ecosystems in such extreme environments. These soils host simple microbial communities that allow easy characterization of the parameters determining microbial establishment and diversification. Soil samples were collected close to three different glacial lakes (Lake Fryxell, Lake Hoare and Lake Joyce) located along a longitudinal gradient from the lower to the upper Taylor Valley. Fungal diversity and functionality of sampled soils were studied through ITS1 metabarcoding sequencing. The correlation between the parameters describing fungal diversity (i.e. total richness, relative richness of dominant taxonomic and functional groups, and community composition) and the edaphic physicochemical parameters (i.e. pH, moisture, C, N, P, Na⁺, K⁺, Mg²⁺ and Ca²⁺, cation exchange capacity, and soil granulometry) was assessed. The fungal communities showed low richness (48 ± 32 OTUs per sample). Their composition was highly diversified even within different sites close to the same lake. The main parameters affecting the diversity and composition of fungal communities were soil texture, in turn influencing the retention of water and nutrients, and physicochemical properties. This is of particular concern for the survival of these communities, given the expected environmental changes due to global warming.     

UV radiation and potential biological effects beneath the perennial ice cover of an antarctic lake

High-resolution spectral scans of solar ultraviolet radiation (UVR) were obtained directly beneath the 4.0–5.0 m thick, perennial ice cover of Lake Hoare, South Victoria Land, Antarctica. Both UVA (320–400 nm) and UVB (280–320 nm) radiation were detectable beneath the ice using a diver-deployed, underwater scanning spectroradiometer which permitted accurate measurement in the 280–340 nm range, while avoiding effects of surface shading and/or hole effects. UVR at wavelengths <310 nm was not detectable below the ice. This lower wavelength UVB appears to penetrate the Lake Hoare ice to depths of no more than 1.5 m during relatively cloud-free austral summer days. Based upon estimated biologically effective UVR dosages and DNA dosimeter data, exposure of benthic and planktonic microbes to the UVR encountered immediately beneath the ice is unlikely to inhibit microbial metabolism. Although waters of oligotrophic antarctic lakes are highly transparent to UVR, the thick, high scattering and optically dense ice covers on many of these lakes offers organisms a degree of protection largely unavailable in temperate and tropical systems. Thinning or complete loss of these overlying ice covers is likely to have major consequences for the structure of antarctic lake microbial communities.     

Stratification and dynamics of microbial loop communities in Lake Fryxell, Antarctica

1. Lake Fryxell, situated in the McMurdo Dry Valleys, Antarctica, offers the opportunity to study microbial loop processes in the absence of crustacean zooplankton and other higher organisms. This is the first study of Lake Fryxell to provide detailed temporal and vertical variations of microbial loop organisms.
2. Protozoan communities are concentrated around the chemocline (9–10 m) in Lake Fryxell. Phototrophic nanoflagellates (PNAN), heterotrophic nanoflagellates (HNAN) and ciliates formed deep maxima of 14 580, 694 and 58 cells mL−1 respectively. Although abundance and biomass at the chemocline was high, diversity of protozoa was low, Plagiocampa accounting for> 80% of the total ciliate biomass.
3. In the mixolimnion (4.5–8 m), protozoa were less abundant, but more diverse, with 24 ciliate morphotypes being identified within this region of the water column. Inter-annual variability of protozoan biomass and abundance was greater in the mixolimnion than at the chemocline due to more variable nutrient and prey concentrations.
4. Physicochemical gradients in Lake Fryxell were very stable because the perennial ice cover reduced wind driven currents. As a consequence, ciliate species occurred in distinct depth strata, Monodinium being most abundant directly beneath the ice cover, Askenasia having maximum abundance at 8 m and Plagiocampa dominating ciliate biomass at the chemocline. The lack of vertical mixing reduced seasonal successions of PNAN and ciliate species. Three cryptophyte species dominated the PNAN community at all times (>79% of total biomass).     

The ciliated protozoa of the monomictic Lake Kinneret (Israel): species composition and distribution during stratification

The ciliate taxa from epilimnion, hypolimnion, and littoral of Lake Kinneret were studied from November to December 1987 during stratification. Two collections of planktonic and benthic ciliates were taken at each of six sampling stations. Benthic ciliates from the littoral zone were collected at seven stations along the lake shore. Densities were determined for each ciliate species and for total protozoans at each station and sampling date. Thirty-six species of ciliates, representing 31 Genera, 18 Orders and 7 Classes, were identified. Planktonic protozoans characteristic of the epilimnion included Coleps hirtus and Vorticella mayeri. The more abundant taxa of benthic ciliates in the profundal of the lake included Saprodinium dentatum, Plagiopyla nasuta and Dexiotricha plagia. The predominant ciliate in the littoral was Pleuronema coronatum. Principal component analysis, performed on the correlation matrix of both sampling stations and species, revealed that epilimnion, hypolimnion, and littoral belt were colonized by different ciliate communities.