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Abundant phytoplankton and bacteria were identified by microscopy and high-throughput 16S rRNA tag Illumina sequencing of samples from water- and ice phases collected during winter at two central European Great Lakes, Balaton and Fertő (Neusiedlersee). Bacterial reads at all sites were dominated (>85%) by Bacteroidetes and Proteobacteria. Amongst phototrophs, microscopy and 16S sequencing revealed that both phytoplankton and cyanobacteria were represented, with a median of 1500 cyanobacterial sequence reads amongst 13 samples analyzed. The sequence analysis compared replicate Balaton and Fertő ice and water samples with an outgroup from three Hungarian soda lakes. In particular, both water and ice from Fertő contained high contributions from cyanobacteria. Two percent of total reads identified to the level of family in water at Fertő were dominated by a single operational taxonomic unit (OTU) of a cyanobacterium within the Rivulariaceae, which was largely absent from ice. Conversely, ice samples from both lakes yielded an abundant OTU assigned to a Flavobacterium sp. known to be associated with freshwater ice. Principal Coordinates Analysis (PCoA) revealed that the ice communities from all sites were similar to one another, and that the water communities did not cluster together. Fluorescence emission spectra obtained at 77 K confirmed the presence of intact cyanobacteria in Fertő water and ice. Photosynthetic characterization of phototrophs resident in water and ice analyzed by assay of acid-stable photosynthetic H14CO3– incorporation showed that communities from both phases were photosynthetically active, thus adding to growing recognition of ice-covered lakes as viable habitat for phototrophs.

Studies on autotrophic picoplankton (APP; 3 μm) to low-temperature and low-light winter environment was confirmed by their higher maximum photosynthetic rate and light utilization parameter. Maximum photosynthetic rate and light saturation parameter increased significantly with both temperature and available light, but with different impact on the two size groups. Twenty-two picoeukaryotic strains were isolated and identified based on 18S rRNA gene sequence analysis. Taxonomic composition of the picoeukaryotic community in the studied shallow lakes was similar to other freshwater lakes in the temperate zone: members of genera Choricystis and Mychonastes were dominant, however, in Lake Balaton, common freshwater taxa such as Stichococcus bacillaris and Nannochloris bacillaris were also found.

‘Chlorella’ and ‘Nannochloris’ were traditional genera of minute coccoid green algae with numerous species described in the past century, including isolates used as experimental test organisms. In the last few years, the introduction of DNA-based phylogenetic analyses resulted in a large number of taxonomic revisions. We investigated and reclassified a taxonomically problematic group within the Trebouxiophyceae (comprising ‘Nannochloris eucaryotum’ UTEX 2502, ‘N. eucaryotum’ SAG 55.87 and ‘Chlorella minutissima’ SAG 1.80), distantly related to the recently described Chloroparva isolates (97.5–97.9 % 18S rRNA gene pairwise similarity). Cryopreserved material of SAG 55.87 was selected as holotype for a novel species – Pseudochloris wilhelmii Somogyi, Felfoldi & Voros – whose phylogenetic position confirmed the proposal of a new genus. Pseudochloris wilhelmii had spherical to oval cells with an average diameter of 2.6 × 2.8 µm and a simple ultrastructure characteristic of small green algae. Vegetative cells ...

Cyanobacteria are a worldwide group of photosynthetic prokaryotes that can cause nuisance blooms in eutrophic waters. It is generally accepted that their resting cells, akinetes, play an important role in the dispersal, recruitment, initiation of blooms and survival under unfavourable conditions, therefore in- formation on the germination, distribution and abundance of akinetes in natural sediments is essential for understanding the ecology and bloom dynamics of N2-fixing cyanobacteria. The present study describes the effect of irradiance on the germination and subsequent growth of N2-fixing filamentous cyanobacteria devel- oped from natural akinete stock in sediment of Lake Balaton (Hungary) with varying phosphorous supply. The research focuses on the invasive Cylindrospermopsis raciborskii and Aphanizomenon flos-aquae the most abundant species of this lake. In the experiments, the germination of ten filamentous N2-fixing cyanobacteria species was observed. The species assemblages of the germinated cyanobacteria populations showed strong light and phosphorus dependence. Anabaena and Anabaenopsis species became dominant in phosphorous-rich conditions, while in phosphorus-deficient environments Aphanizomenon species and C. raciborskii dominated. Among the germinated filaments we have detected Anabaenopsis cunningtonii and Anabaena compacta, which have not been observed in Lake Balaton previously. Our results suggest that among the filamentous hetero- cytic cyanobacteria of this shallow lake the invasive C. raciborskii was the best competitor when phosphorus supply and irradiance were low.

Somogyi B., Felfoldi T., Solymosi K., Makk J., Homonnay Z.G., Horvath G., Turcsi E., Boddi B., Marialigeti K. and Voros L. 2011. Chloroparva pannonica gen. et sp. nov. (Trebouxiophyceae, Chlorophyta) – a new picoplanktonic green alga from a turbid, shallow soda pan. Phycologia 50: 1–10. DOI: 10.2216/10-08.1 We describe Chloroparva pannonica Somogyi, Felfoldi & Voros gen. et sp. nov., a new trebouxiophycean picoplanktonic alga isolated from a turbid, shallow soda pan in Hungary. The cells are spherical to oval, less than 2 µm in diameter, with simple ultrastructure typical to small green algae. Cells divide by autosporulation, forming two daughter cells per autosporangium. Cell wall structure consists of an outer trilaminar layer, an inner microfibrillar layer and an electron-transparent layer covering the plasma membrane. The trilaminar layer of the mother cell wall often persists around the autospores. Typical chlorophyte pigments have been found, including chlorophyll a and b and lutein as the ...

The photoautotrophic picoplankton (PPP) of ten shallow, hyposaline soda lakes located in three different geographical regions in the Carpathian Basin (Central Europe) was characterized. These lakes, which frequently dry out completely, are extremely rich in PPP. Epifluorescence microscopy was applied to determine picocyanobacterial and picoeukaryotic cell abundance and PCR-based molecular techniques (denaturing gradient gel electrophoresis and cloning with phylospecies delineation) to identify the members of PPP. Most of these lakes were eu- and hypertrophic with varying contribution of picocyanobacteria to the total PPP cell number. We found an unusually high PPP abundance with peaks of 8.16 × 106 cells mL-1 for picoeukaryotes and 1.78 × 107 cells mL-1 for picocyanobacteria. The majority of the retrieved PPP sequences belonged to picocyanobacteria (nonmarine Synechococcus/ Cyanobium), while others showed similarity to eukaryotic algal plastids (close to Trebouxiophycean isolates). Molecular analysis revealed significant genetic diversity in the PPP fraction of these lakes and showed that the closest relatives of our picocyanobacterial clones were recovered from different habitats, indicating seemingly no correlation between the 'saline' ecotypes and their phylogenetic position. Our results also confirmed that PPP might exploit different aquatic ecosystems and be successful even in the case of abrupt changes of environmental parameters (in our case, salinity). According to our knowledge, this is the first survey focusing on the identification of the PPP community members in turbid and alkaline lakes with extraordinarily high picoplankton productivity.

Laboratory cultures of Desmodesmus armatus (R. Chod.) Hegew. were grown under different levels of photosynthetically active radiation (PAR) supplemented with 3.75 mW · cm–2 UV-A radiation. Growth rate was monitored daily, chlorophyl-a concentration, total carotenoid content, cell number and the relative abundance of different coenobial forms was determined at the end of each experiment. Exposure to UV-A radiation resulted in an increasing inhibition of growth towards higher PAR levels, reaching 100% at 400 µmol · m–2 · s–1. Cellular carotenoid content was higher in the presence of UV-A radiation, on the other hand no differences were observed in cellular chlorophyll-a concentration. UV-A radiation also induced changes in coenobium formation with a decreasing proportion of 4-celled coenobia and an increase in the abundance of 2-celled and teratologic coenobia, suggesting that high intensity UV-A radiation may influence cell cycle events or morphology development. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The decomposition of leaf and steam litter of reed (P. australis) was measured both in the field and in the laboratory. The breakdown rates, the total carbon and the stable carbon isotope dynamics of reed litter were determined. The stable carbon isotope ratios of isolated humic substances (fulvic and humic acids) were also analysed. The δ13C value in reed remains increased from –26‰ to –24‰ for stems and from –27‰ to –26‰ for leaves. The dissolved fulvic and humic acids isolated from the experimental bottles (mean δ13C was –27.6‰) and the reservoir water were depleted in 13C (mean δ13C was –28.6‰) relative to the reed remains. The results show that reed litter is an important source of coloured aquatic humic substances. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Between 1996 and 1998 phytoplanktonic primary production and bacterioplankton production were measured monthly at five sampling stations in the lower Kis-Balaton reservoir. The open water area of the reservoir was rich in phytoplankton and had hypertrophic characteristics, but inside the reed stand (80% of the surface area) phytoplankton biomass and production were substantially (30–50 times) lower. The algal removal efficiency of the lower Kis-Balaton reservoir was 96%. The reservoir had a considerably smaller effect on bacterioplankton removal than on the phytoplankton. The decrease of biomass and production of bacterioplankton in the through-flowing water was approximately 60%. Inside the reed stand the biomass and the production of planktonic bacteria exceeded that of the phytoplankton by several times, suggesting that the release of biodegradable dissolved organic (humic) substances from macrophytes stimulated the metabolism of bacterioplankton. The significant reduction of phytoplankton inside the dense reed stand was primarily the result of the shading effect of the reeds. In the open water area a shading experiment demonstrated that a 1-week residence period for planktonic algae in the reed-covered area was sufficient for their complete elimination. The decomposition of planktonic algae, reed material and the lack of primary production inside the reed stand created oxygen-deficient and phosphorus-rich conditions during the vegetative period. These results suggest that reed-covered water bodies can effectively retain suspended solids and planktonic algae, but because of decomposition processes they cannot retain biologically-available phosphorus.

The aim of the present study was to assess the effect of ultraviolet radiation (UVR, 280–400 nm) on phytoplankton primary production in Lake Balaton, a shallow lake with the largest surface area in Central Europe possessing unique optical properties. The experiments were performed in situ in summer 1999 in the western basin with generally higher primary production rates and light attenuation coefficient due to higher algal biomass and in the eastern basin of the lake. Primary production was measured using 14C-technique, samples collected on-site were incubated at different depths for 4 h centred around noon. Underwater photosynthetically active radiation (PAR, 400–700) was measured prior to incubation. The incubation was carried out in three treatments using cut-off filters to exclude various portions of the UVR spectrum. Surface photoinhibition reached 72–78% in the samples exposed to PAR and UVR as compared to the control treatment from which UVR was excluded, however, areal primary production showed only an 8–14% decrease in the presence of UVR. Although the filters applied absorb ca. 15% PAR as well, comparison of the treatments pointed out that it was UVAR (320–400 nm), and not UVBR (280–320 nm) that predominantly inhibited photosynthesis.

Morphological characteristics of Cylindrospermopsis raciborskii were investigated in batch culture under P-starvation and in continuous cultures at two growth rates with different N forms. The species displayed an enormous morphological variability under controlled condition. In the P-starved batch culture very short and extreme morphology trichomes with conical or drop-shape heterocytes were formed. In continuous culture extremely long, twisted filaments were found near the maximum growth rate. Rarely, C. raciborskii formed intercalary heterocytes.

The contribution of ammonium, nitrate and N 2 to the nitrogen demand of summer phytoplankton has been estimated in two basins of Lake Balaton. The uptake rates of ammonium and nitrate were determined using 15 N methodology. N 2 -fixation rates were measured by the acetylene-reduction technique. The light dependence of uptake and N 2 -fixation was described with an exponential saturation equation. The decrease of I k values from mid-summer to autumn coincided with an increasing contribution of cyanobacteria to the algal population, suggesting a relatively lower light requirement than that of other species. The dark uptake velocity of ammonium and nitrate were 30-63 % and 1-40 % of light saturated uptake, respectively. The contribution of ammonium to the daily nitrogen supply of algae varied between 50-90%. Nitrate uptake was much less important with a maximum of 40 %. Nitrogen fixation was significant only in the western basin at the time of highest primary production and greatest N 2 -fixing cyanobacterial contribution. Nitrogen uptake experiments and loading data suggest, that nitrogen regeneration is by far the most important process in establishing and supporting the standing crop of phytoplankton in the lake during summer.

Aliquots of a water sample taken from the mesotrophic basin of Lake Balaton in August were incubated in three identical (2 liter) chemostats. Nutrients (35 μg PO 4 P l -1 week 1 and 350 μg NO 3 -N. l -1 week -1 ) were added by weekly pulses to the first culture and by continuous supply to the other two. The third culture was enriched continuously with Na-acetate (5 mg. l -1 week -1 ). Pulsed nutrient supply resulted in permanent coexistence of three algal species (Mougeotia sp., Synedra acus v. radians and Lyngbya limnetica) while the continuous supply (second culture) led to the predominance of Lyngbya limnetica. The Na-acetate addition experiment showed, that in the absence of carbon limitation, picophytoplankton were able to compete successfully with bacteria. The results indicate that indigenous phytoplankton can only thrive when bacterial growth is limited by carbon.

La croissance et la composition cellulaire de C. meneghiniana soumis a des concentrations limitantes en P, N et Si en culture continue a des taux de dilution differents sont discutes. Le contenu cellulaire en carbone, azote et phosphore par cellule ou par unite de volume augmente avec le taux de croissance dans toutes les conditions precitees. Le taux maximum de croissance depend du type de nutriment limitant. Ces valeurs sont tres proches de celles calculees a partir des modeles de Droop, Caperon, Goldman et Monod. Dans le Danube, les concentrations en N et P sont beaucoup plus elevees que les besoins pour la croissance et 1' assimilation de C. meneghiniana. Cela indique que ni N ni P ne sont limitants pour la croissance de C. meneghiniana en milieu naturel.

A strain of Cyclotella meneghiniana was isolated from the River Danube. The optimum irradiance and temperature for growth were determined in batch cultures. The optimum temperature for growth was 25 °C. The estimated saturated irradiance was near 125 µE m-2 s-1. The calculated Ik for growth and photosynthesis were 30 ± 3 and 44 ± 6 µE m-2 s-1 respectively. A continuous culture technique was used to test the growth and cell composition at the transition state between batch, nutrient sufficiency, and a low rate of nutrient supply. Algal response was characterized by high intracellular N and C with negative growth rate. At a low dilution rate (0.06 d-1), the continuous culture theory was not followed. This critical rate is characterized by low chlorophyll-a content per cell and may depend on the mode of nutrition.

N 2 -fixation and 15 N-labelled ammonium and nitrate uptake were measured during a bloom of C. raciborskii in two basins of Lake Balaton. In the eastern basin when the chlorophyll-a content reached 70 μg/l the N 2 -fixation was low (59 mg N m -2 day -1 ) and the N uptake was dominated by ammonium (393 mg N m -2 day -1 ). The nitrate uptake was of minor importance in both basins (43 and 55 mg N m -2 day -1 , respectively). In the western basin, the chlorophyll-a content was 160 μg/l and recirculation of regenerated forms of nitrogen could not support the demand. Although the ammonium uptake was high (357 mg N m -2 day -1 ), the fixed nitrogen (407 mg N m -2 day -1 ) exceeded it. The daily integrated carbon and nitrogen uptake ratios were 5.2 (weight/ weight) in both basins.

Unicellular autofluorescent picoplankton ranging from 0.6 to 0.9 ym in diameter were isolated from Lake Maggiore and from Lake Balaton. The cyanobacterial isolates contain two accessory pigments: phycoerythrin (PE) and phycocyanin (PC) respectively. The in vivo spectral properties of the two clones were compared to identify characteristics of the pigments. In vivo fluorescence excitation and emission spectra revealed that clones with PE are composed only of phycoerythrobilin chromophores and lack phycourobilin. Glycerol treatment enhances the fluorescence yield up to 3 times and improves the detection sensitivity of PE particularly at 436 and 520 nm and of PC at 600 nm. The vertical profiles of the underwater irradiance at different wavelengths were measured in both lakes to study the light quality of the natural environment of the two strains. Growth rates of both clones growing at different light intensities and wavelengths, selected by the same filters used for vertical profiles, were estimated. The results showed a difference in growth rate of phycoerythrin and phycocyanin containing cells exposed to an equal quantum flux of preferential illumination. In particular the maximum growth rate was reached by PE cells exposed to green light and by PC cells exposed to red light.

1. The importance of various forms of nitrogen to the nitrogen supply of phytoplankton has been investigated in the mesotrophic eastern and eutrophic western basin of Lake Balaton. 2. Uptake rates of ammonium, urea, nitrate and carbon were measured simultaneously. The uptake rates were determined using N-15 and C-14 methodologies, and N-2-fixation was measured using the acetylene-reduction method. The light dependence of uptake was described with an exponential saturation equation and used to calculate surface-related (areal) daily uptake. 3. The contribution of ammonium, urea and nitrate to the daily nitrogen supply of phytoplankton varied between 11 and 80%, 17 and 73% and 1 and 15%, respectively. N- 2-fixation was negligible in the eastern basin and varied between 5 and 30% in the western region of the lake. The annual external nitrogen load was only 10% of that utilized by algae. 4. The predominant process supplying nitrogen to the phytoplankton in the lake is the rapid recycling of ammonium and urea in the water column, The importance of the internal nutrient loading is emphasized.

The N:P ratios (TN:TP, NO3-N:TP and DIN:SRP) in the shallow, hypertrophic Marcali reservoir varied about 1, far below the physiological optimum for the planktonic algae. Moreover, N2-fixing cyanobacteria were absent in the vegetation period. One possible explanation for this phenomenon could be the filtration of the stocked silver carp (Hypophthalmichthys molitrix Val.), a phytoplankton feeder, and bighead carp (Aristichthys nobilisRich.), an omnivorous plankton feeder population (600 kg ha–1) which strongly reduces the large sized algae.

A comparison of the composition of natural phytoplanktoncommunities with the gut content of silver carp (Hypophthalmichthys molitrix) showed that this fish speciescan nottake up algae smaller than 10 µm. The species compositionofalgal assemblages in the gut content of the silver carpusuallydiffers significantly from the composition of the >10 µmsizefraction of the natural phytoplankton assemblages. Thisdifferenceis mainly a result of the taxon selective digestion of silvercarp.In vitro experiments with digestive enzymes of thisspeciesresulted in a very fast (1–2 minutes) disintegration ofnon-mucilaginous cyanobacteria. The digestion of diatoms andcryptophytes was also effective, but mucilaginouscyanobacteria andgreen algae (Chlorococcales) proved practicallyindigestible.

Feeding experiments were carried out with fry of silver carp fed on cultured algae, Scenedesmus ellipsoideus, Anacystis nidulans, Anabaena flos-aquae , and on phytoplankton collected in Lake Balaton. Analyses of the intestinal contents of silver carp showed that blue-green algae and diatoms were ingested and digested to the greatest extent. Colonies of blue-greens and big forms of greens were ingested, but poorly digested. Fish were not able to ingest algae smaller than 10 μm in size. Silver carp fed on blue-greens consumed 11–13% of their body weight (wet wt) in a 40 mg l −1 algal suspension. Filtration rate, at the same food density, ranged between 2.6 and 3.3 l g −1 (body weight) 4 h −1 . Algae labeled with 14 C passed through the alimentary canals in an average time of 3.2 h. Feeding of silver carp was most intense in the afternoon and lowest at night. Fecal algae ( Scenedesmus ) did not increase either the biomass or the primary production at a fecal concentration of 1 mg l −1 . At a fecal concentration of 3 mg l −1 , however, there was a slight increase in both the biomass and the primary produciton. At 10 mg l −1 fecal concentration the biomass and primary production of living cells almost doubled. Fecal algae ( Anabaena ) increased the primary production at 10 mg l −1 , but at 50 mg l −1 the biomass was three times and the primary production four times higher as compared with the control.

In April 2014, dual bloom of green algae and purple bacteria occurred in a shallow, alkaline soda pan (Kiskunság National Park, Hungary). The water was only 5 cm deep, in which an upper green layer was clearly separated from a near-sediment purple one. Based on microscopy and DNA-based identification, the upper was inhabited by a dense population of the planktonic green alga, Oocystis submarina Lagerheim, while the deeper layer was formed by purple, bacteriochlorophyll-containing bacteria, predominated by Thiorhodospira and Rhodobaca. Additional bacterial taxa with a presumed capability of anoxygenic phototrophic growth belonged to the genera Loktanella and Porphyrobacter. Comparing the bacterial community of the purple layer with a former blooming event in a nearby soda pan, similar functional but different taxonomic composition was revealed. Members from many dominant bacterial groups were successfully cultivated including potentially new species, which could be the result of the application of newly designed media. Electronic supplementary material The online version of this article (10.1007/s00792-019-01098-4) contains supplementary material, which is available to authorized users.

Winter phytoplankton communities in the shallow alkaline pans of Hungary are frequently dominated by picoeukaryotes, sometimes in particularly high abundance. In winter 2012, the ice-covered alkaline Zab-szék pan was found to be extraordinarily rich in picoeukaryotic green algae (42-82 × 10(6) cells ml(-1)) despite the simultaneous presence of multiple stressors (low temperature and light intensity with high pH and salinity). The maximum photosynthetic rate of the picoeukaryote community was 1.4 μg C μg chlorophyll a (-1) h(-1) at 125 μmol m(-2) s(-1). The assimilation rates compared with the available light intensity measured on the field show that the community was considerably light-limited. Estimated areal primary production was 180 mg C m(-2) d(-1). On the basis of the 18S rRNA gene analysis (cloning and DGGE), the community was phylogenetically heterogeneous with several previously undescribed chlorophyte lineages, which indicates the ability of picoeukaryotic communities to maintain high genetic diversity under extreme conditions.