Feeding dynamics of the copepod Diacyclops thomasi before, during and following filamentous cyanobacteria blooms in a large, shallow temperate lake

Cyanobacteria blooms are an increasing problem in temperate freshwater lakes, leading to reduced water quality and in some cases harmful effects from toxic cyanobacteria species. To better understand the role of zooplankton in modulating cyanobacteria blooms, from 2008 to 2010 we measured water quality and plankton abundance, and measured feeding rates and prey selectivity of the copepod Diacyclops thomasi before, during and following summertime cyanobacteria blooms in a shallow, eutrophic lake (Vancouver Lake, Washington, USA). We used a combined field and experimental approach to specifically test the hypothesis that copepod grazing was a significant factor in establishing the timing of cyanobacteria bloom initiation and eventual decline in Vancouver Lake. There was a consistent annual succession of zooplankton taxa, with cyclopoid copepods (D. thomasi) dominant in spring, followed by small cladocerans (Eubosmina sp.). Before each cyanobacteria bloom, large cladocerans (Daphnia retrocurva, Daphnia laevis) peaked in abundance but quickly disappeared, followed by brief increases in rotifers. During the cyanobacteria blooms, D. thomasi was again dominant, with small cladocerans abundant in autumn. Before the cyanobacteria blooms, D. thomasi substantially consumed ciliates and dinoflagellates (up to 100% of prey biomass per day), which likely allowed diatoms to flourish. A shift in copepod grazing toward diatoms before the blooms may have then helped to facilitate the rapid increase in cyanobacteria. Copepod grazing impact was the highest during the cyanobacteria blooms both years, but focused on non-cyanobacteria prey; copepod grazing was minimal as the cyanobacteria blooms waned. We conclude that cyclopoid copepods may have an indirect role (via trophic cascades) in modulating cyanobacteria bloom initiation, but do not directly contribute to cyanobacteria bloom decline.     

Factors influencing cyanobacteria community structure in Chara-lakes

For practical reasons, cyanobacteria attract scientific interest in eutrophicated, often degraded water bodies. Much less attention is paid to blue-greens in less fertilized lakes, including a group of macrophytic lakes with vegetation dominated by charophytes (Chara-lakes). In this study, therefore, two small, mid-forest Chara-lakes with negligible recreational use were compared with two large Chara-lakes subject, along with their drainage basin, to higher human pressure. An attempt was made to find out whether there are any differences in the qualitative and quantitative structure of cyanobacteria and in cyanobacteria functional groups between the studied Chara-lakes. In addition, the lakes ecological status was assessed based on their total phytoplankton and cyanobacteria biomass.Small mid-forest Chara-lakes were distinguished by Chroococcales taxa, while Oscillatoriales and Nostocales preferred large recreationally used water bodies with catchment areas changed by human activity. Aphanothece minutissima, Merismopedia tenuissima, Aphanizomenon gracile, Aphanocapsa holsatica and Cyanodictyon planctonicum differentiated among the studied Chara-lakes.Although cyanobacteria dynamics did not differ Chara-lakes from eutrophic water bodies, the dominance of cyanobacteria was only detected in the two large Chara-lakes characterised by a worse ecological status compared to the small mid-forest ones.Alkalinity, water temperature, colour and, to a lesser extent, TP and TN:TP ratio are postulated to be water properties which, in addition to extensive charophyte meadows, control the development of cyanobacteria in the studied Chara-lakes.     

A comparison of the sensitivities of Daphnia magna and Daphnia pulex to six different cyanobacteria

Cyanobacteria have become an important environmental concern due to their ability to produce a wide range of natural toxins. At present, very few studies describe concentration response curves for cyanobacteria other than Microcystis. However, field evidence highlights that both cyanobacterial concentration as well as cyanobacterial species composition vary considerably with season and year. Therefore, the aim of this study was to investigate the effects of different cyanobacteria at various concentrations of these cyanobacteria in the diet on the reproduction of Daphnia pulex and Daphnia magna. Those two species were chosen to assess whether the cyanobacteria-daphnid dynamics could be generalized for the Daphnia genus. Results demonstrated that both slope and EC50 of the concentration response curves depend upon the Daphnia species, the cyanobacteria species and the potential interaction between the two. This has two major consequences. First, the differences in sensitivity to cyanobacteria between D. magna and D. pulex depend upon concentration of the specific cyanobacteria. Second, we noted different mechanisms of toxicity for the two zooplankton species, a more general mechanism of toxicity for D. pulex and a more specific one for D. magna. Our data therefore suggest that results of studies investigating effects of cyanobacteria at different concentrations cannot be generalized across species. Furthermore, mechanisms of toxicity are not only cyanobacteria specific, but also dependent on the exposed species, even for rather closely related species such as in the Daphnia genus. Whenever possible, we therefore propose to combine a multi-species approach together with a full concentration response analysis to reach more general conclusions concerning the effects of cyanobacteria on zooplankton.     

Phylogenomics Uncovers Evolutionary Trajectory of Nitrogen Fixation in Cyanobacteria

Biological nitrogen fixation (BNF) by cyanobacteria is of significant importance for the Earth’s biogeochemical nitrogen cycle but is restricted to a few genera that do not form monophyletic group. To explore the evolutionary trajectory of BNF and investigate the driving forces of its evolution, we analyze 650 cyanobacterial genomes and compile the database of diazotrophic cyanobacteria based on the presence of nitrogen fixation gene clusters (NFGCs). We report that 266 of 650 examined genomes are NFGC-carrying members, and these potentially diazotrophic cyanobacteria are unevenly distributed across the phylogeny of Cyanobacteria, that multiple independent losses shaped the scattered distribution. Among the diazotrophic cyanobacteria, two types of NFGC exist, with one being ancestral and abundant, which have descended from diazotrophic ancestors, and the other being anaerobe-like and sparse, possibly being acquired from anaerobic microbes through horizontal gene transfer. Interestingly, we illustrate that the origin of BNF in Cyanobacteria coincide with two major evolutionary events. One is the origin of multicellularity of cyanobacteria, and the other is concurrent genetic innovations with massive gene gains and expansions, implicating their key roles in triggering the evolutionary transition from nondiazotrophic to diazotrophic cyanobacteria. Additionally, we reveal that genes involved in accelerating respiratory electron transport (coxABC), anoxygenic photosynthetic electron transport (sqr), as well as anaerobic metabolisms (pfor, hemN, nrdG, adhE) are enriched in diazotrophic cyanobacteria, representing adaptive genetic signatures that underpin the diazotrophic lifestyle. Collectively, our study suggests that multicellularity, together with concurrent genetic adaptations contribute to the evolution of diazotrophic cyanobacteria.     

Transplantation of marine sponges to different conditions of light and current

Transplantation was employed to determine how marine sponges grow in different in situ conditions of light and current. The growth of Verongia aerophoba (Schmidt), which contained symbiotic cyanobacteria. was enhanced in light, particularly when sediment was excluded by a clear shield. V. cavernicola Vacelet and Chondrosia reniformis Nardo. which did not contain cyanobacteria, grew preferentially in the shade and were inhibited by light. The growth of Petrosia ficiformis (Poirct) and Chondrilla nucula Schmidt, which may or may not contain cyanobacteria. did not appear to be favoured by either light or shade. The growth of sponges in lower current speeds was considerably reduced ; this reduction was. however, partially reversed in those sponges with cyanobacteria when grown in the light. Presumably symbiotic cyanobacteria are beneficial to sponges growing in the light because they transfer photosynthetically fixed nutrient and shield the underlying tissue. Significant morphological differences were observed in sponges grown under different environmental conditions.     

Targeted deep sequencing reveals high diversity and variable dominance of bloom-forming cyanobacteria in eutrophic lakes

Cyanobacterial blooms in eutrophic lakes are severe environmental problems worldwide. To characterize the spatiotemporal heterogeneity of cyanobacterial blooms, a high-throughput method is necessary for the specific detection of cyanobacteria. In this study, the cyanobacterial composition of three eutrophic waters in China (Taihu Lake, Dongqian Lake, and Dongzhen Reservoir) was determined by pyrosequencing the cpcBA intergenic spacer (cpcBA-IGS) of cyanobacteria. A total of 2585 OTUs were obtained from the normalized cpcBA-IGS sequence dataset at a distance of 0.05. The 238 most abundant OTUs contained 92% of the total sequences and were classified into six cyanobacterial groups. The water samples of Taihu Lake were dominated by Microcystis, mixed Nostocales species, Synechococcus, and unclassified cyanobacteria. Besides, all the samples from Taihu Lake were clustered together in the dendrogram based on shared abundant OTUs. The cyanobacterial diversity in Dongqian Lake was dramatically decreased after sediment dredging and Synechococcus became exclusively dominant in this lake. The genus Synechococcus was also dominant in the surface water of Dongzhen Reservoir, while phylogenetically diverse cyanobacteria coexisted at a depth of 10 m in this reservoir. In summary, targeted deep sequencing based on cpcBA-IGS revealed a large diversity of bloom-forming cyanobacteria in eutrophic lakes and spatiotemporal changes in the composition of cyanobacterial communities. The genus Microcystis was the most abundant bloom-forming cyanobacteria in eutrophic lakes, while Synechococcus could be exclusively dominant under appropriate environmental conditions.     

Phytoplankton composition modifies predator-driven life history evolution in Daphnia

Organisms experience competing selective pressures, which can obscure the mechanisms driving evolution. Daphnia ambigua is found in lakes where a predator, the alewife (Alosa pseudoharengus) either does (anadromous) or does not (landlocked) migrate between marine and freshwater. We previously reported an association between alewife variation and life history evolution in Daphnia. However, differences in alewife migration indirectly influence phytoplankton composition for Daphnia. In ‘anadromous lakes’, Daphnia are present in the spring and experience abundant high-quality green algae. Intense predation by young-of-the-year anadromous alewife quickly eliminates these Daphnia populations by early summer. Daphnia from ‘landlocked lakes’ and lakes without alewife (‘no alewife lakes’) are present during the spring and summer and are more likely to experience high concentrations of sub-optimal cyanobacteria during the summer. To explore links between predation, resources, and prey evolution, we reared third-generation laboratory-born Daphnia from all lake types on increasing cyanobacteria concentrations. We observed several significant ‘lake type × resource’ interactions whereby the differences among lake types depended upon cyanobacteria concentrations. Daphnia from anadromous lakes developed faster, were larger at maturation, produced more offspring, and had higher intrinsic rates of increase in the absence of cyanobacteria. Such trends disappeared or reversed as cyanobacteria concentration was increased because Daphnia from anadromous lakes were more strongly influenced by the presence of cyanobacteria. Our results argue that alewife migration and phytoplankton composition both play a role in Daphnia evolution.     

Association of non-heterocystous cyanobacteria with crop plants

Cyanobacteria have the ability to form associations with organisms from all domains of life, notably with plants, which they provide with fixed nitrogen, among other substances. This study was aimed at developing artificial associations between non-heterocystous cyanobacteria and selected crop plants. We isolated several non-heterocystous cyanobacteria from various rice fields. The cultures were tested for their capacity to produce the plant hormone indole-3-acetic acid (IAA), and the possible role of IAA in the association of cyanobacteria with seedling roots was evaluated. Axenic cultures were co-inoculated with 10-day-old plant seedlings of Triticum aestivum, Vigna radiata and Pisum sativum and incubated for 1 week. Cyanobacterial association with the roots of these seedlings was quantified by measuring chlorophyll-a. Cyanobacterial association with the roots was observed by light microscopy as well as by confocal laser scanning microscopy (CLSM). Based on sequence analysis of the 16S rRNA gene, the isolates were identified as Synechocystis sp., Chroococcidiopsis sp., Leptolyngbya sp., and Phormidium sp. CLSM observations revealed the intimate association of cyanobacteria with the seedling roots as well as invasion of the roots and root cells. Strains producing IAA were more efficient in the colonization of the roots than those that lacked this ability. IAA-producing cyanobacteria possess a tryptophan-dependent pathway, and these cyanobacteria showed IAA synthesis activity in the presence of roots in media lacking tryptophan. Based on the results of this study, we conclude that non-heterocystous cyanobacteria also have the potential for use in agriculture to improve the growth and yield of crop plants that do not naturally form associations with cyanobacteria.     

Contiguous organization of nitrogenase genes in a heterocystous cyanobacterium

The organization of the three structural nitrogen fixation (nif) genes that encode nitrogenase (nif K and nif D) and nitrogenase reductase (nif H) have been examined in a number of cyanobacteria. Hybridization of Anabaena 7120 nif gene probes to restriction endonuclease-digested genomic DNA has shown (a) that cyanobacteria incapable of N₂ fixation have no regions of DNA with significant homology to the three nif probes, (b) that Pseudanabaena sp., a nonheterocystous cyanobacterium, has a contiguous nif KDH gene cluster, and (c) that in contrast with other heterocystous cyanobacteria, Fischerella sp. has a contiguous nif KDH gene cluster.     

Diversity of Microcystin-Producing Cyanobacteria in Spatially Isolated Regions of Lake Erie

The diversity of microcystin-producing cyanobacteria in the western basin of Lake Erie was studied using sequence analysis of mcyA gene fragments. Distinct populations of potentially toxic Microcystis and Planktothrix were found in spatially isolated locations. This study highlights previously undocumented diversity of potentially toxic cyanobacteria.     

Benthic freshwater cyanobacteria as indicators of anthropogenic pressures

Freshwater cyanobacteria are hardly used as biological indicators of anthropogenic pressures, possibly for two main reasons: (a) their response to anthropogenic pressures is often poorly known; (b) reliably identifying cyanobacteria species is a challenge for technicians and researchers. We assessed the usefulness of cyanobacteria species as biological indicators of two human stressors: the high orthophosphate input from urban wastewaters; the high nitrate concentration produced by agricultural land use. We analysed variation in benthic cyanobacterial assemblages at 85 sites in South-Central Spain as a response to eight environmental variables: pH, conductivity, temperature, altitude, nitrate, orthophosphate, irrigation land use and non-irrigation land use.Results revealed that conductivity was the main environmental factor that contributed to differences between assemblages. Orthophosphate was more influential for community composition than nitrate. Changes in species composition related to human pressures suggested that some cyanobacteria species (e.g. Nostoc verrucosum, Phormidium autumnale, Plectonema tomasinianum, Rivularia haematites, Tolypothrix distorta) could be useful tools for the bioindication of anthropogenic pressures; while others species provide more information about natural physicochemical reference conditions (Nostoc caeruleum, Phormidium fonticola). Further research into cyanobacteria and macroalgae assemblages in different impacted scenarios could help improve macrophyte indices.     

Molecular characterization and the effect of salinity on cyanobacterial diversity in the rice fields of Eastern Uttar Pradesh, India

Background

Salinity is known to affect almost half of the world's irrigated lands, especially rice fields. Furthermore, cyanobacteria, one of the critical inhabitants of rice fields have been characterized at molecular level from many different geographical locations. This study, for the first time, has examined the molecular diversity of cyanobacteria inhabiting Indian rice fields which experience various levels of salinity.

Results

Ten physicochemical parameters were analyzed for samples collected from twenty experimental sites. Electrical conductivity data were used to classify the soils and to investigate relationship between soil salinity and cyanobacterial diversity. The cyanobacterial communities were analyzed using semi-nested 16S rRNA gene PCR and denaturing gradient gel electrophoresis. Out of 51 DGGE bands selected for sequencing only 31 which showed difference in sequences were subjected to further analysis. BLAST analysis revealed highest similarity for twenty nine of the sequences with cyanobacteria, and the other two to plant plastids. Clusters obtained based on morphological and molecular attributes of cyanobacteria were correlated to soil salinity. Among six different clades, clades 1, 2, 4 and 6 contained cyanobacteria inhabiting normal or low saline (having EC < 4.0 ds m^-1) to (high) saline soils (having EC > 4.0 ds m^-1), however, clade 5 represented the cyanobacteria inhabiting only saline soils. Whilst, clade 3 contained cyanobacteria from normal soils. The presence of DGGE band corresponding to Aulosira strains were present in large number of soil indicating its wide distribution over a range of salinities, as were Nostoc, Anabaena, and Hapalosiphon although to a lesser extent in the sites studied.

Conclusion

Low salinity favored the presence of heterocystous cyanobacteria, while very high salinity mainly supported the growth of non-heterocystous genera. High nitrogen content in the low salt soils is proposed to be a result of reduced ammonia volatilization compared to the high salt soils. Although many environmental factors could potentially determine the microbial community present in these multidimensional ecosystems, changes in the diversity of cyanobacteria in rice fields was correlated to salinity.     

Bloom-Forming Cyanobacteria Support Copepod Reproduction and Development in the Baltic Sea

It is commonly accepted that summer cyanobacterial blooms cannot be efficiently utilized by grazers due to low nutritional quality and production of toxins; however the evidence for such effects in situ is often contradictory. Using field and experimental observations on Baltic copepods and bloom-forming diazotrophic filamentous cyanobacteria, we show that cyanobacteria may in fact support zooplankton production during summer. To highlight this side of zooplankton-cyanobacteria interactions, we conducted: (1) a field survey investigating linkages between cyanobacteria, reproduction and growth indices in the copepod Acartia tonsa; (2) an experiment testing relationships between ingestion of the cyanobacterium Nodularia spumigena (measured by molecular diet analysis) and organismal responses (oxidative balance, reproduction and development) in the copepod A. bifilosa; and (3) an analysis of long term (1999–2009) data testing relationships between cyanobacteria and growth indices in nauplii of the copepods, Acartia spp. and Eurytemora affinis, in a coastal area of the northern Baltic proper. In the field survey, N. spumigena had positive effects on copepod egg production and egg viability, effectively increasing their viable egg production. By contrast, Aphanizomenon sp. showed a negative relationship with egg viability yet no significant effect on the viable egg production. In the experiment, ingestion of N. spumigena mixed with green algae Brachiomonas submarina had significant positive effects on copepod oxidative balance, egg viability and development of early nauplial stages, whereas egg production was negatively affected. Finally, the long term data analysis identified cyanobacteria as a significant positive predictor for the nauplial growth in Acartia spp. and E. affinis. Taken together, these results suggest that bloom forming diazotrophic cyanobacteria contribute to feeding and reproduction of zooplankton during summer and create a favorable growth environment for the copepod nauplii.     

The diversity and distribution of toxigenic Microcystis spp. in present day and archived pelagic and sediment samples from Lake Erie

The reoccurrence of significant cyanobacterial blooms in Lake Erie during the last 13 years has raised questions concerning the long-term persistence of microcystin-producing cyanobacteria and the presence of natural sediment reservoirs for potentially toxic cyanobacteria in this large lake system. To address these questions, we analyzed phytoplankton and sediment samples which were collected and preserved in the 1970s as well as samples collected in 2004 from locations within Lake Erie. The identification of microcystin-producing cyanobacteria in Lake Erie was examined via PCR amplification of the mcyA gene fragment. Based on the high % sequence similarity, the mcyA sequences from all 1970s phytoplankton and sediment samples were determined to belong to Microcystis spp., in spite of reports suggesting that Lake Erie was dominated by filamentous cyanobacteria in the 1970s. In sediment samples from 2004, signature genes for Microcystis were distributed and preserved not only in the surface sediments but also up to 10–12 cm in depth. Based on cell quantities determined by a quantitative polymerase chain reaction (qPCR) method, 0.18% of eubacteria in the sediments were Microcystis cells, of which 4.8% were potential microcystin producers. In combination with experiments showing that Microcystis cells can be cultured from Lake Erie surface sediments, this paper demonstrates the potential for these sediments to act as a reservoir for pelagic Microcystis populations and that the composition of the population of microcystin-producing cyanobacteria in Lake Erie has not changed remarkably since the 1970s.     

The effect of pre-industrial and predicted atmospheric CO2 concentrations on the development of diazotrophic and non-diazotrophic cyanobacterium: Dolichospermum circinale and Microcystis aeruginosa

Photoautotrophs are capable of consuming high quantities of CO₂, yet scant research exists examining the influence of different CO₂ concentrations on the growth of freshwater diazotrophic or non-diazotrophic cyanobacteria. In this study, we cultured two cyanobacteria taxa (Dolichospermum circinale and Microcystis aeruginosa) within controlled atmospheric CO₂ chambers at pre-industrial, and post-industrial concentrations. Biovolume and chlorophyll a (Chl-a) differed as a consequence of the adjusted CO₂ gradients. Significantly higher biovolume measurements were observed in the elevated CO₂ treatment for the diazotrophic species in the initial experiment. However, a follow-up experiment, with a corrected culture replenishment regime showed Chl-a measurements were greater for the diazotrophic and non-diazotrophic species in the elevated CO₂ treatment. Increasing CO₂ presents a risk to already compromised eutrophic and hyper-eutrophic ecosystems, and we reason increasing CO₂ concentrations could affect photosynthetic performance and CO₂ assimilation of surface dwelling cyanobacteria. Further experimental work is required to establish ecological thresholds for freshwater ecosystems, as pH levels showed a measurable reduction within the elevated CO₂ treatments. As cyanobacteria species may respond quite differently to future CO₂ concentrations similar comparative studies should be carried out that focus on CO₂ dynamics and pH. The findings of the study indicate diazotrophic cyanobacteria growth in particular may benefit from elevated atmospheric CO₂ concentrations.     

Epiphytic Cyanobacteria on Chara vulgaris Are the Main Contributors to N2 Fixation in Rice Fields

The distribution of nitrogenase activity in the rice-soil system and the possible contribution of epiphytic cyanobacteria on rice plants and other macrophytes to this activity were studied in two locations in the rice fields of Valencia, Spain, in two consecutive crop seasons. The largest proportion of photodependent N₂ fixation was associated with the macrophyte Chara vulgaris in both years and at both locations. The nitrogen fixation rate associated with Chara always represented more than 45% of the global nitrogenase activity measured in the rice field. The estimated average N₂ fixation rate associated with Chara was 27.53 kg of N ha⁻¹ crop⁻¹. The mean estimated N₂ fixation rates for the other parts of the system for all sampling periods were as follows: soil, 4.07 kg of N ha⁻¹ crop⁻¹; submerged parts of rice plants, 3.93 kg of N ha⁻¹ crop⁻¹; and roots, 0.28 kg of N ha⁻¹ crop⁻¹. Micrographic studies revealed the presence of epiphytic cyanobacteria on the surface of Chara. Three-dimensional reconstructions by confocal scanning laser microscopy revealed no cyanobacterial cells inside the Chara structures. Quantification of epiphytic cyanobacteria by image analysis revealed that cyanobacteria were more abundant in nodes than in internodes (on average, cyanobacteria covered 8.4% ± 4.4% and 6.2% ± 5.0% of the surface area in the nodes and internodes, respectively). Epiphytic cyanobacteria were also quantified by using a fluorometer. This made it possible to discriminate which algal groups were the source of chlorophyll a. Chlorophyll a measurements confirmed that cyanobacteria were more abundant in nodes than in internodes (on average, the chlorophyll a concentrations were 17.2 ± 28.0 and 4.0 ± 3.8 μg mg [dry weight] of Chara⁻¹ in the nodes and internodes, respectively). These results indicate that this macrophyte, which is usually considered a weed in the context of rice cultivation, may help maintain soil N fertility in the rice field ecosystem.     

Allelopathic effects of the Baltic cyanobacteria Nodularia spumdigena, Aphanizomenon flos-aquae and Anabaena lemmermannii on algal monocultures

Allelopathy, the release of extracellular compounds that inhibit the growth of other microorganisms, may be one factor contributing to the formation and/or maintenance of cyanobacterial blooms. We investigated the allelopathic effects of three cyanobacterial species (Nodularia spumigena, Aphanizomenon flos-aquae and Anabaena lemmermannii) that frequently form mass-occurrences in the Baltic Sea. We exposed monocultures of three phytoplankton species (Thalassiosira weissflogii, Rhodomonas sp. and Prymnesium parvum) to cell-free filtrates of the three cyanobacteria, and quantified allelopathic effects with cell counts. We also investigated the role of the growth phase of cyanobacteria in their allelopathy, by comparing the effects of an exponential and a stationary phase culture of N. spumigena. All tested cyanobacteria inhibited the growth of Rhodomonas sp., but none of them affected P. parvum. The effects on T. weissflogii were more variable, and they were amplified by repeated filtrate additions compared to a single filtrate addition. N. spumigena was more allelopathic in exponential than in stationary growth phase, whereas the culture filtrate was more hepatotoxic in stationary phase. Hepatotoxins were thus probably not involved in the allelopathic effects, which is also indicated by the allelopathic properties of the non-toxic A. flos-aquae and A. lemmermannii. The results demonstrate that the common Baltic cyanobacteria affect some coexisting phytoplankton species negatively. Allelopathy may therefore play a role in interspecific competition and contribute to cyanobacterial bloom maintenance.     

Comparing Acute Effects of a Nano-TiO2 Pigment on Cosmopolitan Freshwater Phototrophic Microbes Using High-Throughput Screening

Production of titanium-dioxide nanomaterials (nano-TiO₂) is increasing, leading to potential risks associated with unintended release of these materials into aquatic ecosystems. We investigated the acute effects of nano-TiO₂ on metabolic activity and viability of algae and cyanobacteria using high-throughput screening. The responses of three diatoms (Surirella angusta, Cocconeis placentula, Achnanthidium lanceolatum), one green alga (Scenedesmus quadricauda), and three cyanobacteria (Microcystis aeruginosa, Gloeocapsa sp., Synechococcus cedrorum) to short-term exposure (15 to 60 min) to a common nano-TiO₂ pigment (PW6; average crystallite size 81.5 nm) with simulated solar illumination were assessed. Five concentrations of nano-TiO₂ (0.5, 2.5, 5, 10, and 25 mg L^-1) were tested and a fluorescent reporter (fluorescein diacetate) was used to assess metabolic activity. Algae were sensitive to nano-TiO₂, with all showing decreased metabolic activity after 30-min exposure to the lowest tested concentration. Microscopic observation of algae revealed increased abundance of dead cells with nano-TiO₂ exposure. Cyanobacteria were less sensitive to nano-TiO₂ than algae, with Gloeocapsa showing no significant decrease in activity with nano-TiO₂ exposure and Synechococcus showing an increase in activity. These results suggest that nanomaterial contamination has the potential to alter the distribution of phototrophic microbial taxa within freshwater ecosystems. The higher resistance of cyanobacteria could have significant implications as cyanobacteria represent a less nutritious food source for higher trophic levels and some cyanobacteria can produce toxins and contribute to harmful algal blooms.     

Molecular genetic identification and seasonal succession of toxigenic cyanobacteria in phytoplankton of the Rybinsk Reservoir (Russia)

Data on the hepato- and neurotoxigenic cyanobacteria in phytoplankton of the Rybinsk Reservoir have been obtained for the first time. Different methods for revealing toxigenic cyanobacteria (light microscopy, PCR analysis, and enzyme-linked immunosorbent assay) demonstrate the same results. Hepatotoxins microcystins and for the first time neurotoxins saxitoxins were detected in the reservoir, whereas cylindrospermopsin and anatoxin-a were not revealed. The presence of mcyE and stxA genes responsible for microcystin and saxitoxin biosynthesis in total phytoplanktonic DNA is demonstrated. The following three genera of cyanobacteria containing mcyE gene are identified: Microcystis (M. aeruginosa, M. viridis), Planktothrix (P. agardhii), and Dolichospermum (Anabaena). It is hypothesized that saxitoxin-producing cyanobacteria Dolichospermum (Anabaena) inhabit the Rybinsk Reservoir.