Planktothrix Rubescens Research Articles

The Red Harmful Plague in Times of Climate Change: Blooms of the Cyanobacterium Planktothrix rubescens Triggered by Stratification Dynamics and Irradiance.

Planktothrix rubescens is a harmful planktonic cyanobacterium, forming concentrated metalimnetic populations in deep oligo- and mesotrophic lakes, even after successful restoration. In Lake Zurich (Switzerland), P. rubescens emerged as a keystone species with annual mass developments since the 1970s. Its success was partly attributed to effects of lake warming, such as changes in thermal stratification and seasonal deep mixing. However, recent observations based on a biweekly monitoring campaign (2009–2020) revealed two massive breakdowns and striking seasonal oscillations of the population. Here, we disentangle positive from negative consequences of secular lake warming and annual variations in weather conditions on P. rubescens dynamics: (i) despite the high survival rates of overwintering populations (up to 25%) during three consecutive winters (2014–2016) of incomplete deep convective mixing, cyanobacterial regrowth during the following stratified season was moderate and not overshooting a distinct standing stock threshold. Moreover, we recorded a negative trend for annual population maxima and total population size, pointing to a potential nutrient limitation after a series of incomplete winter mixing. Thus, the predication of steadily increasing blooms of P. rubescens could not be confirmed for the last decade. (ii) The seasonal reestablishment of P. rubescens was strongly coupled with a timely formation of a stable metalimnion structure, where the first positive net growth in the following productive summer season was observed. The trigger for the vertical positioning of filaments within the metalimnion was irradiance and not maximal water column stability. Repetitive disruptions of the vernal metalimnion owing to unstable weather conditions, as in spring 2019, went in parallel with a massive breakdown of the standing stock and marginal regrowth during thermal stratification. (iii) Driven by light intensity, P. rubescens was entrained into the turbulent epilimnion in autumn, followed by a second peak in population growth. Thus, the typical bimodal growth pattern was still intact during the last decade. Our long-term study highlights the finely tuned interplay between climate-induced changes and variability of thermal stratification dynamics and physiological traits of P. rubescens, determining its survival in a mesotrophic temperate lake.

Remediation Strategies to Control Toxic Cyanobacterial Blooms: Effects of Macrophyte Aqueous Extracts on Microcystis aeruginosa (Growth, Toxin Production and Oxidative Stress Response) and on Bacterial Ectoenzymatic Activities

Increasing toxic cyanobacterial blooms in freshwater demand environmentally friendly solutions to control their growth and toxicity, especially in arid countries, where most drinking water is produced from surface reservoirs. We tested the effects of macrophyte allelochemicals on Microcystis aeruginosa and on the fundamental role of bacteria in nutrient recycling. The effects of Ranunculus aquatilis aqueous extract, the most bioactive of four Moroccan macrophyte extracts, were tested in batch systems on M. aeruginosa growth, toxin production and oxidative stress response and on the ectoenzymatic activity associated with the bacterial community. M. aeruginosa density was reduced by 82.18%, and a significant increase in oxidative stress markers was evidenced in cyanobacterial cells. Microcystin concentration significantly decreased, and they were detected only intracellularly, an important aspect in managing toxic blooms. R. aquatilis extract had no negative effects on associated bacteria. These results confirm a promising use of macrophyte extracts, but they cannot be generalized. The use of the extract on other toxic strains, such as Planktothrix rubescens, Raphidiopsis raciborskii and Chrysosporum ovalisporum, caused a reduction in growth rate but not in cyanotoxin content, increasing toxicity. The need to assess species-specific cyanobacteria responses to verify the efficacy and safety of the extracts for human health and the environment is highlighted.

Integration of In Situ and Remote Sensing Measurements for the Management of Harmful Cyanobacteria Blooms. A Lesson from a Strategic Multiple-Uses Reservoir (Lake Occhito, South Italy)

Harmful cyanobacteria blooms (HCBs) are one of the main water quality threats affecting reservoirs. Guidelines suggest integrating laboratory, real-time in situ, and remote sensing data in the monitoring of HCBs. However, this approach is still little adopted in institutional measuring programs. We demonstrated that this integration improves frequency and spatial resolution of the data collection. Data were from an intense HCB (Planktothrix rubescens), which occurred in a south Italy multiple-uses reservoir (Lake Occhito) between 2008 and 2009 and regarded both the lake and the irrigation network. Laboratory and in situ fluorometric data were related to satellite imagery, using simple linear regression models, to produce surface lake-wide maps reporting the distribution of both P. rubescens and microcystins. In the first node of the distribution network, microcystin concentrations (4–10 µg L−1) reached values potentially able to damage the culture and to accumulate during cultivation. Nevertheless, our study shows a decrease in the microcystin content with the distance from the lake (0.05 µg L−1 km−1), with a reduction of about 80% of the microcystin concentrations at the furthest tanks. Recent improvements in the spatial resolution (i.e., tens of meters) of satellite imagery allow us to monitor the main tanks of large and complex irrigation systems.

Monitoring of cyanobacterial blooms and assessing polymer-enhanced microfiltration and ultrafiltration for microcystin removal in an Italian drinking water treatment plant

The water intake of a drinking water treatment plant (DWTP) in Central Italy was monitored over six bloom seasons for cyanotoxin severity, which supplies drinking water from an oligo-mesotrophic lake with microcystin levels up to 10.3 μg/L. The historical data showed that the water temperature did not show extreme/large seasonal variation and it was not correlated either with cyanobacterial growth or microcystin concentration. Among all parameters, the cyanobacteria growth was negatively correlated with humidity and manganese and positively correlated with atmospheric temperature. No significant correlation was found between microcystin concentration and the climatic parameters. Polymer(chitosan)-enhanced microfiltration (PEMF) and ultrafiltration (PEUF) were further tested as an alternative microcystin removal approach from dense cyanobacteria-rich flows. The dominant cyanobacteria in the water intake, Planktothrix rubescens, was isolated and enriched to simulate cyanobacterial blooms in the lake. The PEMF and PEUF were separately applied to enriched P. rubescens culture (PC) (microcystin = 1.236 μg/L) as well as to the sand filter backwash water (SFBW) of the DWTP where microcystin concentration was higher than 12 μg/L. The overall microcystin removal rates from the final effluent of PC (always <0.15 μg/L) were between 90.1-94.7% and 89.5–95.4% using 4 and 20 mg chitosan/L, respectively. Meanwhile, after the PEMF and PEUF of SFBW, the final effluent contained only 0.099 and 0.057 μg microcystin/L with an overall removal >99%. The presented results are the first from the application of chitosan to remove P. rubescens as well as the implementation of PEMF and PEUF on SFBW to remove cyanobacterial cells and associated toxins.

Inhibited vertical mixing and seasonal persistence of a thin cyanobacterial layer in a stratified lake

Harmful blooms of the filamentous cyanobacteria Planktothrix rubescens have become common in many lakes as they have recovered from eutrophication over the last decades. These cyanobacteria, capable of regulating their vertical position, often flourish at the thermocline to form a deep chlorophyll maximum. In Lake Zurich (Switzerland), they accumulate during stratified season (May–October) as a persistent metalimnetic thin layer (~2 m wide). This study investigated the role of turbulent mixing in springtime layer formation, its persistence over the summer, and its breakdown in autumn. We characterised seasonal variation of turbulence in Lake Zurich with four surveys conducted in April, July and October of 2018 and September of 2019. Surveys included microstructure profiles and high-resolution mooring measurements. In July and October, the thin layer occurred within a strong thermocline (N gtrsim 0.05 s^{-1}) and withstood significant turbulence, observed as turbulent kinetic energy dissipation rates (varepsilon approx 10^{-8} W kg^{-1}). Vertical turbulent overturns –monitored by the Thorpe scale– went mostly undetected and on average fell below those estimated by the Ozmidov scale (L_O approx 1 cm). Consistently, vertical diffusivity was close to molecular values, indicating negligible turbulent fluxes. This reduced metalimnetic mixing explains the persistence of the thin layer, which disappears with the deepening of the surface mixed layer in autumn. Bi-weekly temperature profiles in 2018 and a nighttime microstructure sampling in September 2019 showed that nighttime convection serves as the main mechanism driving the breakdown of the cyanobacterial layer in autumn. These results highlight the importance of light winds and convective mixing in the seasonal cycling of P. rubescens communities within a strongly stratified medium-sized lake.

Lake Ecosystem Robustness and Resilience Inferred from a Climate-Stressed Protistan Plankton Network.

Network analyses of biological communities allow for identifying potential consequences of climate change on the resilience of ecosystems and their robustness to resist stressors. Using DNA metabarcoding datasets from a three-year-sampling (73 samples), we constructed the protistan plankton co-occurrence network of Lake Zurich, a model lake ecosystem subjected to climate change. Despite several documentations of dramatic lake warming in Lake Zurich, our study provides an unprecedented perspective by linking changes in biotic association patterns to climate stress. Water temperature belonged to the strongest environmental parameters splitting the data into two distinct seasonal networks (October–April; May–September). The expected ecological niche of phytoplankton, weakened through nutrient depletion because of permanent thermal stratification and through parasitic fungi, was occupied by the cyanobacterium Planktothrix rubescens and mixotrophic nanoflagellates. Instead of phytoplankton, bacteria and nanoflagellates were the main prey organisms associated with key predators (ciliates), which contrasts traditional views of biological associations in lake plankton. In a species extinction scenario, the warm season network emerged as more vulnerable than the cold season network, indicating a time-lagged effect of warmer winter temperatures on the communities. We conclude that climate stressors compromise lake ecosystem robustness and resilience through species replacement, richness differences, and succession as indicated by key network properties.

Phototransformation kinetics of cyanobacterial toxins and secondary metabolites in surface waters

BackgroundCyanobacteria and their toxins occur in high concentrations during the so-called bloom events in surface waters. To be able to assess the risks associated with cyanobacterial blooms, we need to understand the persistence and fate processes of these toxins and other bioactive metabolites. In this study, we investigated the photochemical fate of 54 cyanopeptides extracted from two strains of Microcystis aeruginosa (PCC7806 and UV006), Planktothrix rubescens, and Dolichospermum flos aquae. We determined half-lives during sunlight exposure in lake water and inspected the effect of pH on transformation kinetics for 27 microcystins, 8 anabaenopeptins, 14 cyanopeptolins, 2 cyclamides, and 3 aeruginosins.ResultsFor cyanopeptides from D. flos aquae and P. rubescens, we observed the highest removal of 28 and 26%, respectively, after 3-h sunlight exposure. Most cyanopeptides produced by the two M. aeruginosa strains were rather persistent with only up to 3% removal. The more reactive cyanopeptides contained amino acids known to undergo phototransformation, including methionine and tyrosine moieties or their derivatives. Photochemical half-lives of 14 tyrosine-containing cyanopeptides decreased by one order of magnitude from nearly persistent conditions at pH 7 (half-life > 70 h) to shorter half-lives at pH 10 (< 10 h).ConclusionsMore work is needed to distinguish the contribution of different photochemical reaction pathways including the contributions to the pH effect. To the best of our knowledge, this is the first assessment of transformation kinetics of such a wide range of cyanopeptides. The abundant and persistent cyanopeptides that have not been studied in detail yet should be prioritized for the evaluation of their ecosystem and human health risks and for their abatement during drinking water treatment.

Die hard in Lake Bourget! The case of Planktothrix rubescens reborn

Blooms of Planktothrix rubescens have been recorded for 15 years in Lake Bourget (France), from 1995 to 2009. Then, the presence of this filamentous and toxic cyanobacterium became anecdotic between 2010 and 2015 and it was thought that its proliferation was over. However, blooms occurred again in 2016 and 2017 despite apparent low phosphorus concentrations in surface waters of the lake. We have attempted to explain the reasons for this come back in order to develop scenarios helpful to stakeholders who are concerned such proliferations may occur in the future. We show that phosphorus input, both from the main tributaries to the lake and possibly from the sediments, were likely the triggers of the new development of the cyanobacterium provided a minimum autumn/winter inoculum of P. rubescens was detected the year before. The subsequent bloom was observed deeper than previous years and associated with a conjunction of factors known to favour the development of this species (i.e., mild winter temperature, water column stability, available light at depth, surface water transparency, low predation, etc.). Although many factors and processes could account for the occurrence and bloom of the cyanobacterium, a plausible scenario is proposed. One thing remains unclear: where does this cyanobacterium “hide” when it is not observed during the routine monitoring surveys and from which place it could initiate its development (nearshore, the pelagic zone, or from the sediment?).

The Relevance of Microcystin Monitoring in Dialysis Centers of Sicilians Cities: An Environmental Study

Background: Sicilian surface basins are among the most important water supply resources available on the island. They are often affected by harmful cyanobacteria blooms as Planktothrix rubescens and Microcystis aeruginosa. Since dialysates are produced using network water, they could contain cyanotoxins. No study has been conducted yet to evaluate the removal efficiency of osmotic systems for cyanotoxins in abnormal conditions at room temperatures of about 40°C. The aim of this study was to monitor the presence of microcystins in Sicilian dialysis center, network waters and, finally, dialysates produced from these waters in an Agrigento dialysis unit where environmental conditions are favorable for algal bloom. Methods: This clinic normally receives surface water from artificial basins, where several times, traces of cyanobacteria have been detected. Moreover, dialysates and underground supply waters of a clinic in Catania were also sampled as control. Samplings were performed in summer 2018, when room temperatures were above 38 °C. A total of 40 samples were analyzed by ELISA assay. Results: Results of our monitoring highlighted concentrations of MCs in waters of several basins among LOD - 155 ng/L, lower than WHO reference value for drinking waters (1,0 µg/L), that decrease up to undetectable levels whereas no MC contamination was detected both in supply waters and dialysates. Conclusion: Although our first set of data outcomes seem to be quite comforting, an improvement of law and a complete census of the water supplies of dialysate centers would be ideal.

Examination of Substrate Specificity of the First Adenylation Domain in mcyA Module Involved in Microcystin Biosynthesis

The cyanotoxin microcystin (MC) is a secondary metabolite, synthesized by nonribosomal peptide synthetase (NRPS) and polyketide synthase (PKS) enzymes. It has many isoforms and the mechanism of its diversity is not well understood. One of the MC synthetase genes, mcyA, codes for the McyA module containing two adenylation (A) domains. The first domain, McyA-A1, generally binds to L-serine (L-ser). Then the N-methyl transferase (NMT) domain converts L-Ser into N-methyldehydroalanine (Mdha), which usually occupies position 7 on the MC molecule. However, various other amino acids (AAs) might also be present at this position. In this study, bioinformatic analyses of selected cyanobacteria were performed to understand whether genetic information in the first adenylation domain of mcyA could explain incorporation of different AAs at position 7 of the MC molecule. Binding pocket signatures of McyA-A1 and putative activated AAs were determined via various bioinformatics tools. Maximum likelihood phylogenetic trees of full length mcyA, mcyA-A1 and 16S rRNA genes were prepared in Mega 6. Phylogenetic analysis of mcyA-A1 nucleotide sequences was in agreement with the predictions of activated AAs by McyA-A1. In comparison with the 16S rRNA and full length mcyA gene trees, mcyA-A1 phylogenetic trees suggested horizontal transfer of the A domain in either Planktothrix agardhii (Gomont) Anagnostidis &amp;amp; Komárek or Planktothrix rubescens (De Candolle ex Gomont) Anagnostidis &amp;amp; Komárek strains. Predictions of activated AAs were generally in agreement with the chemically determined position 7 AAs. However, there were exceptions suggesting the multispecificity of the first A domain of McyA in some cyanobacteria.

Driving factors affecting the phytoplankton functional groups in a deep alkaline lake

This study evaluated the phytoplankton communities based on functional groups to obtain information about the water quality of Lake İznik, Turkey. The phytoplankton consisted of 103 taxa, classified in 12 phytoplankton functional groups (PFGs), with dominancy of 5 species, Chrysosporum ovalisporum, Dolichospermum mendotae, Planktothrix rubescens, Fragilaria capucina and Mougeotia sp. The Shannon-Wiener diversity index (H') was calculated and results ranged between 0.41 and 2.47. The redundancy analysis (RDA) and Spearman's correlation analysis were used to assess the relationships between the PFGs and environmental variables. According to the multiple comparisons of the data, the main efficient factors that determined the seasonal distribution of the PFGs were TP, DO, SiO2, SD, and pH. The ecological requirements of the dominant PFGs (C, D, F, J, H1, Lo, SN, N, P, R, T, and X2) indicated mainly meso-eutrophic waters. Similarly, Carlson's trophic state index (TSI) stated mesotrophy conditions. As a result, the approach of PFGs can be successfully applied in a deep, alkaline lake to understand the water quality and trophic status.

Removal of Positively Buoyant Planktothrix rubescens in Lake Restoration.

The combination of a low-dose coagulant (polyaluminium chloride—‘Floc’) and a ballast able to bind phosphate (lanthanum modified bentonite, LMB—‘Sink/Lock’) have been used successfully to manage cyanobacterial blooms and eutrophication. In a recent ‘Floc and Lock’ intervention in Lake de Kuil (the Netherlands), cyanobacterial chlorophyll-a was reduced by 90% but, surprisingly, after one week elevated cyanobacterial concentrations were observed again that faded away during following weeks. Hence, to better understand why and how to avoid an increase in cyanobacterial concentration, experiments with collected cyanobacteria from Lakes De Kuil and Rauwbraken were performed. We showed that the Planktothrix rubescens from Lake de Kuil could initially be precipitated using a coagulant and ballast but, after one day, most of the filaments resurfaced again, even using a higher ballast dose. By contrast, the P. rubescens from Lake Rauwbraken remained precipitated after the Floc and Sink/Lock treatment. We highlight the need to test selected measures for each lake as the same technique with similar species (P. rubescens) yielded different results. Moreover, we show that damaging the cells first with hydrogen peroxide before adding the coagulant and ballast (a ‘Kill, Floc and Lock/Sink’ approach) could be promising to keep P. rubescens precipitated.

Toward Disentangling the Multiple Nutritional Constraints Imposed by Planktothrix: The Significance of Harmful Secondary Metabolites and Sterol Limitation.

The harmful bloom-forming cyanobacterium Planktothrix is commonly considered to be nutritionally inadequate for zooplankton grazers, resulting in limited top-down control. However, interactions between Planktothrix and zooplankton grazers are poorly understood. The food quality of Planktothrix is potentially constrained by morphological properties (i.e., filament formation), the production of harmful secondary metabolites, and a deficiency in essential lipids (i.e., primarily sterols). Here, we investigated the relative significance of toxin production (microcystins, carboxypeptidase A inhibitors, protease inhibitors) and sterol limitation for the performance of Daphnia feeding on one Planktothrix rubescens and one P. agardhii wild-type/microcystin knock-out mutant pair. Our data suggest that the poor food quality of both Planktothrix spp. is due to deleterious effects mediated by various harmful secondary metabolites and that the impact of sterol limitation is partially or completely superimposed by toxicity. The significance of the different factors seems to depend on the metabolite profile of the considered Planktothrix strain and the Daphnia clone that is used for the experiments. The toxin-responsive gene expression (transporter genes, gpx, and trypsin) and enzyme activity patterns revealed strain-specific food quality constraints and that Daphnia is capable of modulating its physiological responses according to the ingested Planktothrix strain. Future studies need to consider that Planktothrix–grazer interactions are simultaneously modulated by multiple factors to improve our understanding of top-down influences on Planktothrix bloom formation.

Water quality and human health: A simple monitoring model of toxic cyanobacteria growth in highly variable Mediterranean hot dry environments

Due to population growth, urbanization and economic development, demand for freshwater in urban areas is increasing throughout Europe. At the same time, climate change, eutrophication and pollution are affecting the availability of water supplies. Sicily, a big island in southern Italy, suffers from an increasing drought and consequently water shortage. In the last decades, in Sicilian freshwater reservoirs several Microcystis aeruginosa and more recently Planktothrix rubescens blooms were reported.The aims of the study were: (1) identify and quantify the occurring species of cyanobacteria (CB), (2) identify which parameters, among those investigated in the waters, could favor their growth, (3) set up a model to identify reservoirs that need continuous monitoring due to the presences, current or prospected, of cyanobacterial blooms and of microcystins, relevant for environmental and, consequentially, for human health.Fifteen artificial reservoirs among the large set of Sicilian artificial water bodies were selected and examined for physicochemical and microbiological characterization. Additional parameters were assessed, including the presence, identification and count of the cyanobacterial occurring species, the measurement of microcystins (MCs) levels and the search for the genes responsible for the toxins production. Principal Component Analysis (PCA) was used to relate environmental condition to cyanobacterial growth.Water quality was poor for very few parameters, suggesting common anthropic pressures, and PCA highlighted clusters of reservoirs vulnerable to hydrological conditions, related to semi-arid Mediterranean climate and to the use of the reservoir. In summer, bloom was detected in only one reservoir and different species was highlighted among the Cyanobacteria community. The only toxins detected were microcystins, although always well below the WHO reference value for drinking waters (1.0 μg/L). However, molecular analysis could not show the presence of potential cyanotoxins producers since a few numbers of cells among total could be sufficient to produce these low MCs levels but not enough high to be proved by the traditional molecular method applied. A simple environmental risk-based model, which accounts for the high variability of both cyanobacteria growth and cyanotoxins producing, is proposed as a cost-effective tool to evaluate the need for monitoring activities in reservoirs aimed to guarantee supplying waters safety.

Light intensity and spectral distribution affect chytrid infection of cyanobacteria via modulation of host fitness.

Light gradients are an inherent feature in aquatic ecosystems and play a key role in shaping the biology of phytoplankton. Parasitism by chytrid fungi is gaining increasing attention as a major control agent of phytoplankton due to its previously overlooked ubiquity, and profound ecological and evolutionary consequences. Despite this interest, if and how light conditions modulate phytoplankton chytridiomycosis remains poorly studied. We investigated life-history traits of a chytrid parasite, Rhizophydium megarrhizum, under different light intensities and spectral compositions when infecting two closely related planktonic cyanobacteria with different light-harvesting strategies: Planktothrix rubescens and P. agardhii. In general, parasite transmission was highest under light conditions (both intensity and quality) that maximized growth rates for uninfected cyanobacteria. Chytrid encystment on hosts was significantly affected by light intensity and host strain identity. This likely resulted from higher irradiances stimulating the increased discharge of photosynthetic by-products, which drive parasite chemotaxis, and from strain-specific differences at the cell-surface. Comparisons of parasite transmission and host growth rates under different light conditions suggest the potential for epidemic development at higher irradiances, whereas host and parasite could coexist without epidemic outbreaks at lower light levels. These results illustrate the close relationship between parasite transmission and host fitness, which is ultimately modulated by the external environment.

Co-occurrence of anatoxin-a and microcystins in Lake Garda and other deep subalpine lakes

Cyanotoxins are a global concern in freshwaters and eutrophication and climate changes can have synergistic effects in exacerbating the problem. Deep Subalpine Lakes (DSL) are a group of lakes of huge economic and naturalistic importance in Italy. Together with eutrophication (occurred during 1960s and 1970s) and re-oligotrophication (from 1990 onward) these lake have been experiencing warming and increase of the water column stability. These changes have influenced the phytoplankton (comprised the cyanobacteria) community of the lakes. Four DSL lakes (Lakes Garda, Iseo, Como and Lugano) have been studied with the aim of comparing their toxic potential. For one of them (Lake Garda) an 8 years survey was conducted, allowing a long term trend analysis. Toxin analysis was conducted on a monthly basis by targeted LC-MS/MS. A screening for anatoxins, cylindrospermopsins, saxitoxins, microcystins (MCs) and nodularins was carried out. Among all the listed toxins, only one anatoxin and five MCs were detected in the lakes. In particular, the alkaloid Anatoxin-a (ATX) was found dominant Lakes Garda, Iseo and Como and absent in lake Lugano; the MC-[D-Asp3]RR was found as the most abundant MC in all four lakes. Four other minoritary MCs were also found. The two major toxins are produced by two different cyanobacteria (Tychonema bourrellyi and Planktothrix rubescens) which have however similar ecological traits. Peaks of these toxins occur in warmer months (typically between May and September) in the thermocline layer (around 20 m, in the considered lakes). In summer 2016, the highest concentrations of ATX and total microcystins (MCs) were registered in Lake Iseo (1100 and 430ngL-1, respectively), while in the other lakes values were approximately twice lower. In the lakes where it is present, ATX peak levels are much higher than MCs, thus highlighting the necessity of considering ATX in the procedures of risk assessment. The importance of ATX is expected to further grow in the future with respect to MCs, as demonstrated by the long term trend analysis carried out in Lake Garda that showed a clear decline for MCs from 2009 till 2016 and a relative constancy for ATX.

Inhibition of Extracellular Enzymes Exposed to Cyanopeptides.

Harmful cyanobacterial blooms in freshwater ecosystems produce bioactive secondary metabolites including cyanopeptides that pose ecological and human health risks. Only adverse effects of one class of cyanopeptides, microcystins, have been studied extensively and have consequently been included in water quality assessments. Inhibition is a commonly observed effect for enzymes exposed to cyanopeptides and has mostly been investigated for human biologically relevant model enzymes. Here, we investigated the inhibition of ubiquitous aquatic enzymes by cyanobacterial metabolites. Hydrolytic enzymes are utilized in the metabolism of aquatic organisms and extracellularly by heterotrophic bacteria to obtain assimilable substrates. The ubiquitous occurrence of hydrolytic enzymes leads to the co-occurrence with cyanopeptides especially during cyanobacterial blooms. Bacterial leucine aminopeptidase and alkaline phosphatase were exposed to cyanopeptide extracts of different cyanobacterial strains ( Microcystis aeruginosa wild type and microcystin-free mutant, Planktothrix rubescens) and purified cyanopeptides. We observed inhibition of aminopeptidase and phosphatase upon exposure, especially to the apolar fractions of the cyanobacterial extracts. Exposure to the dominant cyanopeptides in these extracts confirmed that purified microcystins, aerucyclamide A and cyanopeptolin A inhibit the aminopeptidase in the low mg L-1 range while the phosphatase was less affected. Inhibition of aquatic enzymes can reduce the turnover of nutrients and carbon substrates and may also impair metabolic functions of grazing organisms.

The formation of a metalimnetic oxygen minimum exemplifies how ecosystem dynamics shape biogeochemical processes: A modelling study

Metalimnetic oxygen minima are observed in many lakes and reservoirs, but the mechanisms behind this phenomena are not well understood. Thus, we simulated the metalimnetic oxygen minimum (MOM) in the Rappbode Reservoir with a well-established two-dimensional water quality model (CE-QUAL-W2) to systematically quantify the chain of events leading to its formation. We used high-resolution measured data to calibrate the model, which accurately reproduced the physical (e.g. water level and water temperature), biogeochemical (e.g. nutrient and oxygen dynamics) and ecological (e.g. algal community dynamics) features of the reservoir, particularly the spatial and temporal extent of the MOM. The results indicated that around 60% of the total oxygen consumption rate in the MOM layer originated from benthic processes whereas the remainder originated from pelagic processes. The occurrence of the cyanobacterium Planktothrix rubescens in the metalimnion delayed and slightly weakened the MOM through photosynthesis, although its decaying biomass ultimately induced the MOM. Our research also confirmed the decisive role of water temperature in the formation of the MOM since the water temperatures, and thus benthic and pelagic oxygen consumption rates, were higher in the metalimnion than in the hypolimnion. Our model is not only providing novel conclusions about the drivers of MOM development and their quantitative contributions, it is also a new tool for understanding and predicting ecological and biogeochemical water quality dynamics.

Monitoring of cyanobacteria for water quality: doing the necessary right or wrong?

Cyanobacteria are an essential biological component of phytoplankton water quality assessment. However, there are some problems associated with the widely used everyday practices of sampling, estimation and use of cyanobacteria when calculating phytoplankton indices assessing water quality. Many indices were developed during the implementation of the Water Framework Directive, considered the most innovative European environmental legislation. Most indices include cyanobacteria as a composition or bloom metric. Problems with the indices concern the exclusion of most chroococcalean taxa from cyanobacterial biovolume estimations in lakes and reservoirs of the Mediterranean region, treatment of the mucilage of colonial chroococcalean taxa in biovolume estimations and overlooking of deep-water cyanobacterial blooms due to sampling depth. These problems may lead to a biased view of water quality. In this paper we argue in favour of including all cyanobacteria taxa and their mucilage in biovolume estimations and considering a sampling depth that covers deep-water maxima, such as those formed by Planktothrix rubescens or colonial chroococcalean taxa.

The potential of zooplankton in constraining chytrid epidemics in phytoplankton hosts.

Fungal diseases threaten natural and man‐made ecosystems. Chytridiomycota (chytrids) infect a wide host range, including phytoplankton species that form the basis of aquatic food webs and produce roughly half of Earth's oxygen. However, blooms of large or toxic phytoplankton form trophic bottlenecks, as they are inedible to zooplankton. Chytrids infecting inedible phytoplankton provide a trophic link to zooplankton by producing edible zoospores of high nutritional quality. By grazing chytrid zoospores, zooplankton may induce a trophic cascade, as a decreased zoospore density will reduce new infections. Conversely, fewer infections will not produce enough zoospores to sustain long‐term zooplankton growth and reproduction. This intricate balance between zoospore density necessary for zooplankton energetic demands (growth/survival), and the loss in new infections (and thus new zoospores) because of grazing was tested empirically. To this end, we exposed a cyanobacterial host (Planktothrix rubescens) infected by a chytrid (Rizophydium megarrhizum) to a grazer density gradient (the rotifer Keratella cf. cochlearis). Rotifers survived and reproduced on a zoospore diet, but the Keratella population growth was limited by the amount of zoospores provided by chytrid infections, resulting in a situation where zooplankton survived but were restricted in their ability to control disease in the cyanobacterial host. We subsequently developed and parameterized a dynamical food‐chain model using an allometric relationship for clearance rate to assess theoretically the potential of different‐sized zooplankton groups to restrict disease in phytoplankton hosts. Our model suggests that smaller‐sized zooplankton may have a high potential to reduce chytrid infections on inedible phytoplankton. Together, our results point out the complexity of three‐way interactions between hosts, parasites, and grazers and highlight that trophic cascades are not always sustainable and may depend on the grazer's energetic demand.