Evaluating the influence of light intensity in mcyA gene expression and microcystin production in toxic strains of Planktothrix agardhii and Microcystis aeruginosa

Polyphasic Approach and Potential Cyanotoxin Production by Planktothrix from the Río Grande de Comitán and Montebello Lakes National Park, Southern Mexico

The development of anthropic activities during recent years has led to an increase in nutrient fluxes in the Río Grande de Comitán and Montebello Lakes National Park, Mexico. In turn, this has modified the dynamics of the biotic community, specifically favoring the presence of cyanobacteria tolerant to contamination. The continual and massive presence of Planktothrix species (spp.) in the system suggests a potential detrimental impact for economic issues and human health. In this study, we identify the morphological and molecular characteristics of Planktothrix populations from seven tropical (1,380-1,740 masl, 23.0-25.5°C) and calcareous lakes and two ponds from a water treatment plant. We also assess the ecological drivers that could be related to the presence of cyanotoxins in the system. The ecological preferences, morphology, 16S rRNA structure, and 16S-23S rRNA internal transcribed spacer found evidence for three species: P. agardhii distributed in neutral to slightly basic water (pH = 7.7-8.7), and P. spiroides and Planktothrix sp. in alkaline waters (pH = 9.1). The presence of the mcyE gene and its validation by liquid chromatography confirmed the presence of two microcystin variants (MC-RR and MC-LR) in at least three populations of P. agardhii. These microcystins put the health of the ecosystem and its inhabitants at risk, a condition that should be addressed and resolved with a water management and detoxification strategy in the basin.

Current research scenario for biological effect of exogenous factors on microcystin synthesis

In natural water bodies, numerous cyanobacteria have the potential to intracellularly synthesize cyanotoxins, among which microcystin (MC) is the ubiquitous toxin that has been well known to be carcinogenic for hepatocytes. MC synthesis is a complex process, which involves about 10 non-ribosomal proteins encoded by the mcy gene cluster. In the natural environments containing MC-producing cyanobacteria, a variety of external factors can affect the generation of MC by mediating the expression of synthesizing genes. These factors can be generally divided into biotic factors (e.g., daphnia, virioplankton, MC-degrading bacteria, algicidal bacteria) and abiotic factors (e.g., nutrients, physical factors, chemicals, phytochemicals, essential trace elements), which are of great significance to the effective reduction of MC. Furthermore, comparison of MC-synthesizing genes in different cyanobacterial strains was performed, and the related factors affecting MC synthesis were summarized. Then, the problems and gaps regarding the biological effect of exogenous factors on microcystin synthesis were discussed. This review article may provide new ideas for addressing the challenges and bottlenecks of MC management.

The genetic and ecophysiological diversity of Microcystis

Microcystis is a cyanobacterium that forms toxic blooms in freshwater ecosystems around the world. Biological variation among taxa within the genus is apparent through genetic and phenotypic differences between strains and via the spatial and temporal distribution of strains in the environment, and this fine-scale diversity exerts strong influence over bloom toxicity. Yet we do not know how varying traits of Microcystis strains govern their environmental distribution, the tradeoffs and links between these traits, or how they are encoded at the genomic level. Here we synthesize current knowledge on the importance of diversity within Microcystis and on the genes and traits that likely underpin ecological differentiation of taxa. We briefly review spatial and environmental patterns of Microcystis diversity in the field and genetic evidence for cohesive groups within Microcystis. We then compile data on strain-level diversity regarding growth responses to environmental conditions and explore evidence for variation of community interactions across Microcystis strains. Potential links and tradeoffs between traits are identified and discussed. The resulting picture, while incomplete, highlights key knowledge gaps that need to be filled to enable new models for predicting strain-level dynamics, which influence the development, toxicity and cosmopolitan nature of Microcystis blooms.

Review on Cyanobacterial Studies in Portugal: Current Impacts and Research Needs

Cyanobacteria have long been associated with harmful effects on humans, animals and aquatic biota. Cyanotoxins are their most toxic metabolite. This review summarizes the current research, impacts and future needs in cyanobacterial studies undertaken in Portugal, the southernmost country of Europe, and with a recent multiplication of cyanotoxicity due to climate change events. Microcystins are still the most prevalent, studied and the only regulated cyanotoxins in Portuguese freshwater systems much like most European countries. With the development of some tools, particularly in molecular studies, the recent discovery of cylindrospermopsins, anatoxins and saxitoxins, both genes and toxins, in North and Center ecosystems of our country highlight current impacts that overall communities are facing with increased risks of exposure and uptake to cyanotoxins. Research needs encompass the expansion of studies at all aspects due to the uprising of these cyanotoxins and reinforces the urgent need of increasing the frequency of surveillance to achieve tangible effects of cyanotoxins in Portugal to ultimately implement regulations on cylindrospermopsins, anatoxins and saxitoxins worldwide.

Potential applications of CRISPR/Cas for next-generation biomonitoring of harmful algae blooms: A review

Research on harmful algal and cyanobacterial blooms (HABs and CHABs) has risen dramatically due to their increasing global distribution, frequency, and intensity. These blooms jeopardize public health, ecosystem function, sustainability and can have negative economic impacts. Numerous monitoring programs have been established using light microscopy, liquid chromatography coupled to mass spectrometry (LC-MS), ELISA, and spectrophotometry to monitor HABs/CHABs outbreaks. Recently, DNA/RNA-based molecular methods have been integrated into these programs to replace or complement traditional methods through analyzing environmental DNA and RNA (eDNA/eRNA) with techniques such as quantitative polymerase chain reaction (qPCR), fluorescent in situ hybridization (FISH), sandwich hybridization assay (SHA), isothermal amplification methods, and microarrays. These have enabled the detection of rare or cryptic species, enhanced sample throughput, and reduced costs and the need for visual taxonomic expertise. However, these methods have limitations, such as the need for high capital investment in equipment or detection uncertainties, including determining whether organisms are viable. In this review, we discuss the potential of newly developed molecular diagnosis technology based on Clustered Regularly Interspaced Short Palindromic Repeats/Cas proteins (CRISPR/Cas), which utilizes the prokaryotic adaptative immune systems of bacteria and archaea. Cas12 and Cas13-based platforms can detect both DNA and RNA with attomolar sensitivity within an hour. CRISPR/Cas diagnostic is a rapid, inexpensive, specific, and ultrasensitive technology that, with some further development, will provide many new platforms that can be used for HABs/CHABs biomonitoring and research.

Correlation between specific groups of heterotrophic bacteria and microcystin biodegradation in freshwater bodies of central Europe

Microcystins produced by several toxic cyanobacterial strains constitute an important problem for public health. Bacterial degradation of these hepatotoxins may play an important role in natural ecosystems, however the nature of the process is very poorly understood. The aim of our study was to investigate the possible interactions between cyanotoxin producers and degraders. Samples collected from 24 water bodies in western Poland were analysed to determine the chemo-physical parameters, phytoplankton content, bacterial community structure and microcystin-biodegradation potency. A redundancy analysis identified a positive correlation between the capacity of a community to degrade microcystin LR (MC-LR) and temperature, pH, chlorophyll a concentration and the abundance of MC-producers. The relative abundance of classes F38, TM7-3 and the order WCHB1-81c (Actinobacteria) was significantly higher in the lakes with MC-biodegradation potency. Some specific bacterial genera belonging to Acidobacteria, Chloroflexi, Gemmatimonadetes, Firmicutes and TM7 were closely correlated with the occurrence of Microcystis spp. Furthermore, the MC biodegradation process was connected with the same bacterial groups. Thus, our approach allowed us to provide a broader picture of some specific relations between microcystin producers and potential microcystin degraders. A more comprehensive analysis of the existing correlations may be helpful in our understanding of natural mechanisms of MC elimination using bacteria such as MC-degraders.

Improved detection of mcyA genes and their phylogenetic origins in harmful algal blooms

Microcystins, a group of cyanotoxins produced by cyanobacterial strains, have become a significant microbial hazard to human and animal health due to increases in the frequency and intensity of cyanobacterial harmful algal blooms (CyanoHABs). Many studies have explored the correlation between microcystin concentrations and abundances of toxin-producing genes (e.g., mcyA genes) measured using quantitative PCR, and discrepancies between toxin concentrations and gene abundances are often observed. In this study, the results show that these discrepancies are at least partially due to primer sets that do not capture the phylogenetic diversity of naturally present toxin-producers. We designed three novel primer gene probes based on known mcyA genes to improve the detection and quantification of these genes in environmental samples. These primers were shown to improve the identification of mcyA genes compared to previously published primers in freshwater metagenomes, cyanobacterial isolates, and lake water samples. Unlike previously published primers, our primer sets could selectively amplify and resolve Microcystis, Anabaena, and Planktothrix mcyA genes. In lake water samples, abundance estimations of mcyA genes were found to correlate strongly with microcystin concentrations. Based on our results, these primers offer significant improvements over previously published probes to accurately identify and quantify mcyA genes in the environment. There is an increasing need to develop models based on microbial information and environmental factors to predict CyanoHABs, and improved primers will play an important role in aiding monitoring efforts to collect reliable and consistent data on toxicity risks.

Development of a new extraction method based on high-intensity ultra-sonication to study RNA regulation of the filamentous cyanobacteria Planktothrix

Efficient RNA extraction methods are needed to study transcript regulation. Such methods must lyse the cell without degrading the genetic material. For cyanobacteria this can be particularly challenging because of the presence of the cyanobacteria cell envelope. The great breath of cyanobacterial shape and size (unicellular, colonial, or filamentous multicellular) created a variety of cell lysis methods. However, there is still a lack of reliable techniques for nucleic acid extraction for several types of cyanobacteria.

Here we designed and tested 15 extraction methods using physical, thermic or chemical stress on the filamentous cyanobacteria Planktothrix agardhii. Techniques based on the use of beads, sonication, and heat shock appeared to be too soft to break the Planktothrix agardhii cell envelope, whereas techniques based on the use of detergents degraded the cell envelope but also the RNA. Two protocols allowed to successfully obtain good-quality RNA. The first protocol consisted to manually crush the frozen cell pellet with a pestle and the second was based on the use of high-intensity ultra-sonication. When comparing these two, the high-intensity ultra-sonication protocol was less laborious, faster and allowed to extract 3.5 times more RNA compared to the liquid nitrogen pestle protocol. The high-intensity ultra-sonication protocol was then tested on five Planktothrix strains, this protocol allowed to obtain >8.5 μg of RNA for approximatively 3.5 × 10⁸ cells. The extracted RNA were characterized by 260/280 and 260/230 ratio > to 2, indicating that the samples were devoid of contaminant, and RNA Quality Number > to 7, meaning that the integrity of RNA was preserved with this extraction method. In conclusion, the method we developed based on high-intensity ultra-sonication proved its efficacy in the extraction of Planktothrix RNA and could be helpful for other types of samples.

Cyanotoxins and Cyanobacteria Cell Accumulations in Drinking Water Treatment Plants with a Low Risk of Bloom Formation at the Source

Toxic cyanobacteria have been shown to accumulate in drinking water treatment plants that are susceptible to algal blooms. However, the risk for plants that do not experience algal blooms, but that receive a low influx of cells, is not well known. This study determined the extent of cell accumulation and presence of cyanotoxins across the treatment trains of four plants in the Great Lakes region. Samples were collected for microscopic enumeration and enzyme-linked immunosorbent assay (ELISA) measurements for microcystins, anatoxin-a, saxitoxin, cylindrospermopsin, and β-methylamino-L-alanine (BMAA). Low cell influxes (under 1000 cells/mL) resulted in significant cell accumulations (over 1 × 10⁵ cells/mL) in clarifier sludge and filter backwash samples. Microcystins peaked at 7.2 µg/L in one clarifier sludge sample, exceeding the raw water concentration by a factor of 12. Anatoxin-a was detected in the finished drinking water of one plant at 0.6 µg/L. BMAA may have been detected in three finished water samples, though inconsistencies among the BMAA ELISAs call these results into question. In summary, the results show that plants receiving a low influx of cells can be at risk of toxic cyanobacterial accumulation, and therefore, the absence of a bloom at the source does not indicate the absence of risk.

Arsenic uptake, transformation, and release by three freshwater algae under conditions with and without growth stress

This study was carried out using indoor controlled experiments to study the arsenic (As) uptake, biotransformation, and release behaviors of freshwater algae under growth stress. Three freshwater algae, Microcystis aeruginosa, Anabaena flosaquae, and Chlorella sp., were chosen. Two types of inhibitors, e.g., Cu²⁺ and isothiazolinone, were employed to inhibit the growth of the algae. The algae were cultivated to a logarithmic stage in growth media containing 0.1 mg/L P; then, 0.8 mg/L As in the form of arsenate (iAs^V) was added, while both inhibitors were simultaneously added at dosages of 0.1 and 0.3 mg/L, with no addition of inhibitors in the control. After 2 days of exposure, the average growth rate (μ_2d) was measured to represent the growth rates of the algae cells; the extra- and intracellular As concentrations in various forms, i.e., arsenate, arsenite (iAs^III), monomethyl arsenic (MMA), and dimethyl arsenic (DMA), were also measured. Without inhibitors, the average growth rate followed the order of M. aeruginosa, Chlorella sp., and A. flosaquae, with the growth rate of M. aeruginosa significantly higher than that of the other two algae. However, when Cu²⁺ was added as an external inhibitor, the order of the average growth rate for the three algae became partially reversed, suggesting differentiation of the algae in response to the inhibitor. This differentiation can be seen by the reduction in the average growth rate of M. aeruginosa, which was as high as 1730% at the 0.3-mg/L Cu²⁺ dosage when compared with the control, while for the other two algae, much fewer changes were seen. The great reduction in M. aeruginosa growth rate was accompanied by increases in extracellular iAs^V and iAs^III and intracellular iAs^V concentrations in the algae, indicating that As transformation is related to the growth of this algae. Much fewer or neglectable changes in growth were observed that were consistent with the few changes in the extra- and intracellular As speciation for the other two algae with Cu²⁺ inhibition and all the three algae with isothiazolinone inhibition, corroborating the above hypothesis again. All the algae tested in this study demonstrated great abilities for As transformation and release, as seen by the much higher rates of 86.11-99.98% and 81.11-99.89% for transformation and release when compared to the control, respectively. When inhibitors were added, the transformation and release values of only A. flosaquae decreased remarkably down to 72.37-86.79% and 64.67-85.24%, respectively, while no changes were seen for these values in the other two algae, indicating that growth stress did not affect the As transformation and release of the other algae. The biological productivity of As by the three algae followed the order of M. aeruginosa, Chlorella sp., and A. flosaquae, which was generally consistent with the As transformation and release in conditions with and without inhibitors, suggesting that the As behavior in the algae that was related to growth stress largely differed among algae species.

Interactions between nitrogen form, loading rate, and light intensity on Microcystis and Planktothrix growth and microcystin production

The toxin-producing, bloom-forming cyanobacterial genera Microcystis and Planktothrix require fixed nitrogen (N), such as nitrate, ammonium, or organic N (e.g., urea) for growth and production of microcystins (MC). Bioavailable N can enter lakes in pulses via tributary discharge and through in-lake recycling, which can maintain low N concentrations. Additionally, light intensity has been suggested to play a role in MC production. This study examined how three forms of N (nitrate, ammonium, and urea) interacted with N loading rate (one large pulse vs. many small pulses) and light intensity to stimulate Microcystis and Planktothrix growth and MC production using nutrient enrichment experiments. Enrichments of nitrate, ammonium, and urea resulted in greater cyanobacterial biovolumes and MC concentrations than phosphorus-only enrichments, and there was no difference between pulse (100 μmol/L) and press treatments (8.3 μmol/L every 4 h). Analysis of mcyD transcripts showed significant up-regulation within 4 h of ammonium and urea enrichment. High light intensities (300 μmol photons/m²/s) with N enrichment resulted in greater cyanobacterial biovolumes and MC concentrations than lower light intensities (30 and 3 μmol photons/m²/s). Overall, the results suggest Microcystis and Planktothrix can use many forms of N and that high light intensities enhance MC production during elevated N concentrations. Moreover, the results here further demonstrate the importance of considering N, as well as P, in management strategies aimed at mitigating cyanobacterial blooms.

Simultaneous Microcystis Algicidal and Microcystin Degrading Capability by a Single Acinetobacter Bacterial Strain

Measures for removal of toxic harmful algal blooms often cause lysis of algal cells and release of microcystins (MCs). In this study, Acinetobacter sp. CMDB-2 that exhibits distinct algal lysing activity and MCs degradation capability was isolated. The physiological response and morphological characteristics of toxin-producing Microcystis aeruginosa, the dynamics of intra- and extracellular MC-LR concentration were studied in an algal/bacterial cocultured system. The results demonstrated that Acinetobacter sp. CMDB-2 caused thorough decomposition of algal cells and impairment of photosynthesis within 24 h. Enhanced algal lysis and MC-LR release appeared with increasing bacterial density from 1 × 10³ to 1 × 10⁷ cells/mL; however, the MC-LR was reduced by nearly 94% within 14 h irrespective of bacterial density. Measurement of extracellular and intracellular MC-LR revealed that the toxin was decreased by 92% in bacterial cell incubated systems relative to control and bacterial cell-free filtrate systems. The results confirmed that the bacterial metabolite caused 92% lysis of Microcystis aeruginosa cells, whereas the bacterial cells were responsible for approximately 91% reduction of MC-LR. The joint efforts of the bacterium and its metabolite accomplished the sustainable removal of algae and MC-LR. This is the first report of a single bacterial strain that achieves these dual actions.