Interspecific competition reveals Raphidiopsis raciborskii as a more successful invader than Microcystis aeruginosa

New records on toxic cyanobacteria from Brazil: Exploring their occurrence and geography

Cyanobacterial Harmful Algal Blooms (CyanoHABs) pose a significant threat to communities globally, impacting ecosystems and public health. This study provides an in-depth review of the current state of cyanotoxins and the distribution of CyanoHABs species in Brazil, while also detailing the methods used for their detection. Four hundred and twenty-one incidents were analyzed from 1993 to 2021, compiling cyanotoxin records and toxic CyanoHABs occurrences. The investigation begins with the first detection of microcystins in 1994 and highlights pivotal moments, like the 1996 "Caruaru Syndrome" outbreak. This event encouraged research and updated cyanotoxin-monitoring guidelines. The Brazilian drought period of 2015-2016 exacerbated cyanobacterial growth and saxitoxin levels, coinciding with Zika-related microcephaly. This study delves into methods used for cyanotoxin analysis, including ELISA, bioassays, HPLC, and LC-MS. Additionally, we investigated the toxicity of 37 cyanobacterial strains isolated from various Brazilian environments. Extracts were tested against Artemia salina and analyzed by LC-MS. Results revealed toxicity in extracts from 49 % of cyanobacterial strains. LC-MS results were analyzed using GNPS MS/MS molecular networking for comparing experimental spectra with those of cyanotoxin standards against in-house databases and the existing literature. Our research underscores the variability in cyanotoxin production among species and over time, extending beyond microcystins. LC-MS results, interpreted through the GNPS platform, revealed six cyanotoxin groups in Brazilian strains. Yet, compounds present in 75 % of the toxic extracts remained unidentified. Further research is crucial for fully comprehending the impact of potentially harmful organisms on water quality and public health management strategies. The study highlights the urgent need for continuously monitoring cyanobacteria and the cyanotoxin inclusion of management in public health policies.

Assessment of cyanotoxins in water and fish in an African freshwater lagoon (Lagoon Aghien, Ivory Coast) and the application of WHO guidelines

In comparison with northern countries, limited data are available on the occurrence and potential toxicity of cyanobacterial blooms in lakes and ponds in sub-Saharan countries. With the aim of enhancing our knowledge on cyanobacteria and their toxins in Africa, we performed a 17-month monitoring of a freshwater ecosystem, Lagoon Aghien (Ivory Coast), which is used for multiple practices by riverine populations and for drinking water production in Abidjan city. The richness and diversity of the cyanobacterial community were high and displayed few variations during the entire survey. The monthly average abundances ranged from 4.1 × 104 to 1.8 × 105 cell mL-1, with higher abundances recorded during the dry seasons. Among the five cyanotoxin families analyzed (anatoxin-a, cylindrospermopsin, homoanatoxin, microcystins, saxitoxin), only microcystins (MC) were detected with concentrations ranging from 0 to 0.364 μg L-1 in phytoplankton cells, from 32 to 1092 μg fresh weight (FW) kg-1 in fish intestines, and from 33 to 383 μg FW kg-1 in fish livers. Even if the MC concentrations in water and fish are low, usually below the thresholds defined in WHO guidelines, these data raise the issue of the relevance of these WHO guidelines for sub-Saharan Africa, where local populations are exposed throughout the year to these toxins in multiple ways.

Recent Advances in the Ecology of Bloom-Forming Raphidiopsis (Cylindrospermopsis) raciborskii: Expansion in China, Intraspecific Heterogeneity and Critical Factors for Invasion

Water blooms caused by the invasive cyanobacterium Raphidiopsis raciborskii occur in many reservoirs in the tropical and subtropical regions of China. In recent decades, this species has spread rapidly to temperate regions. Phenotypic plasticity and climate warming are thought to promote the worldwide dispersion of R. raciborskii. However, investigations into the genetic and phenotypic diversities of this species have revealed significant intraspecific heterogeneity. In particular, competition between R. raciborskii and Microcystis aeruginosa was highly strain dependent. Although the concept of an ecotype was proposed to explain the heterogeneity of R. raciborskii strains with different geographic origins, microevolution is more reasonable for understanding the coexistence of different phenotypes and genotypes in the same environment. It has been suggested that intraspecific heterogeneity derived from microevolution is a strong driving force for the expansion of R. raciborskii. Additionally, temperature, nutrient fluctuations, and grazer disturbance are critical environmental factors that affect the population establishment of R. raciborskii in new environments. The present review provides new insights into the ecological mechanisms underlying the invasion of R. raciborskii in Chinese freshwater ecosystems.

Cyanotoxins and water quality parameters as risk assessment indicators for aquatic life in reservoirs

We assessed the extent of pollution in an essential public water supply reservoir (southeastern Brazil). An environmental monitoring study was performed at the Billings Reservoir (at the water catchment site) to assess the water quality in 2017, 2018, and 2019. Physicochemical parameters were analyzed, quantifying the total cyanobacteria and the cyanotoxins microcystins (MCs) and saxitoxins (SXTs), as well as their possible ecological risk to the aquatic environment. We also determined metals and metalloids (As, Ba, Cd, Pb, Cu, Cr, Fe, Mn, Ni, Zn, and Sb) and fecal bacteria (Escherichia coli). Monthly samplings were performed for 2017, 2018, and 2019 (totaling 36 sampling campaigns). Metals, metalloids, and E. coli values were below the maximum limit allowed by the Brazilian legislation. High concentrations of total cyanobacteria (3.07 × 10⁴ - 3.23 × 10⁵ cells/mL), microcystin variants MC-LR (0.67-23.63 μg/L), MC-LA (0.03-8.66 μg/L), MC-RR (0.56-7.92 μg/L), and MC-YR (0.04-1.24 μg/L), as well as the saxitoxins GTX2 (0.18-5.37 μg/L), GTX3 (0.13-4.40 μg/L), and STX (0.12-2.92 μg/L) were detected. From an ecotoxicological point of view, the estimated values for the risk quotient (RQ) for microcystins and saxitoxins were largely greater than 1, indicating a high risk to aquatic life. Therefore, further efforts need to be made to delay the eutrophication of the reservoir.

Cyanobacterial Harmful Algal Blooms in Aquatic Ecosystems: A Comprehensive Outlook on Current and Emerging Mitigation and Control Approaches

An intensification of toxic cyanobacteria blooms has occurred over the last three decades, severely affecting coastal and lake water quality in many parts of the world. Extensive research is being conducted in an attempt to gain a better understanding of the driving forces that alter the ecological balance in water bodies and of the biological role of the secondary metabolites, toxins included, produced by the cyanobacteria. In the long-term, such knowledge may help to develop the needed procedures to restore the phytoplankton community to the pre-toxic blooms era. In the short-term, the mission of the scientific community is to develop novel approaches to mitigate the blooms and thereby restore the ability of affected communities to enjoy coastal and lake waters. Here, we critically review some of the recently proposed, currently leading, and potentially emerging mitigation approaches in-lake novel methodologies and applications relevant to drinking-water treatment.

Ecophysiological Aspects and sxt Genes Expression Underlying Induced Chemical Defense in STX-Producing Raphidiopsis raciborskii (Cyanobacteria) against the Zooplankter Daphnia gessneri

Cyanobacteria stand out among phytoplankton when they form massive blooms and produce toxins. Because cyanotoxin genes date to the origin of metazoans, the hypothesis that cyanotoxins function as a defense against herbivory is still debated. Although their primary cellular function might vary, these metabolites could have evolved as an anti-predator response. Here we evaluated the physiological and molecular responses of a saxitoxin-producing Raphidiopsis raciborskii to infochemicals released by the grazer Daphnia gessneri. Induced chemical defenses were evidenced in R. raciborskii as a significant increase in the transcription level of sxt genes, followed by an increase in saxitoxin content when exposed to predator cues. Moreover, cyanobacterial growth decreased, and no significant effects on photosynthesis or morphology were observed. Overall, the induced defense response was accompanied by a trade-off between toxin production and growth. These results shed light on the mechanisms underlying zooplankton-cyanobacteria interactions in aquatic food webs. The widespread occurrence of the cyanobacterium R. raciborskii in freshwater bodies has been attributed to its phenotypic plasticity. Assessing the potential of this species to thrive over interaction filters such as zooplankton grazing pressure can enhance our understanding of its adaptive success.

Occurrence of Raphidiopsis raciborskii blooms in cool waters: Synergistic effects of nitrogen availability and ecotypes with adaptation to low temperature

Raphidiopsis raciborskii is a diazotrophic and potentially toxic cyanobacterium. To date, this species has successfully invaded many regions from the tropics to sub-tropical and temperate regions, typically forming blooms at temperatures greater than 25 °C. However, there have been a few cases in which R. raciborskii blooms have occurred at low temperatures (below 15 °C), but its cause and mechanisms remain unclear. In this study, field investigations revealed that R. raciborskii blooms occurred at 10-15 °C in Lake Xihu, Yunnan, China. The biomass of R. raciborskii was found to be positively related to nitrate concentrations in this lake. Three strains of R. raciborskii, two isolated from Lake Xihu (CHAB 6611 and CHAB 6612) and one from Lushui Reservoir in central China (CHAB 3409), were used for growth experiments at 15 °C. The three strains exhibited genotypic (16S rRNA and ITS-L genes) and physiological differences in response to nitrogen concentrations at low temperature. The growth rates of strains CHAB 6611 and CHAB 6612 increased with nitrogen concentration while CHAB 3409 could not grow at 15 °C. Furthermore, the growth and phenotypic responses of CHAB 6611 and CHAB 6612 to nitrogen concentrations were different, despite the closer genetic relationship shared by these two strains. Thus, increased nitrogen concentration in water may enhance the biological availability and utilization of nitrogen by R. raciborskii, which is the external promoter, leading to improving the resistance of R. raciborskii to low temperature. The internal cause is the presence of ecotypes in R. raciborskii populations with adaptation to low temperature. With increasing global eutrophication, the distribution range of R. raciborskii as well as the scale of its blooms will increase. As such, the risk of exposure of aquatic biota and humans to cylindrospermopsin is also expected to increase.

Invasion and toxin production by exotic nostocalean cyanobacteria (Cuspidothrix, Cylindrospermopsis, and Sphaerospermopsis) in the Nakdong River, Korea

The extent and frequency of harmful cyanobacterial blooms are increasing, owing to the climate change caused by global warming, and some harmful filamentous cyanobacteria that were first reported in the tropics are spreading to temperate regions, such as North America, Europe, and Northeast Asia. Although these exotic invasive cyanobacteria have a high toxigenic potential, they are not targeted in management plans in many countries. This study analyzed the occurrence of and potential toxin and off-flavor secondary metabolite production by invasive nostocalean cyanobacteria in the Nakdong River in Korea, which is a temperate region. The occurrence of four species belonging to three genera of cyanobacteria was confirmed in the Nakdong River. The quantities of cyanobacteria in the Nakdong River were mostly low, fewer than 1,000 cells mL^-1. Twenty-four strains belonging to four species in three genera of cyanobacteria were isolated from the Nakdong River. Analysis revealed no off-flavor secondary metabolite production by any of the isolates, and those belonging to Cylindrospermopsis raciborskii, Sphaerospermopsis aphanizomenoides, and S. reniformis were identified as nontoxic strains. However, anatoxin-a production was observed in two of the eleven isolates of Cuspidothrix issatschenkoi. Given the sites and the timing of its occurrence, C. issatschenkoi had the highest potential for toxin production among the invasive nostocalean cyanobacteria appearing in the Nakdong River.