Cyanobacteria commonly form large blooms in waterbodies; they can produce cyanotoxins, with toxic effects on humans and animals, and volatile compounds, causing bad tastes and odors (T&O) at naturally occurring low concentrations. Notwithstanding the large amount of literature on either cyanotoxins or T&O, no review has focused on them at the same time. The present review critically evaluates the recent literature on cyanotoxins and T&O compounds (geosmin, 2-methylisoborneol, β-ionone and β-cyclocitral) to identify research gaps on harmful exposure of humans and animals to both metabolite classes. T&O and cyanotoxins production can be due to the same or common to different cyanobacterial species/strains, with the additional possibility of T&O production by non-cyanobacterial species. The few environmental studies on the co-occurrence of these two groups of metabolites are not sufficient to understand if and how they can co-vary, or influence each other, perhaps stimulating cyanotoxin production. Therefore, T&Os cannot reliably serve as early warning surrogates for cyanotoxins. The scarce data on T&O toxicity seem to indicate a low health risk (but the inhalation of β-cyclocitral deserves more study). However, no data are available on the effects of combined exposure to mixtures of cyanotoxins and T&O compounds and to combinations of T&O compounds; therefore, whether the co-occurrence of cyanotoxins and T&O compounds is a health issue remains an open question.

In recent years macrophytes have been considered promising tools in the biocontrol of harmful cyanobacteria blooms (cyanoHABs). In this study, the inhibitory effect of aqueous extracts of Ranunculus aquatilis and Nasturtium officinale on Microcystis aeruginosa growth was assessed via six treatments (0%, 0.1%, 0.25%, 0.50%, 0.75%, and 1% extracts). Chlorophyll a and carotenoid content were analyzed and changes in cell and colonial morphology of M. aeruginosa cultures were observed. Also, to reveal potential allelochemical compounds, total phenols (TPs), total flavonoids (TFs), and tannins (TTs) were analyzed in both extracts. The obtained results showed that M. aeruginosa growth was significantly inhibited by R. aquatilis and N. officinale aqueous extracts in a concentration-dependent way. After 8 days of treatment, the highest inhibition rates reached 100% and 75.74% respectively. The Chlorophyll a and carotenoid concentrations were decreased compared to the control group. Colonial and cell and colonial morphology changes were observed under the treatment group with 1% of aqueous extract accompanied by sedimentation of the cyanobacterial cells. This study shows that M. aeruginosa growth inhibition was induced by the total polyphenol, flavonoids and tannins. It was concluded that theses macrophytes may control M. aeruginosa and may be useful to control harmful blooms in lake-reservoirs.

The health risks linked to the consumption of microcystin-accumulating crops have been increasing worldwide in toxic cyanobloom-occurring regions. The bioaccumulation of microcystins (MCs) in agricultural produce at environmentally realistic concentrations is poorly investigated. In this field study, we assessed the health risks of MCs in raw water used for irrigating fruit crops (bioaccumulation) and watering farm animals in the Lalla Takerkoust agricultural region (Marrakesh, Morocco). Thus, MCs were extracted from water and fruit samples and quantified by enzyme-linked immunosorbent assay in order to calculate the health risk indicators. MCs posed a high health-risk level to poultry and horses, with estimated daily intakes (EDI) being 14- and 19-fold higher than the recommended limits (3.1 and 2.3 μg MC-LR L⁻¹), respectively. Furthermore, pomegranate posed the same level of risk, with EDI being 22- and 53-fold higher than the limit dose (0.04 μg MC-LR kg⁻¹) for adults and children, respectively. There was an urgent need for guidelines regarding water use and management in MC-polluted areas, besides the setup of nature-based tools for toxin removal from raw water used in farming practices. Moreover, MCs could contaminate the human food chain, which implies further investigations of their potential accumulation in livestock- and poultry-based food.

The intensive human activities extensively contaminated water sources making its treatment a problem of paramount importance, especially with the increasing of global population and water scarcity. The application of natural coagulants has become a promising and environmentally friendly alternative to conventional ones. This study was aimed at evaluating the efficiency of four plant extracts namely Agave americana, Carpobrotus acinaciformis, Austrocylindropuntia subulate, and Senicio anteuphorbium as natural coagulants to remove Microcystis aeruginosa cyanobacterium from water. The effects of pH (4, 5, 6, 7, 8 9, and 10) and coagulant dose (5, 10, 15, 20, 25, and 30 mg/L) on the coagulation efficiency were investigated. Results showed that plant‐based extracts exhibited high coagulant abilities significantly contributing to the removal of M. aeruginosa cells up to 80% on a case‐by‐case basis. The ecotoxicity (Daphnia magna, Aliivibrio fischeri, Raphidocelis subcapitata, and Sorghum saccharatum) was absent or presented very slight acute toxicity up to 12.5 mg/L being S. anteuphorbium the least toxic.Practitioner Points: Nature‐based plant extracts showed removal rates up to 80%. Lower pH and A. subulate and S. anteuphorbium were the most efficient coagulants Toxicity effects were plant extracts‐based and dose function. A. subulate and S. anteuphorbium were the least toxic extracts.

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.