Abstract
Microcystins (MCs) produced in eutrophic waters may decrease crop yield, enter food chains and threaten human and animal health. The main objective of this research was to highlight the role of rhizospheric soil microbiota to protect faba bean plants from MCs toxicity after chronic exposure. Faba bean seedlings were grown in pots containing agricultural soil, during 1 month under natural environmental conditions of Marrakech city in Morocco (March–April 2018) and exposed to cyanobacterial extracts containing up to 2.5 mg·L−1 of total MCs. Three independent exposure experiments were performed (a) agricultural soil was maintained intact “exposure experiment 1”; (b) agricultural soil was sterilized “exposure experiment 2”; (c) agricultural soil was sterilized and inoculated with the rhizobia strain Rhizobium leguminosarum RhOF34 “exposure experiment 3”. Overall, data showed evidence of an increased sensitivity of faba bean plants, grown in sterilized soil, to MCs in comparison to those grown in intact and inoculated soils. The study revealed the growth inhibition of plant shoots in both exposure experiments 2 and 3 when treated with 2.5 mg·L−1 of MCs. The results also showed that the estimated daily intake (EDI) of MCs, in sterilized soil, exceeded 2.18 and 1.16 times the reference concentrations (0.04 and 0.45 µg of microcysin-leucine arginine (MC-LR). Kg−1 DW) established for humans and cattle respectively, which raises concerns about human food chain contamination.
Highlights
Over the last decades, bloom-forming cyanobacteria have been increasing in frequency and intensity in eutrophic freshwater ecosystems worldwide, in which they produce and release, various deleterious secondary metabolites, into the extracellular environment [1,2,3]
Upon conducting a visual inspection of V. faba plants exposed for 28 days to cyanobacterial bloom extract containing 2.5 mg·L−1 microcystin-leucine arginine (MC-LR), no deleterious effects in faba bean plants were perceptible compared to the controls
At the end of the experiment, the effects of MCs on faba bean growth were studied by comparing the means of stem length (SL), shoot dry weight (SDW) and root dry weight (RDW) between the control and treatment groups at different exposure experiments (Table 1)
Summary
Bloom-forming cyanobacteria have been increasing in frequency and intensity in eutrophic freshwater ecosystems worldwide, in which they produce and release, various deleterious secondary metabolites (i.e., cyanotoxins), into the extracellular environment [1,2,3]. MCs are hepatotoxic cyclic heptapeptides produced by several bloom-forming cyanobacteria belonging to several genera such as Microcystis, Nostoc, Oscillatoria, Planktothrix and Dolichospermum [8,9,10,11,12,13] Owing to their ring structure, MCs are chemically stable under several field and laboratory conditions with half-life ranging from 0.4 to 22 days [14,15,16,17,18,19] and total concentrations varying from less than 1 μg·L−1 to 29,000 μg·L−1 in surface waters [20,21,22,23,24,25,26,27,28,29,30]. This paper suggested the use of symbiotically MC-tolerant and degrading bacteria as a new bioremediation tool and Toxins 2021, 13, 118 environmentally-relevant procedure to mitigate MCs phytotoxic effects and transfer to the crop plants and to the human and animal food chain
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