Impacts of Rac- and S-metolachlor on cyanobacterial cell integrity and release of microcystins at different nitrogen levels

Widely used herbicide metolachlor can promote harmful bloom formation by stimulating cyanobacterial growth and driving detrimental effects on their chytrid parasites

Metolachlor (MET) is a widely used herbicide that can adversely affect phytoplanktonic non-target organisms, such as cyanobacteria. Chytrids are zoosporic fungi ubiquitous in aquatic environments that parasitize cyanobacteria and can keep their proliferation in check. However, the influence of organic pollutants on the interaction between species, including parasitism, and the associated ecological processes remain poorly understood. Using the host-parasite system consisting of the toxigenic cyanobacterium Planktothrix agardhii and its chytrid parasite Rhizophydium megarrhizum, we investigated the effects of environmentally relevant concentrations of MET on host-parasite interactions under i) continuous exposure of chytrids and cyanobacteria, and ii) pre-exposure of chytrids. During a continuous exposure, the infection prevalence and intensity were not affected, but chytrid reproductive structures were smaller at the highest tested MET concentration. In the parasite's absence, MET promoted cyanobacteria growth possibly due to a hormesis effect. In the pre-exposure assay, MET caused multi- and transgenerational detrimental effects on parasite fitness. Chytrids pre-exposed to MET showed reduced infectivity, intensity, and prevalence of the infection, and their sporangia size was reduced. Thus, pre-exposure of the parasite to MET resulted in a delayed decline of the cyanobacterial cultures upon infection. After several parasite generations without MET exposure, the parasite recovered its initial fitness, indicating that detrimental effects are transient. This study demonstrates that widely used herbicides, such as MET, could favor cyanobacterial bloom formation both directly, by promoting cyanobacteria growth, and indirectly, by inhibiting their chytrid parasites, which are known to play a key role as top-down regulators of cyanobacteria. In addition, we evidence the relevance of addressing multi-organism systems, such as host-parasite interactions, in toxicity assays. This approach offers a more comprehensive understanding of the effects of pollutants on aquatic ecosystems.

A strategy to reduce the dose of multichiral agricultural chemicals: The herbicidal activity of metolachlor against Echinochloa crusgalli

Agricultural chemicals are normally used as mixtures of several isomers, e.g., enantiomers. In theory, in order to minimize the pesticides dose, it is desirable to use the most target-active isomer. Metolachlor is a typical multichiral herbicide belonging to amide herbicides. An asymmetric carbon atom and a chiral axis yield four stereoisomers. In this study, a novel laboratory method was developed to prepare the S-metolachlor and the four stereoisomers using high performance liquid chromatography. The separated isomers had a purity of >99%, with their absolute configurations assigned by electronic circular dichroism. The enantioseparation by ultra performance convergence chromatography tandem mass spectrometry was also performed for the rapid and sensitive detection of metolachlor stereoisomers. The enantioselective herbicidal activity toward the target weed (Echinochloa crusgalli) was systematically assessed for the first time by measuring the morphology of the weed after treatment with rac-, S-metolachlor and the four stereoisomers, respectively. Among the commercial pesticides, S-metolachlor was more effective in weed inhibition than rac-metolachlor, and to the four stereoisomers, the herbicidal activities were ranked as: SS > SR ≫ RS > RR, and the RR-isomer even had some stimulative effect to the weed growth at lower concentration (1 ppm). Thus, we concluded that in these cases, the chiral carbon feature played a major role in herbicidal activity rather than the chiral axis feature, and the higher bioactivity of the S-isomers was confirmed by more effective uptake and stronger interaction with target enzymes that were involved in the gibberellic acid biosynthesis. Although the SS-isomer shows the highest herbicidal activity, controlling the major chiral feature is still much easier and more economical than controlling two chiral features.

Response of bloom-forming cyanobacterium Microcystis aeruginosa to 17β-estradiol at different nitrogen levels

Co-existence of cyanobacterial harmful algal blooms (CyanoHABs) and steroid estrogens (SEs) has been an increasing concern in eutrophic waters. The cellular responses and biodegradation of 17β-estradiol (E2) in cyanobacterium Microcystis aeruginosa were investigated at different nitrogen levels. During the 10-d experiment, the growth of M. aeruginosa was stimulated by 10-100 μg L^-1 of E2 at the lowest nitrogen level of 0.5 mg L^-1, whereas the presence of E2 inhibited the cyanobacterial growth at 5 mg L^-1 of nitrogen. With nitrogen concentration increased to 50 mg L^-1, the impact of E2 on levels of growth rate and chlorophyll a (Chla) alleviated. Exposure to E2 also promoted the superoxide dismutase activity of M. aeruginosa, coupled with cellular oxidative damage as indicated by the increasing malondialdehyde content. A sufficient nitrogen supply mitigated the oxidative stress of E2 through enhancing the synthesis of detoxification-related enzymes. Simultaneously, the secretion of tryptophan-like substances in loosely- and tightly-bound extracellular polymeric substances was triggered for adapting to an E2 addition in the short term. Moreover, significant biodegradation of E2 was observed, and the process followed a first-order kinetic reaction. The obtained half-lives decreased with nitrogen levels and ranged from 2.47 to 2.81 and 3.39-5.04 d, respectively, at 10 and 100 μg L^-1 of E2. These results provide a better understanding of the potential effects of SEs on CyanoHABs formation, as well as the important role of CyanoHABs on SEs removal in aquatic ecosystems, which should be fully considered in the control of combined pollution.

Enantioselective Toxicity of Chiral Herbicide Metolachlor to Microcystis aeruginosa

The enantioselective effects of chiral herbicides on aquatic organisms have received increasing attention. As one kind of freshwater algae responsible for most algal blooms, Microcystis aeruginosa can produce hepatotoxic microcystin and cause serious health concerns for drinking water. Thus, the effects of chiral herbicides on M. aeruginosa are of vital significance but poorly understood, especially as the structures of chiral herbicides become more complex. In this study, the enantioselective effects of four metolachlor enantiomers based on carbon center and axis chirality on M. aeruginosa were investigated for the first time at an enantiomeric level. The results of the investigation into algal growth inhibition, chlorophyll a content, and cell integrity indicated that ( S)-metolachlor [( S)-Met] was significantly more toxic than any other isomer. The toxicity ranking of different enantiomers at the highest concentration (15 mg/L) against M. aeruginosa was ( S)-Met > (α R,1' S)-Met > (α S,1' S)-Met > (α S,1' R)-Met > (α R,1' R)-Met, with (α S,1' S)-Met and (α R,1' S)-Met displaying a synergistic effect. Additionally, the Fe distribution in M. aeruginosa presented distinct enantioselectivity, which may contribute to the enantioselective toxicity of metolachlor. Furthermore, metolachlor upregulated the expression of genes mcyD and mcyH in an enantioselective manner, indicating that this herbicide can potentially promote the synthesis and efflux of microcystin, thus aggravating agricultural water contamination to different extents. Overall, this study will help to understand the ecotoxicity of metolachlor at a deeper level and provide theoretical insights into the enantioselective behaviors of metolachlor.

Effect of hydrogen peroxide on Microcystic aeruginosa: Role of cytochromes P450

Cyanobacterial bloom has been rising as a worldwide issue owing to its adverse effects to water quality and ecological health. To solve this problem, hydrogen peroxide (H₂O₂) has been considered as a potential algaecide because no by-products are generated after treatment and because it kills cyanobacteria selectively. In addition, cytochromes P450 (CYPs) was reported to be related with H₂O₂, but the roles of CYPs in the regulation of H₂O₂ in cyanobacteria have yet to be investigated. In this study, the CYPs suicide inhibitor 1-aminobenzotriazole (ABT) was added to the representative cyanobacteria Microcystis aeruginosa (M. aeruginosa) exposed to H₂O₂. The results showed that CYPs mediates the effects of H₂O₂ on M. aeruginosa. To be exact, the addition of ABT induced greater inhibitory effects on the growth and higher reactive oxygen species levels in M. aeruginosa comparing to those treated with H₂O₂ alone. At the same time, photosynthetic parameters significantly decreased, and the content of extracellular microcystins (MCs) increased but the total MCs decreased due to the combined effect of H₂O₂ and ABT. ABT also intensified the aggregation of Fe, which might explain the effects on photosynthesis and synthesis of MCs. Furthermore, the transcriptional levels of MCs-synthesis genes (mcyA and mcyD) decreased but MCs-release gene (mcyH) increased, and photosynthetic genes (psaB, psbD1 and rbcL) decreased, which confirmed the effects on the MC production/release and electron transport of photosynthesis, respectively. In summary, this study illuminated the mediation role of CYPs in the adverse effects on M. aeruginosa induced by H₂O₂, thus providing new theoretical basis for the explanation of H₂O₂ as potential algaecide.