Key Takeaways & Executive Findings
- •• • Malathion exhibits a biphasic effect on Microcystis aeruginosa: 100 mg·L−1 reduces cell density to 8% of control, while 0.01 mg·L−1 increases cell density by 47%, indicating hormesis that can exacerbate algal blooms at environmentally relevant concentrations. • • Low-dose malathion (0.01 mg·L−1) stimulates photosynthesis and metabolism, elevating chlorophyll-a by 51% and ATP content by 98%, which directly boosts algal proliferation and bloom potential in aquatic systems. • • Sub-inhibitory malathion upregulates microcystin synthesis genes (mcyA, mcyB), increasing intracellular microcystin production to 1.6-fold of controls, thereby amplifying toxin-related health risks even at low exposure levels. • • High malathion concentration (100 mg·L−1) causes >80% membrane damage, triggering microcystin release into water, which poses acute toxicity risks and complicates water treatment.
Abstract
The extensive agricultural application of organophosphorus pesticides (OPPs) has established them, alongside nitrogen and phosphorus nutrients, as core pollutants in agricultural non-point source contamination, necessitating urgent clarification of their composite ecological effects on aquatic ecosystems. This study systematically investigated the concentration gradient effects and ecological risk mechanisms of malathion, a heavily utilized OPP, on the bloom-forming cyanobacterium Microcystis aeruginosa. Results demonstrated a pronounced concentration-dependent biphasic effect: high concentrations (100 mg·L−1) suppressed algal growth, reducing cell density to 8% of the control group, whereas low concentrations (0.01 mg·L−1) markedly stimulated algal proliferation (47% cell density increase) through photosynthetic system activation (51% chlorophyll-a elevation), metabolic enhancement (98% ATP content increase), and mild oxidative stress induction. Regarding microcystin regulation, low-concentration exposure upregulated microcystin synthesis genes (mcyA, mcyB), elevating intracellular microcystin production to 1.6-fold of controls. Conversely, high concentrations triggered microcystin release via severe membrane integrity disruption (>80% membrane damage). Risk assessment demonstrated that environmentally relevant malathion levels (0.01 mg·L−1) pose dual threats: exacerbating ecological risks by promoting algal blooms and amplifying health hazards through intensified microcystin synthesis and release. These findings provide critical theoretical insights for evaluating OPP ecotoxicity and formulating cyanobacterial bloom control strategies in agricultural non-point source pollution management.
1. Introduction
Agricultural non-point source pollution is dominated by co-occurring organophosphorus pesticides (OPPs) and nitrogen/phosphorus nutrients, yet their combined ecological effects on aquatic primary producers remain poorly constrained. Conventional risk assessments focus on acute toxicity thresholds, overlooking sub-lethal stimulatory effects that can drive harmful algal blooms. This study addresses this bottleneck by systematically characterizing the concentration-dependent response of Microcystis aeruginosa to malathion, a widely used OPP, linking physiological metrics to toxin production and ecological risk.
Existing water quality guidelines for OPPs are based on direct toxicity to non-target organisms, but fail to account for hormetic effects that enhance cyanobacterial growth and microcystin synthesis at trace concentrations. By integrating cell density, photosynthetic activity, ATP content, membrane integrity, and microcystin gene expression, this work provides a mechanistic framework for reassessing OPP ecological risks, informing bloom control strategies and regulatory thresholds in agricultural watersheds.
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LI Sisi, DAO Guohua, PAN Xuejun (2026). Physiological and biochemical characteristics and ecological risk assessment of Microcystis aeruginosa under Malathion stress. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025010902
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Frequently Asked Questions
What are the specific mechanisms by which low-dose malathion (0.01 mg·L−1) stimulates Microcystis aeruginosa growth and toxin production?
Low-dose malathion activates the photosynthetic system (chlorophyll-a +51%), enhances metabolic activity (ATP +98%), and induces mild oxidative stress, collectively increasing cell density by 47%. It also upregulates microcystin synthesis genes mcyA and mcyB, leading to a 1.6-fold increase in intracellular microcystin content.
How does high-dose malathion (100 mg·L−1) affect cell viability and microcystin release?
High-dose malathion causes severe membrane damage (>80% membrane damage), leading to cell lysis and a drastic reduction in cell density to 8% of control. This membrane disruption triggers the release of intracellular microcystins into the surrounding water, posing acute toxicity risks.
What are the ecological implications of the biphasic dose-response observed in this study?
The biphasic response indicates that environmentally relevant low concentrations of malathion can promote algal blooms and increase microcystin production, while high concentrations cause cell death and toxin release. This dual threat complicates risk assessment and management of OPP contamination in aquatic ecosystems.
How do these findings inform regulatory thresholds for malathion in agricultural runoff?
Current thresholds based on acute toxicity may underestimate risks from low-dose stimulation. The study suggests that even at 0.01 mg·L−1, malathion can enhance bloom formation and toxin synthesis, necessitating stricter limits or consideration of sub-lethal effects in water quality criteria.
What are the potential limitations of extrapolating these laboratory results to field conditions?
Laboratory conditions do not fully replicate natural environmental factors such as nutrient variability, microbial interactions, and sunlight. However, the observed effects at 0.01 mg·L−1, which is within reported environmental concentrations, suggest that similar stimulatory effects could occur in the field, warranting further mesocosm studies.
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