Cyanobacteria, often called blue-green algae, are photosynthetic microorganisms that proliferate aggressively in nutrient-rich waters, a process known as eutrophication. Driven by agricultural runoff, sewage discharge, and warming climates, these blooms have become one of the most visible symptoms of degraded freshwater systems worldwide. Beyond the ecological disruption they cause, many bloom-forming cyanobacteria produce secondary metabolites that are toxic to animals and humans. Among these, microcystin-LR stands out as the most potent and most frequently detected variant. It is a cyclic heptapeptide that primarily targets the liver in vertebrates by inhibiting protein phosphatases, but its effects extend across many biological systems, and it is notably stable in the environment, persisting through light exposure and other degrading conditions long after a bloom has collapsed.

While the hazards of microcystin-LR to fish, zooplankton, and even humans have been documented extensively, benthic invertebrates living in the sediments beneath blooms have received far less attention. Chironomid larvae, the aquatic juveniles of non-biting midges, are among the most abundant and ecologically important of these bottom-dwelling organisms. They burrow in sediments, process organic matter, serve as a food source for fish and other predators, and are widely used as bioindicators in freshwater monitoring programs. Their position at the interface between contaminated sediments and the food web makes them a critical species for understanding how cyanobacterial toxins move and act in ecosystems. Yet until now, the sublethal consequences of realistic microcystin-LR exposure in these larvae remained largely unexplored.

To close this gap, a team of researchers led by Tamara Petronijević of the University of Niš in Serbia, working with colleagues at the Institute for Biological Research Siniša Stanković in Belgrade, designed a controlled exposure experiment using fourth-instar larvae of Chironomus riparius, a standard test organism in ecotoxicology. The larvae were exposed to four concentrations of microcystin-LR: 5, 10, 15, and 30 micrograms per liter. These values were chosen deliberately to reflect concentrations documented in natural freshwater systems during and after bloom events, rather than the artificially high doses that dominate much of the toxicological literature. This design choice matters, because risk assessments built on unrealistically high exposures can either overstate or, more insidiously, completely miss the subtle effects that occur at doses organisms actually experience.

The researchers employed a multi-biomarker approach, measuring several independent indicators of physiological stress in the same organisms. The first was hemoglobin, a molecule of particular relevance to chironomids. Unlike most insect larvae, Chironomus larvae possess extracellular hemoglobin that allows them to thrive in the oxygen-poor sediments they inhabit, and changes in hemoglobin levels are a sensitive indicator of compromised oxygen transport. The team also measured markers of oxidative stress, including advanced oxidation protein products, or AOPP, which signal protein damage caused by reactive oxygen species; malondialdehyde, or MDA, a product of lipid peroxidation that reflects damage to cell membranes; and the activities of the antioxidant enzymes catalase and superoxide dismutase, which form the first line of cellular defense against oxidative attack. Finally, they assessed DNA damage, the most consequential of the endpoints, since genotoxic injury can impair development, reproduction, and survival.

The results were striking. DNA damage increased significantly in larvae at every concentration tested, from the lowest dose of 5 micrograms per liter to the highest, and the response followed a clear dose-dependent pattern. This means that even the mildest, most environmentally plausible exposure produced measurable genotoxic effects, and that the damage grew consistently worse as the concentration rose. In a broader toxicological context, this finding aligns with a growing body of evidence that microcystins can damage genetic material through both direct and indirect mechanisms, including oxidative stress and interference with DNA repair pathways. For a benthic organism that lives continuously in contact with contaminated sediment and water, chronic genotoxic pressure could erode population fitness over time even in the absence of overt mortality.

Hemoglobin told a complementary story. Levels of the oxygen-carrying protein declined as microcystin-LR concentrations increased, with statistically significant reductions recorded at 5 and 10 micrograms per liter. Because chironomid larvae depend on hemoglobin to exploit hypoxic sediments, a reduction in this protein could constrain their ability to occupy their preferred microhabitats, forcing them into shallower, better-oxygenated zones where predation risk is higher. Previous research on related species has shown that hemoglobin expression in chironomids is sensitive to a range of contaminants, and the new data extend that pattern to cyanobacterial toxins, suggesting that impaired oxygen transport may be a key pathway through which microcystin-LR undermines larval fitness.

The antioxidant system showed a more nuanced response. Catalase activity decreased in a concentration-dependent manner across the exposure range, a result the authors interpret as evidence that the toxin may suppress this defensive enzyme rather than provoke it. Superoxide dismutase activity and MDA levels both showed increasing trends with rising toxin concentration, although these changes did not reach statistical significance. Meanwhile, AOPP levels, which reflect oxidatively damaged proteins, rose significantly at the 10 micrograms per liter exposure. Taken together, these patterns sketch a picture of oxidative imbalance: the toxin appears to push larvae toward protein oxidation and lipid damage while simultaneously weakening part of the enzymatic machinery that would normally contain the damage. Such a combination, if sustained, could compound the genotoxic effects by allowing reactive oxygen species to attack DNA unchecked.

The ecological implications extend beyond individual larvae. Chironomids occupy a central position in freshwater food chains, linking microbial communities and detritus in the sediment to fish, amphibians, and other predators above. Physiological stress in larvae — whether expressed as reduced oxygen transport, oxidative damage, or DNA injury — can translate into slower growth, delayed emergence, altered behavior, and reduced abundance, all of which propagate upward through the food web. There is also the question of trophic transfer: microcystins are known to accumulate in aquatic organisms and pass to their predators, meaning that stressed or contaminated chironomids may serve as a vector delivering toxins to higher trophic levels. The authors caution, however, that their findings rest on acute exposure under laboratory conditions, and that confirmation under chronic, environmentally realistic exposure scenarios is required before the full ecological weight of the results can be judged.

Nevertheless, the study marks an important step forward in cyanotoxin risk assessment. By anchoring exposure concentrations to real-world measurements and by reading the toxicological signal across multiple biological levels simultaneously, the work demonstrates that sublethal effects of microcystin-LR are not confined to laboratory extremes but begin at concentrations larvae plausibly encounter during bloom events. As eutrophication intensifies and cyanobacterial blooms expand under a warming climate, the findings underscore that the organisms most at risk may not be the most visible ones. Hidden in the sediments, quietly processing the aftermath of every bloom, the larvae of Chironomus riparius may be recording the true cost of freshwater toxic algae long before fish kills or drinking water advisories capture public attention.

Subject of Research: Sublethal ecotoxicological effects of the cyanobacterial toxin microcystin-LR on Chironomus riparius larvae in freshwater ecosystems

Article Title: Assessing the risk of cyanobacterial toxins in freshwater ecosystems: microcystin-LR exposure in Chironomus riparius larvae

Article References: Petronijević, T., Stojanović, J., Vitorović, J., Zdravković, D. S., Kolarević, M. K., Milošević, Đ., & Stanković, N. (2026). Assessing the risk of cyanobacterial toxins in freshwater ecosystems: microcystin-LR exposure in Chironomus riparius larvae. Ecotoxicology, 35(8), Article 168. https://doi.org/10.1007/s10646-026-03148-y

Image Credits: AI Generated

DOI: 10.1007/s10646-026-03148-y

Keywords: microcystin-LR, cyanobacterial toxins, Chironomus riparius, freshwater ecosystems, DNA damage, oxidative stress, biomarkers, hemoglobin, ecotoxicology, eutrophication, chironomid larvae, sublethal effects