The research was led by Patrícia Alexandre Evangelista with support from FAPESP. It combined several approaches, including environmental monitoring, studies of how pollutants build up in organisms, analyses of genetic damage in aquatic life, and experiments using plants to remove contaminants. This broad strategy allowed the team to better understand both the scale of the problem and possible ways to address pollution linked to human and veterinary drug use.

Pollution Sources and Seasonal Patterns

Samples were collected near the Santa Maria da Serra dam, close to the Barra Bonita reservoir, where contaminants from across the river basin tend to gather. This region receives inputs from treated sewage, household wastewater, aquaculture operations, pig farming, and agricultural runoff.

The researchers analyzed water, sediment, and fish during both the rainy and dry seasons. They monitored 12 commonly used antibiotics from groups such as tetracyclines, fluoroquinolones, sulfonamides, and phenols. “The results showed a clear pattern of seasonality. During the rainy season, most antibiotics had concentrations below detection limits. In the dry season, however, when water volume decreases and contaminants become concentrated, different compounds were detected,” says Evangelista.

Measured levels ranged from nanograms per liter in water to micrograms per kilogram in sediment. Some antibiotics, including enrofloxacin and certain sulfonamides, were found in sediment at higher levels than those reported in similar studies worldwide. Because the sediment is rich in organic matter and nutrients like phosphorus, calcium, and magnesium, it can store these compounds and potentially release them back into the environment over time.

Banned Antibiotic Found in Fish

“One of the most significant findings of the study was the detection of chloramphenicol in lambari fish (Astyanax sp.) collected from local fishermen in the Barra Bonita region. Chloramphenicol is an antibiotic whose use in livestock is prohibited in Brazil precisely because of the risks associated with its toxicity,” the researcher states.

This substance appeared only during the dry season, at levels of tens of micrograms per kilogram. Since lambari fish are widely consumed in the region, this raises concerns about possible exposure to antibiotics through food.

Evangelista explains that chloramphenicol and enrofloxacin were selected for detailed lab experiments because of their importance to both environmental and human health. “Enrofloxacin is widely used in animal husbandry, including aquaculture, as well as in human medicine. Chloramphenicol, on the other hand, is still used in humans despite being banned for food-producing animals and serves as a historical marker of persistent contamination,” she explains.

Can Aquatic Plants Remove Antibiotics?

The team also explored whether Salvinia auriculata, a floating plant often considered invasive, could help clean contaminated water.

In controlled experiments, the plant was exposed to both typical environmental concentrations and levels 100 times higher for enrofloxacin and chloramphenicol. Carbon-14-radiolabeled compounds were used to precisely track how the antibiotics moved through the water, plant, and fish.

“The results showed the high efficiency of Salvinia in removing enrofloxacin. In treatments with higher plant biomass, more than 95% of the antibiotic was removed from the water within a few days. The half-life of the compound dropped to about two to three days. In the case of chloramphenicol, removal was slower and partial. The plant was able to remove 30% to 45% of the antibiotic from the water, with half-lives ranging from 16 to 20 days, indicating the greater persistence of the compound in the environment,” the researcher reports.

Imaging techniques showed that the antibiotics mainly accumulated in the plant’s roots, suggesting that root absorption and filtration play a key role.

Complex Effects on Fish Exposure

One of the more challenging findings involved how these antibiotics behave inside fish. Experiments showed that lowering the amount of antibiotics in the water does not always reduce how much fish absorb.

Enrofloxacin tended to stay dissolved in the water and was eliminated relatively quickly by lambari fish, with a half-life of about 21 days and low accumulation in tissues. Chloramphenicol behaved very differently. It persisted much longer in the fish, with a half-life exceeding 90 days and a strong tendency to build up in tissues.

The presence of Salvinia auriculata changed these dynamics. While the plant reduced antibiotic levels in the water, it sometimes increased how quickly fish absorbed them. One possible explanation is that the plant alters the chemical form of the antibiotics, making them easier for fish to take in.

“This shows that using plants as ‘sponges’ for contaminants is not a trivial matter. The presence of the macrophyte changes the entire system, including the way the organism comes into contact with the contaminant,” Evangelista notes.

DNA Damage in Fish and Potential Protection

The study also examined genetic damage in fish. Chloramphenicol significantly increased DNA damage, measured by changes in blood cells such as micronuclei and other abnormalities. However, when Salvinia auriculata was present, this damage decreased and approached levels seen in control groups. For enrofloxacin, the plant did not significantly reduce genetic effects.

“The interpretation we propose is that, in the case of chloramphenicol, the plant may generate fewer genotoxic byproducts or release antioxidant compounds into the rhizosphere, reducing oxidative stress in the fish. On the other hand, enrofloxacin is chemically more stable and may produce persistent and potentially toxic metabolites whose action is not neutralized by the macrophyte,” the researcher comments.

Promise and Limits of Nature-Based Solutions

Evangelista emphasizes that Salvinia auriculata is not a simple fix for antibiotic pollution. While it shows potential, there are important limitations. One concern is how to manage the plant after it absorbs contaminants. If the biomass is not properly removed and treated, it could release antibiotics back into the environment.

Even so, aquatic plants may offer a low-cost, nature-based option for reducing pollution, especially in places where advanced treatment methods like ozonation or other oxidative processes are too expensive.

“The study shows that the problem is real, measurable, and complex. And any strategy to address it must consider not only the removal of the contaminant, but also its biological and ecological effects,” the researcher concludes.

Growing Environmental and Public Health Concerns

“The detection of antibiotic residues in the water, sediments, and fish of the Piracicaba River shows just how harmful human activities can be. The resistance of microorganisms to antibiotics can lead to the emergence of superbugs in the environment. The research yielded positive results with low-cost environmental solutions and enabled a better understanding of the integrated functioning of aquatic ecosystems and the use of effective natural techniques for impact mitigation,” adds Valdemar Luiz Tornisielo, supervisor of Evangelista’s research and co-author of the article.

The radiolabeled molecules used in the study were provided by the International Atomic Energy Agency (IAEA).