Antibiotic-Metal Complexes Prove More Toxic to Freshwater Algae Than Either Contaminant Alone

When antibiotics and heavy metals meet in polluted water, they can chemically bind together into complexes whose effects on aquatic life differ sharply from those of the parent compounds. A laboratory study published in Ecotoxicology has synthesized four such complexes of the widely used antibiotic ciprofloxacin — with iron(III), cobalt(II), nickel(II), and copper(II) — and found that each one inhibited the growth of a freshwater alga more strongly than either the antibiotic or the free metal ions on their own. Of the four, the ciprofloxacin-copper(II) complex showed the strongest toxic effect.

The research team, Dipak Baral, Ajaya Bhattarai, and Narendra Kumar Chaudhary, prepared the four ciprofloxacin-metal complexes and characterized them using several complementary techniques. Fourier-transform infrared spectroscopy (FT-IR) and UV-visible spectroscopy were used to probe the bonding between the antibiotic and each metal, while MALDI-TOF mass spectrometry and inductively coupled plasma mass spectrometry (ICP-MS) provided information on the complexes’ molecular masses and metal content.

To compare toxicity, the researchers exposed the freshwater alga Spirogyra columbiana to ciprofloxacin alone, to free metal ions, and to each of the four complexes under controlled laboratory conditions. After 72 hours of exposure, they measured two endpoints by UV-visible spectroscopy: algal growth inhibition and chlorophyll content. Both measurements track the health of the alga’s photosynthetic machinery, and the absorbance assays confirmed impaired photosynthesis following pollutant exposure.

The results showed a consistent pattern: the complexes were more toxic than either of their individual components. Ciprofloxacin-copper(II) stood out as the strongest inhibitor of growth among the four complexes tested. The authors attribute this enhanced toxicity to three linked mechanisms: increased lipophilicity of the complexes relative to the free antibiotic, improved bioavailability to the algal cells, and oxidative stress driven by elevated levels of reactive oxygen species (ROS) — chemically reactive molecules that can damage cell membranes, proteins, and DNA.

The broader implication is one of environmental risk assessment. Antibiotic residues and heavy metals frequently co-occur in rivers, lakes, and wastewater-affected waterways, yet regulatory testing often evaluates contaminants individually. If pairing a common antibiotic with common metals produces complexes more toxic than either component alone, single-substance testing could underestimate the ecological hazard these mixtures pose to algae and other primary producers.

As with all controlled laboratory studies, the findings come with scope limitations. The results derive from a single algal species, Spirogyra columbiana, exposed under artificial conditions with fixed endpoints of growth and chlorophyll measured after 72 hours; real-world waterways involve fluctuating concentrations, additional contaminants, and many species. The proposed mechanisms — lipophilicity, bioavailability, and ROS-induced oxidative stress — are offered by the authors as explanations for the observed toxicity pattern, based on the measurements reported in the study. Even so, the work adds concrete experimental evidence that chemical interactions between co-occurring pollutants can elevate ecological risk, strengthening the case for assessing pollutant mixtures rather than isolated compounds when judging the health of freshwater ecosystems.

Subject of Research: Ecotoxicity of ciprofloxacin-metal complexes against freshwater algae

Article Title: Synthesis, spectroscopic characterization, and ecotoxicity evaluation of ciprofloxacin-metal (Fe, Co, Ni, Cu) complexes on freshwater algae

Article References: Baral, D., Bhattarai, A., & Chaudhary, N. K. (2026). Synthesis, spectroscopic characterization, and ecotoxicity evaluation of ciprofloxacin-metal (Fe, Co, Ni, Cu) complexes on freshwater algae. Ecotoxicology, 35(9), Article 193. https://doi.org/10.1007/s10646-026-03178-6

Image Credits: AI Generated

DOI: 10.1007/s10646-026-03178-6

Keywords: ciprofloxacin, heavy metals, metal complexes, ecotoxicology, freshwater algae, Spirogyra columbiana, oxidative stress, reactive oxygen species, chlorophyll, growth inhibition, environmental risk assessment, water pollution