Fish on the Front Line: Oxidative Stress Reveals Hidden Pollution Hotspots on the Bulgarian Black Sea Coast

Along the sun-drenched shores of the Bulgarian Black Sea coast, a quiet biochemical battle is being fought inside the cells of commercially important fish. A new pilot study, published in the journal BioRisk, has measured the fingerprints of oxidative stress in four key species — round goby, red mullet, sprat and horse mackerel — captured at three contrasting localities, and its findings suggest that where a fish lives may matter as much as what it is. The research offers one of the first comparative portraits of how pollution pressure translates into molecular damage in fish from this stretch of the sea, and it points to bottom-dwelling species as particularly sensitive sentinels of coastal environmental health.

Oxidative stress is a concept that sounds abstract but describes a very concrete danger. When marine organisms encounter environmental pressures — chemical contaminants, temperature swings, hypoxia, eutrophication — their cells ramp up production of reactive oxygen species, or ROS. In modest amounts, these molecules serve as signaling agents that help organisms adapt to change. But when exposure is acute or prolonged, ROS generation overwhelms the antioxidant defense system, and the resulting imbalance damages lipids, proteins and DNA. In fish, this damage can impair growth, development and reproduction, with consequences that ripple upward through populations and ultimately shape entire community structures.

The research team, sampling fish during trawl selectivity experiments, chose four species that occupy distinct ecological niches. The round goby (Neogobius melanostomus) is a slow-swimming demersal carnivore hugging the seabed. Red mullet (Mullus barbatus) and horse mackerel (Trachurus mediterraneus) are benthopelagic species, living near but not on the bottom. Sprat (Sprattus sprattus) is a fast-swimming pelagic plankton feeder. Individuals of each species — nine to twelve per sample — were randomly taken from pelagic midwater otter trawl catches at three sites: Maslen Nos, Staro Oryahovo and Nessebar Bay. The sites differ markedly in character: Maslen Nos lies within a protected area under the European ecological network NATURA 2000 and is regarded as having good ecological conditions, while Nessebar is a bustling tourist center whose surrounding waters are presumed to carry a heavier load of anthropogenic stressors.

Once the fish were shock-frozen on board and transported to the laboratory, the researchers dissected out liver and gill tissue and subjected them to a battery of spectrophotometric assays. They measured lipid peroxidation (LPO), the oxidative degradation of lipids that serves as a classic marker of membrane damage; glutathione (GSH), a key cellular antioxidant; and the activities of the enzymes superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and glutathione-S-transferase (GST), which together form the front line of antioxidant and detoxification defense. They also measured acetylcholinesterase (AChE), an enzyme essential for nerve impulse transmission whose suppression is a well-established indicator of neurotoxic exposure. Fulton’s condition factor, a biometric index of general fish health computed from body weight and length, rounded out the assessment.

The results revealed a striking geography of stress. Round gobies and red mullets caught in Nessebar Bay showed clear signs of oxidative stress, with significantly elevated lipid peroxidation and GST activity accompanied by the lowest AChE activity in both liver and gills compared with fish from the other two sites. In sharp contrast, round gobies from Maslen Nos were the least affected of all: they displayed the lowest LPO levels and GST activities alongside the highest concentrations of glutathione and the highest SOD activity, together with peak AChE activity — a biochemical profile consistent with fish living in a clean, well-protected environment. Red mullets from Maslen Nos and Staro Oryahovo showed no significant differences between sites, suggesting those two localities impose comparable, milder stress loads on this species.

Sprat told a similar but subtler story. Individuals from Nessebar Bay exhibited significantly lower glutathione levels, higher activities of catalase, glutathione peroxidase and GST, and reduced AChE activity in the gills relative to sprat from the other localities — another clear signature of oxidative stress. Meanwhile, horse mackerel emerged as the most biochemically uniform species: no significant differences in any of the measured biomarkers were found among fish from the three sites, hinting that either the species is more resilient, or that its pelagic, mobile lifestyle buffers it from site-specific pressures that concentrate near the seabed.

The organ-level patterns were equally revealing. Gills, which sit in direct contact with the water and whatever it carries, generally showed higher lipid peroxidation than liver across all species except round goby. Livers, by contrast, hosted roughly twice the SOD activity of gills in every species studied, and catalase activity ran two to five times higher in the livers of round goby, red mullet and sprat. The researchers attribute these differences to the unique composition and metabolic profile of each organ. Notably, the extremely high GST values recorded in the livers of fish from Nessebar Bay point to an activated detoxification metabolism for xenobiotics — foreign chemical compounds that the authors suggest most likely enter the fish from contaminated sediments, a plausible pathway given that many pollutants accumulate in low water layers and bottom sediments where benthic species feed and shelter.

Multidimensional scaling, a statistical technique for visualizing similarity among multivariate data, reinforced the species-level contrasts. Red mullet and round goby each separated clearly from the other species in the two-dimensional ordination plane, reflecting their distinctive oxidative stress reactions, while horse mackerel and sprat clustered closer together. The analysis also revealed two coordinated lines of defense within the fish: a first line driven by the antioxidant enzymes catalase and superoxide dismutase, which grouped together, and a second line reflected by the looser grouping of glutathione, glutathione peroxidase and AChE, indicating that detoxification processes and neurotransmission capacity respond in tandem to toxic pollutant exposure.

Lifestyle and metabolism help explain why the species diverged. Fast-swimming plankton feeders such as sprat and horse mackerel consume more oxygen and run higher metabolic rates, generating more ROS and requiring stronger baseline antioxidant protection — consistent with the significantly higher LPO and GST found in their gills. Sluggish benthic species like round goby and red mullet, however, inhabit the most contaminated marine environments precisely because pollutants settle into sediments. The study’s conclusion that round goby and red mullet are more vulnerable to environmentally induced oxidative stress than horse mackerel and sprat therefore carries a practical implication: demersal fish may be the most convenient and informative species for monitoring the state of the marine environment and the risk of impaired fish fitness.

Encouragingly, the Fulton’s condition factor data showed that round goby and red mullet from all three localities were in good condition (K greater than 1), with the highest values at Maslen Nos, whereas sprat and horse mackerel fell below 1 at every site. The antioxidant defenses of the vulnerable benthic species, the authors conclude, appear robust enough to let them tolerate and adapt to their habitats even under stress. But the team is careful to frame this as a pilot assessment: comprehensive evaluation of oxidative stress in economically important fish across the Bulgarian Black Sea will require further, broader studies — especially as the sea faces mounting pollution from metals, polycyclic aromatic hydrocarbons and eutrophication. For now, the goby and the mullet of Nessebar Bay stand as molecular witnesses to the pressures building along one of Europe’s most beloved coastlines.

Subject of Research: Oxidative stress biomarkers in commercially important fish species from different ecological localities along the Bulgarian Black Sea coast

Article Title: Comparative study on the oxidative stress of commercially important fish species from localities with different ecological conditions along the Bulgarian Black Sea coast

Article References: Alexandrova, A., Raev, Y., Dimitrov, D., Chipev, N., Tsvetanova, E., Georgieva, A., & Raykov, V. (2022). Comparative study on the oxidative stress of commercially important fish species from localities with different ecological conditions along the Bulgarian Black Sea coast. BioRisk, 17, 117-126. https://doi.org/10.3897/biorisk.17.77300

Image Credits: AI Generated

DOI: 10.3897/biorisk.17.77300

Keywords: oxidative stress, Black Sea, round goby, red mullet, sprat, horse mackerel, lipid peroxidation, antioxidant enzymes, acetylcholinesterase, marine pollution, biomarkers, Bulgarian coast