Histopathological alterations in various fish organs have a pronounced value in aquatic toxicology and are widely used in environmental monitoring. The aim of this study was to evaluate whether histopathological alterations in fish gills can discriminate seasonal variations in environmental conditions within the same aquatic ecosystem, and if so, which alterations contributed the most to seasonal differentiation. Microscopic examination of common bream Abramis brama gills displayed various alterations in gill structure, including epithelial hypertrophy, hyperplasia, mucous and chloride cell alterations, epithelial lifting, necrosis, hyperemia and aneurism. These alterations were subsequently quantified by a semi-quantitative analysis in order to detect differences in the intensity of the mentioned alterations. Epithelial hypertrophy, hyperplasia, epithelial lifting and necrosis varied significantly between seasons with only necrosis being significantly higher in the first season. Discriminant canonical analysis displayed that epithelial hyperplasia, mucous cell alterations, epithelial lifting and necrosis contributed the most to discrimination between seasons. Overall, this study demonstrates that histopathological biomarkers in fish gills can be used in discriminating seasonal variations in water quality within the same aquatic ecosystem.

For 50 years persistent cyanobacterial blooms have been observed in Lake Ludoš (Serbia), a wetland area of international significance listed as a Ramsar site. Cyanobacteria and cyanotoxins can affect many organisms, including valuable flora and fauna, such as rare and endangered bird species living or visiting the lake. The aim was to carry out monitoring, estimate the current status of the lake, and discuss potential resolutions. Results obtained showed: (a) the poor chemical state of the lake; (b) the presence of potentially toxic (genera Dolichospermum, Microcystis, Planktothrix, Chroococcus, Oscillatoria, Woronichinia and dominant species Limnothrix redekei and Pseudanabaena limnetica) and invasive cyanobacterial species Raphidiopsis raciborskii; (c) the detection of microcystin (MC) and saxitoxin (STX) coding genes in biomass samples; (d) the detection of several microcystin variants (MC-LR, MC-dmLR, MC-RR, MC-dmRR, MC-LF) in water samples; (e) histopathological alterations in fish liver, kidney and gills. The potential health risk to all organisms in the ecosystem and the ecosystem itself is thus still real and present. Although there is still no resolution in sight, urgent remediation measures are needed to alleviate the incessant cyanobacterial problem in Lake Ludoš to break this ecosystem out of the perpetual state of limbo in which it has been trapped for quite some time.

Rapid urbanization and increased anthropogenic activities have led to the release of an increasing number of pollutants, including metals, into freshwater ecosystems, posing a significant threat to aquatic life. In this study, the bioaccumulation of metals and hepatic enzyme activities in northern pike (Esox lucius) from two contrasting freshwater ecosystems in Serbia, the lotic Tisza River and the lentic Bela Crkva Reservoirs, were investigated. A total of 22 specimens (11 per site) were sampled in autumn 2024. The liver tissue was analyzed for the concentrations of 11 metals (As, Cd, Cr, Co, Cu, Pb, Li, Fe, Zn, Mn, Ni) and the activities of the liver enzymes (ALT, AST, AP, GGT). The results showed ecosystem-specific patterns of metal accumulation, with the northern pike in the rivers showing significantly higher Cu and Fe levels, while the fish in the reservoirs showed increased Zn concentrations. Enzymatic biomarkers showed different responses between ecosystems, with river fish showing increased ALT and AST activities, indicating hepatocellular stress, while reservoir fish showed increased GGT, indicating enhanced detoxification processes. The body condition factor was negatively correlated with liver Mn and Zn concentrations, emphasizing its utility as an integrative bioindicator of metal-induced stress. No significant sex-specific differences in metal or enzyme levels were found. These results suggest the suitability of northern pike as a sentinel species for environmental monitoring in freshwaters and highlight the different physiological adaptations to local metal stress in lentic and lotic habitats.

Nitrate pollution in freshwater has become an increasing concern for both environmental sustainability and human health, especially in water reuse systems and intensive aquaculture. Photocatalytic reduction in nitrate to nitrogen gas (N₂) represents a promising low-chemical treatment strategy that can operate under sunlight or LED irradiation, and in general, enable nitrate removal without generating concentrated waste streams. Over the past decade, the development of advanced photocatalytic materials, including heterojunction semiconductors, plasmonic catalysts, and single-atom co-catalysts, has significantly enhanced visible-light absorption and overall photocatalytic performance. Despite these advances in photocatalyst design and synthesis, several critical challenges still limit the large-scale implementation of photocatalytic nitrate reduction to N₂. First, selectivity toward N₂ remains limited, as competing reaction pathways often lead to the formation of undesirable byproducts, such as nitrite (NO₂⁻), ammonium (NH₄⁺), and nitrous oxide (N₂O). Second, nitrogen reaction pathways are often uncertain, because many studies lack isotopic labeling or nitrogen mass balances, making it difficult to verify that the detected N₂ originates from nitrate reduction. Third, practical implementation is restricted by several technical challenges, including catalyst fouling or leaching, limitations in reactor design, excessive addition of hole scavengers, and the relatively high energy demand associated with indoor LED-driven systems. This review critically surveys advances from 2015 to 2025 in photocatalytic materials and reaction mechanisms for nitrate conversion to N₂. It highlights best practices for reliable product quantification and reaction pathway validation, and evaluates the feasibility of integrating these systems into recirculating aquaculture systems (RAS), where effective nitrate management is essential. In addition, the potential role of modern inline nitrate sensors (optical and electrochemical) and automated process control is discussed, outlining pathways toward hybrid photocatalytic–biological nitrate removal systems for sustainable aquaculture applications.

The analysis of the physical-chemical, biological and microbiological parameters of the water quality of Lake Palić and Lake Ludaš showed a poor ecological status belonging to class V with the dominant presence of cyanobacteria in the composition of the phytoplankton community, which is an indicator of potentially risky water pollution with cyanotoxins. Heterogeneous photocatalysis based on ZnO is an innovative method for the mineralization of cyanotoxins, mycotoxins, pharmaceuticals, pesticides and other micropollutants from aquatic ecosystems. ZnO nanoparticles synthesized according to the principle of „green chemistry” with non-toxic precursors using the tartaric acid combustion method showed 100% photocatalytic efficiency in the removal of several types of micropollutants after only 60 min of visible light irradiation.