Detoxification of Fenoxaprop‐P‐Ethyl Herbicide: A Study on Physiological and Metabolic Adaptation of <i>Anabaena laxa</i> and <i>Nostoc muscorum</i>

Fenoxaprop-p-ethyl is a widely employed aryloxyphenoxypropionate herbicide that inhibits acetyl-CoA carboxylase (ACCase), thus interfering with fatty acid biosynthesis in target organisms. While its effects on terrestrial plants are well-documented, its impact on nontarget aquatic microorganisms, particularly cyanobacteria, which serve as the foundation of many aquatic ecosystems, remains inadequately characterized. Consequently, this study was undertaken to evaluate the differential uptake and metabolic responses of two cyanobacterial species, Anabaena laxa and Nostoc muscorum, upon exposure to fenoxaprop-p-ethyl. Cyanobacterial cultures were subjected to 200 mg/L of fenoxaprop-p-ethyl under controlled experimental conditions. Bioaccumulation was quantified, and a comprehensive analysis of photosynthetic parameters was conducted, including chlorophyll-a, carotenoids, CO₂ fixation, Rubisco, and PEPC activity. Additional biochemical profiling encompassed carbohydrates, organic acids, amino acids, and fatty acid composition. Anabaena exhibited a 26.3% higher accumulation of the herbicide compared to Nostoc (4.70 vs. 3.72 μg/g). Both species demonstrated substantial reductions in chlorophyll-a (57.4% in Anabaena, 47.2% in Nostoc) along with increased carotenoid production, with Nostoc displaying superior defensive capabilities (67.4% vs. 37.6% increase). Carboxylation enzyme activities were more severely inhibited in Anabaena. Despite ACCase inhibition, both species exhibited notable increases in total fatty acids, with distinct species-specific patterns in the accumulation of saturated, monounsaturated, and polyunsaturated fatty acids. Metabolic reconfiguration was further evidenced by significant accumulations of carbohydrates, organic acids, and selective amino acids, particularly branched-chain amino acids. The results highlight distinct species-specific metabolic adaptations to herbicide-induced stress, with Nostoc displaying more robust stress response mechanisms despite lower herbicide uptake. These findings provide valuable insights into the resilience of cyanobacteria to agrochemical exposure and underscore the potential ecological implications for aquatic microbial communities in agricultural watersheds.