| Summary: | In cyanobacteria, the interplay of ATP and lactate dynamics underpins cellular energetics; their pronounced shifts in response to zero-valent iron (nZVI) nanoparticles and ampicillin highlight the nuanced metabolic adaptations to environmental challenges. In this study, we investigated the impact of nZVIs and ampicillin on <i>Fremyella diplosiphon</i> cellular energetics as determined by adenosine triphosphate (ATP) content, intracellular and extracellular lactate levels, and their impact on cell morphology as visualized by transmission electron microscopy. While a significant increase in ATP concentration was observed in 0.8 mg/L ampicillin-treated cells compared to the untreated control, a significant decline was noted in cells treated with 3.2 mg/L nZVIs. ATP levels in the combination regimen of 0.8 mg/L ampicillin and 3.2 mg/L nZVIs were significantly elevated (<i>p</i> < 0.05) compared to the 3.2 mg/L nZVI treatment. Intracellular and extracellular lactate levels were significantly higher in 0.8 mg/L ampicillin, 3.2 mg/L nZVIs, and the combination regimen compared to the untreated control; however, extracellular lactate levels were the highest in cells treated with 3.2 mg/L nZVIs. Visualization of morphological changes indicated increased thylakoid membrane stacks and inter-thylakoidal distances in 3.2 mg/L nZVI-treated cells. Our findings demonstrate a complex interplay of nanoparticle and antibiotic-induced responses, highlighting the differential impact of these stressors on <i>F. diplosiphon</i> metabolism and cellular integrity.
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