Background <p>Diverse environmental stressors, including rising temperatures, human-induced changes in soil pH, increased soil salinity due to anthropogenic practices and climate change, and soil degradation resulting in phosphorus depletion, pose significant challenges to ecosystems and microbial communities.</p> Objectives <p>This study investigated the toxicological impact of various environmental stressors, including temperature, pH, and sodium chloride and potassium dihydrogen phosphate concentrations, on <i>Rhodobacter sphaeroides</i> by assessing its reactive oxygen species (ROS) levels and carotenoid production.</p> Methods <p>Cell growth was monitored spectrophotometrically at 660&#xa0;nm. Total carotenoids were quantified by acetone extraction and absorbance at 480&#xa0;nm. Specific carotenoids were extracted with acetone:methanol or hexane and quantified using reported millimolar extinction coefficients. Hydrogen peroxide and peroxidase activities were determined using the Amplex^® Red Hydrogen Peroxide/Peroxidase Assay Kit. Intracellular ROS levels were assessed via the fluorescent probe CM-H₂DCFDA.</p> Results <p>Stress conditions generally inhibited growth and elevated ROS and peroxidase activity compared with moderate conditions. Total carotenoid production increased under high-temperature stress, peaking at 35 °C, and under alkaline pH conditions, consistent with their antioxidant function. Spheroidenone, a potent ROS scavenger, accumulated under high-temperature, alkaline, high-salinity, and low-to-moderate phosphate stress conditions, underscoring its importance in stress adaptation and the tolerance of <i>R. sphaeroides</i>.</p> Conclusion <p>This study concludes that <i>R. sphaeroides</i> responds to stress with increased hydrogen peroxide and peroxidase activity. Furthermore, while overall carotenoid production varies with nutrient stress, the potent antioxidant spheroidenone consistently rises to mitigate oxidative damage across most stress conditions.</p>