Trophic Condition Shapes UVC Responses in Euglena gracilis

Area of Science:

• * Environmental Science
• * Microbiology
• * Cell Biology

Background:

• * Short-wavelength ultraviolet radiation, including ultraviolet C (UVC), can damage biological systems by inducing DNA damage, oxidative stress, and disrupting photosynthesis.
• * While UVC is a common laboratory stressor, its effects on phototrophic protists under varying trophic conditions are not well understood.
• * Understanding these effects is crucial for assessing the ecological impact of UV radiation on aquatic microorganisms.

Purpose of the Study:

• * To investigate the influence of trophic conditions (photoautotrophy vs. mixotrophy) on the tolerance of *Euglena gracilis* to repeated UVC exposure.
• * To compare the physiological responses, including growth, pigment content, oxidative stress, and lipid peroxidation, of *E. gracilis* under different trophic states when exposed to UVC.
• * To determine if mixotrophic growth enhances the resilience of *E. gracilis* to chronic UVC stress.

Main Methods:

• * *Euglena gracilis* cultures were grown under photoautotrophic and ethanol-supported mixotrophic conditions.
• * Cultures were exposed to daily pulses of UVC (254 nm) for five consecutive days.
• * Measurements included cell density, photosynthetic pigments (chlorophyll a, carotenoids), intracellular oxidative stress (using 2',7' dichlorodihydrofluorescein diacetate), and lipid peroxidation (thiobarbituric acid reactive substances). Qualitative fluorescence microscopy was also employed.

Main Results:

• * Repeated UVC exposure significantly reduced cell density in both trophic conditions, with a more pronounced effect under photoautotrophy.
• * Photoautotrophic cultures exhibited higher levels of intracellular oxidative stress compared to mixotrophic cultures after UVC treatment.
• * Mixotrophic *E. gracilis* maintained higher cell densities and showed lower oxidative stress signals under UVC, indicating enhanced tolerance. Pigment levels remained more stable in mixotrophic cultures compared to photoautotrophic ones under UVC stress.

Conclusions:

• * Trophic condition significantly shapes the response of *Euglena gracilis* to repeated UVC stress.
• * Ethanol-supported mixotrophy confers improved physiological robustness and UVC tolerance in *E. gracilis* compared to photoautotrophy under the tested experimental conditions.
• * These findings have implications for understanding the ecological adaptability of phototrophic protists in environments with fluctuating UV radiation levels.