Assessment of photosynthetic activity is one of the quick and simple methods of verification whether the studied environmental factors have a stressful effect on photosynthetically active organisms. High-intensity light can be a stress factor that could have a potential impact on the maximum productivity of photosystem II. The purpose of the conducted research was to observe changes in photosynthetic activity of the lichen Cladonia mitis and the bryophyte Pleurozium schreberi exposed to artificial high-energy lighting under laboratory culture conditions. The obtained results showed variability of photosynthetic activity over time, depending on the amount of light energy supplied. C. mitis and P. schreberi at full exposure (light energy: 52.03 W m ^-2 and photosynthetically active radiation 167.24 μmol m ^-2) showed a slow downward trend in photosynthetic activity, while at half the light intensity periodic fluctuations were observed without changes in the controls. Long-term and high-light intensity exposure of photosynthetically active organisms may cause gradual degradation of the photosynthetic apparatus, which in turn leads to cell death. Low values of photosynthetic activity may indicate a situation in which, due to excess light, the rate of photosystem II damage exceeds the rate of its repair. This leads to irreversible damage to the photosynthetic apparatus.