Microspectrophotometry and Raman Investigations of the Effects of Hexavalent Chromium on the Photosynthetic and Photoreceptive Apparatus of Euglena gracilis

Heavy metals such as copper and zinc serve as essential trace elements for photosynthetic organisms at appropriate concentrations. However, at elevated levels, these metals (along with non-essential metals like chromium, lead, mercury, and cadmium) exert severe toxic effects on aquatic life. Heavy metal toxicity primarily relates to oxidative damage in living systems through a direct increase in reactive oxygen species (ROS) and reduction in cellular antioxidant capacity. Previous research on algae of different types with different coverings led us to complete the comparative framework. For this purpose and to assess biotechnological potential, we investigated chromium effects on Euglena gracilis, which possesses a unique pellicle covering, to determine whether it could serve as a chromium biosensor or bioremediation agent. Using Raman spectroscopy and absorption microspectrophotometry (MSP), we found that chromium concentrations of up to 500 μM had no effect on Euglena chlorophyll or carotenoid profiles, consistent with the pellicle preventing chromium entry and protecting the photosynthetic apparatus. However, concentrations > 10 μM severely inhibited growth through extracellular interference with essential nutrient utilization (ammonium phosphate and vitamin B12). Growth inhibition was reversible upon transfer to fresh medium, confirming that cellular machinery remained intact. These results suggest that E. gracilis cannot serve as a chromium biosensor (photosynthetic apparatus unaffected) or bioremediation agent (no chromium internalization), but its ability to maintain photosynthetic functionality in chromium-contaminated environments suggests the potential for alternative applications in polluted water biomass production.