Title : Short-term exposure to Diclofenac cause adverse effects on Green Alga Chlamydomonas reinhardtii
The popular nonsteroidal anti-inflammatory drug diclofenac (DCF) is one of the most common contaminants of the water environment. Its toxicity to non-target organisms is often reported to be associated with long-term (chronic) exposure. However, our results suggest that DCF can cause harmful effects even during short-term treatment. The objective of the work was therefore to estimate the acute effect of DCF on green alga Chlamydomonas reinhardtii, which is an accepted model of research at the physiological, biochemical and molecular level.
To achieve the goal mentioned above, Chlamydomonas reinhardtii CC-1690 was treated for 6h with DCF in concentration equal to the toxicological parameter EC25/24 (25% inhibition of population growth after 24h of treatment). We have found that DCF caused oxidative stress in treated cultures, in which the level of H2O2 produced by the cells reached 176% of control. Even though DCF caused no effect on the photosynthetic oxygen evolution, the detailed analysis of parameters of chlorophyll a fluorescence in vivo (OJIP test) revealed that DCF decreased the photosynthetic “vitality” of the cells (P.I. parameter) by 20% as compared to control and diminished the fraction of active PS II reaction centres (RCM parameter) by 17% as compared to control. However, when energy flux through each particular active reaction center (RC) was considered, it appeared that in DCF-treated cells the energy absorption (ABS/RC), energy trapping (TR0/RC), and electron transport (ET0/RC) were unchanged, while non-photochemical energy dissipation (DI0/RC) tend to increase. The above is in a line with our previous finding, that DCF cause transformation some RCs into “heat sinks” (silent reaction centres) while the other ones retain the same activity as in the control conditions. As regards mitochondrial activity, respiratory oxygen consumption in DCF-treated cells tend to increase (about 140% of control), however, the mitochondrial membrane potential (MMP) decreased by 17%. According to literature data, this effect could be caused by uncoupling of substrate oxidation and ADP phosphorylation and an increase in oxygen consumption via the diminishment of the constraining effect of the proton gradient on electron transport.
In conclusion, we have found that DCF can cause rapid, adverse effects on green algae cells. Oxidative stress symptoms as well as disturbance in photosynthetic and respiratory processes were seen already after 6h of exposure which indicates, that DCF poses a real threat for green algae not only due to chronic, but also acute exposure to this drug.
Acknowledgements: This work was partially supported by the National Science Centre Poland [grant UMO-2019/35/B/NZ9/01567].