Impact of Rare Earth Elements in sediments on the growth and photosynthetic efficiency of the benthic plant Myriophyllum aquaticum

Purpose Rare Earth Elements (REEs) are becoming more present in our everyday life. With this work, we aimed to study and compare the toxic responses of the REEs lanthanum (La), cerium (Ce), neodymium (Nd), and gadolinium (Gd) to the macrophyte Myriophyllum aquaticum. The scope was to evaluate if these elements trigger a response on the photosynthetic system (PSII), which causes inhibition of the growth rate of the plant. Methods We measured the fluorescence yield by pulse-amplitude-modulated chlorophyll fluorometer (PAM) which enabled simultaneous high-resolution fluorescence measurements of the whorls daily for the whole duration of the test (10 days) and fresh weight change (FWC) at the end of the test. Results Our findings suggest that La significantly decreased FWC at the highest concentration (500 mg kg(-1)) but did not cause any significant effects on the fluorescence yield. Ce and Nd significantly decreased the chlorophyll fluorescence between days 2 and 4, and after that the yield was not significantly different with respect to the control. Of all the REEs tested in this study, Gd showed the most negative effect as the whorls exhibited chlorosis/necrosis and the fresh weight at the end of the test decreased significantly compared to the same plant at day 0. The yield of M. aquaticum showed time-dependent effects for Gd at the highest concentration. Conclusion Gd was the most toxic REE, strongly affecting both the yield and FWC. The measurement of the fluorescence yield of the PSII is a useful effect observation and of high environmental importance. The difference in sensitivity between the functional and growth endpoints may give hints about the mode of action of contaminants to aquatic plants.