Physiological and morphological response of marine diatom Cylindrotheca closterium (Bacillariophyceae) exposed to Cadmium

Abstract

ABSTRACT The impact of the heavy metal cadmium on the lightly silicified marine diatom Cylindrotheca closterium was studied with regard to growth dynamics, organic matter production and morphological characteristics. C. closterium cultures were able to sustain positive growth in the tested range of 0–1000 μg Cd l–1 with a significant decrease in specific growth rate in the exponential growth phase at the highest tested Cd concentration. The concentrations of both dissolved (DOCcell) and particulate organic carbon per cell (POCcell) in C. closterium cultures exposed to 500 and 1000 µg l–1 Cd more than doubled compared with the control culture. Enhanced organic matter production as a feedback response to Cd exposure may thus contribute to cell detoxification, supported by the electrochemical data showing that Cd was not significantly accumulated by or associated with diatom cells except for the culture exposed to 1000 μg Cd l–1. In order to gain more insight into the effects of cadmium on diatom morphological features, the cell surface of C. closterium was characterized at the nanoscale on a single cell level using atomic force microscopy (AFM). Changes induced by Cd were observed as irregular patterns of silica spheres on more silicified parts of the cell, girdle band and around the raphe. These changes clearly show that beside the physiological response, C. closterium also showed a morphological response which is probably due to the interference of Cd with the diatom biosilification process. This study overall contributes to better understanding of the effects of cadmium on diatoms and shows that morphological characteristics assessed by AFM may be a valuable indicator for metal contamination.