Copper Uptake, Intracellular Localization, and Speciation in Marine Microalgae Measured by Synchrotron Radiation X‑ray Fluorescence and Absorption Microspectroscopy

Metal toxicity to aquatic organisms depends on the speciation of the metal and its binding to the critical receptor site(s) (biotic ligand) of the organism. The intracellular nature of the biotic ligand for Cu in microalgal cells was investigated using the high elemental sensitivity of microprobe synchrotron radiation X-ray fluorescence (SR-XRF) and X-ray absorption near-edge spectroscopy (XANES). The marine microalgae, <i>Ceratoneis closterium</i>, <i>Phaeodactylum tricornutum</i>, and <i>Tetraselmis</i> sp. were selected based on their varying sensitivities to Cu (72-h 50% population growth inhibitions of 8–47 μg Cu/L). Intracellular Cu in control cells was similar for all three species (2.5–3.2 × 10^–15 g Cu/cell) and increased 4-fold in <i>C. closterium</i> and <i>Tetraselmis</i> sp. when exposed to copper, but was unchanged in <i>P. tricornutum</i> (72-h exposure to 19, 40, and 40 μg Cu/L, respectively). Whole cell microprobe SR-XRF identified endogenous Cu in the central compartment (cytoplasm) of control (unexposed) cells. After Cu exposure, Cu was colocated with organelles/granules dense in P, S, Ca, and Si and this was clearly evident in thin sections of <i>Tetraselmis</i> sp. XANES indicated coexistence of Cu­(I) and Cu­(II) in control and Cu-exposed cells, with the Cu ligand (e.g., phytochelatin) in <i>P. tricornutum</i> different from that in <i>C. closterium</i> and <i>Tetraselmis</i> sp. This study supports the hypothesis that Cu­(II) is reduced to Cu­(I) and that polyphosphate bodies and phytochelatins play a significant role in the internalization and detoxification of Cu in marine microalgae.