Intracellular distribution and speciation of indium in the green microalgae Chlamydomonas reinhardtii

The interactions between indium and aquatic organisms are poorly documented despite indium increasing use. This study aimed to improve current knowledge of indium effects and intracellular fate in microalgae. The model green algae Chlamydomonas reinhardtii were thus exposed for 96 h to total indium concentrations ranging from 0.08 to 36.5 µM and examined for their growth and subcellular parameters as well as for their indium subcellular distribution and speciation using differential ultracentrifugation and size-exclusion chromatography coupled with inductively coupled plasma mass spectrometry (SEC-ICP-MS), respectively. The strong adsorption of indium onto flask walls and its low solubility limited its stability and tested concentrations in the exposure media. Under these conditions of low bioavailability, indium did not have any significant effects on algal growth and subcellular parameters. Internalized indium was observed to plateau at a maximum value of 18.5 amol.cell^-1 from 8 µM total indium concentration. This internalized indium was mainly localized in granular (53%) and organelle fractions (33%), with only minor association with heat-stable or denatured proteins. Analysis of cytosol fraction by SEC-ICP-MS further revealed that indium was predominantly bound to biomolecules >470 kDa and to low-molecular-weight ligands <10 kDa. These results demonstrate for the first time that internalized indium could bind to sensitive sites in algal cells and that algae could employ intracellular granules to sequester part of the accumulated indium. Additional studies using molecular mass spectrometry based hyphenated techniques are needed to identify the biomolecules responsible for indium complexation in the cytosol in order to better understand indium reactivity within algal cells.