The roles of silicon in combating cadmium challenge in the Marine diatom Phaeodactylum tricornutum

Marine phytoplankton possess a sophisticated homeostatic network to counteract metal toxicity. Changes in environmental conditions such as ambient nutrient concentrations can significantly impact their intrinsic metal sensitivity. In this study, we evaluated the role of silicon (Si) in counteracting cadmium (Cd) toxicity in the marine diatom Phaeodactylum tricornutum. We first demonstrated that Si enrichment dramatically enhanced Cd tolerance and changed the Cd accumulation in the diatom. Our modeling suggested that Si-enriched cells adsorbed more Cd but had a higher Cd elimination rate than the Si-starved cells. Examinations by atomic force microscopy and X-ray photoelectron spectroscopy revealed that the Si-enriched cells had better silification and more SiO^- in the cell walls, which markedly lowered the surface potential of the diatom cells and allowed them to attract more Cd. Although the Si-enriched cells tended to have a high Cd burden when facing Cd stress, they suppressed the increase of intracellular Cd by both down-regulating the influx transporter ZIP and up-regulating the efflux transporter ATPase5-1B. Our study shows the significant roles Si plays in maintaining metal homeostasis and combating Cd challenge in marine diatoms.