Silicon's role in enhancing cadmium substitution for zinc in Thalassiosira weissflogii: implications for heavy metal detoxification in marine environments

Abstract

Diatoms can substitute cadmium (Cd) for zinc (Zn) in Zn-dependent enzymes, but the role of silicon (Si) in this process remains underexplored. This study investigates how Si availability affects the ability of Cd to support the growth and physiology of Thalassiosira weissflogii under Zn-deficient conditions. Cultures were grown in Zn-free seawater containing 86 μmol L⁻¹ (high) or 17.2 μmol L⁻¹ (low) Si and supplemented with either Cd or Zn. Under Zn limitation, Cd enhanced growth by 18.6 % and chlorophyll a synthesis by 22.4 %, though Zn was more effective. High Si increased photosynthetic efficiency, with the maximum quantum yield of photosystem II (F_v/Fm) and the maximum electron transport rate (ETR_max) rising by 6 % and 15 %, respectively. Activities of carbonic anhydrase (CAA) and alkaline phosphatase (APA) increased under Cd but remained lower than under Zn. Expression of the CDCA gene, which encodes a Cd-specific carbonic anhydrase isoform, was downregulated by both Cd and Zn, consistent with its induction under Zn starvation. Low Si led to larger cell size, while high Si promoted carbon accumulation and reduced biogenic silica. These effects may be attributed to Si’s role in stabilizing frustule architecture and optimizing enzymatic metal use. Overall, Si modulates Cd substitution for Zn in diatoms, offering insights into trace metal adaptation and the potential for bioremediation in polluted marine environments.