Cellular and genetic responses of Phaeodactylum tricornutum to seawater acidification and copper exposure

The ongoing decline in seawater pH, driven by the absorption of excess atmospheric CO₂, represents a major environmental issue. This reduction in pH can interact with metal pollution, resulting in complex effects on marine phytoplankton. In this study, we examined the combined impacts of seawater acidification and copper (Cu) exposure on the marine diatom Phaeodactylum tricornutum. Our data indicate that elevated pCO₂ had a minor effect on the growth and photochemistry and overall performance of P. tricornutum. However, seawater acidification significantly influenced cell size, surface roughness, and adhesion. Higher pCO₂ levels led to increased Cu accumulation in P. tricornutum under low ambient Cu concentrations, while significantly reducing Cu accumulation. The smaller cell size and reduced negative charge on the cell surface may explain the decreased Cu accumulation and toxicity. In response to metal stress, P. tricornutum upregulated Cu efflux to mitigate the increased Cu stress in acidified seawater. The expression of the metal transporter gene CTR1 and the reductase gene FRE1 were significantly downregulated, while ATPase5-1B was upregulated in cells exposed to elevated Cu concentrations at 1200 μatm pCO₂. Our study provides useful insights into the interactions between metals and diatoms in an increasingly acidified ocean.