Metabolism, elemental and isotopic composition of the coastal diatom Pseudo-nitzschia australis under Cu and Zn exposure

Coastal marine systems are critical environments that can be affected by harmful algal blooms caused by the diatom Pseudo-nitzschia australis. This species produces a neurotoxin, domoic acid (DA), which poses significant risks to human health and raises societal and environmental concerns. The factors determining the toxigenicity of P. australis strains remain unclear, particularly concerning the effects of copper (Cu) and zinc (Zn). Cu and Zn are essential trace metals that are ubiquitous in coastal environments but can become toxic at nanomolar ionic concentrations. While Cu is known to enhance DA production in some Pseudo-nitzschia species, the effect of Zn on the DA metabolic pathway has not been studied. This study aims to investigate the effects of Cu²⁺ and Zn²⁺ on the metabolism of the toxic diatom P. australis. We examined the physiology, elemental composition, and isotopic composition of a coastal P. australis strain isolated from the North Biscay region (France). We present results from 10-day laboratory-controlled culture experiments, exposing the diatom to pico- to nanomolar concentrations of Cu²⁺ and Zn²⁺. The results reveal direct effects of these metals on DA production by P. australis. We report Cu, Zn, and carbon intracellular quotas, along with the first measurements of Cu isotopic composition (δ⁶⁵Cu) in cultured cells under varying metal exposure conditions. Our findings demonstrate metal-specific physiological responses in P. australis, with distinct δ¹³C and δ⁶⁵Cu isotope fractionation patterns depending on cell metabolism, including the DA production regime.