Abstract: The mechanisms by which heavy metal pollution impacts marine primary producers, particularly microalgal growth and photosynthesis, remain poorly understood. We investigated the effects of heavy metals (Cd, Ni, Sb, and Cr) on the marine diatom Phaeodactylum tricornutum using a controlled laboratory experiment. We evaluated algal density, chlorophyll content, fluorescence parameters, superoxide dismutase (SOD) activity, and energy metabolism under three concentrations of Cd²⁺, Sb³⁺, Ni²⁺, Cr⁶⁺ (1.16, 11.6, and 116 μmol·L). The results showed that P. tricornutum exhibited strong resilience to heavy metal stress, with algal density remaining stable across all treatments. Under Cd²⁺, Ni²⁺, and Sb³⁺ stress, the maximum electron transport rate (ETR_max) of algal cells decreased, while the initial quantum yield (α) increased, ultimately maintaining the stability of the chlorophyll fluorescence parameter (F_v/F_m). Meanwhile, heavy metals variably inhibited pigment synthesis and photosynthesis while significantly elevating SOD activity to mitigate oxidative stress. Despite these adaptive responses, photosynthetic efficiency remained consistently suppressed. Each heavy metal induced distinct intracellular effects. Cd²⁺ disrupted photosynthetic proteins. Ni²⁺ replaced Fe²⁺ in photosynthetic pigments. Sb³⁺ was enzymatically oxidized and extruded. Cr⁶⁺ was photoreduced before acting on proteins. These findings clarify the diverse physiological responses of marine primary producers to heavy metal stress, providing essential theoretical and experimental insights for assessing the ecological risks of heavy metal pollution on primary productivity in marine.

Key words: heavy metal pollution, Phaeodactylum tricornutum, photosynthetic parameter, growth