Ultrastructural and physiological effects of mercuric chloride on the marine seaweed Ulva australis Areschoug

Mercury represents a highly toxic pollutant of major concern in aquatic ecosystems, capable of affecting primary producers, including macroalgae, through structural, physiological and biochemical disturbance. Seaweeds, owing to their direct exposure to dissolved contaminants and their sensitivity to environmental stress, represent a model of choice for ecotoxicological assessment. In this study, the effects of mercuric chloride (HgCl2) on the marine macroalga Ulva australis were investigated through multiple approaches combining morphological, ultrastructural and physiological analyses. Thalli of U. australis collected from the Venice Lagoon (Chioggia) were exposed under laboratory conditions to two mercury concentrations treatment (5 µg/L and 25 µg/L), alongside untreated control samples, and analyzed after 2, 6 and 14 days of exposure. Responses were assessed through light (LM) and transmission electron microscopy (TEM), respiratory measurements, pulse-amplitude-modulated (PAM) fluorometry, and pigment analyses. Although no evident macroscopic morphological alterations were observed, light microscopy revealed concentration- and time-dependent cellular damages, while TEM analyses showed chloroplast disorganization, thylakoid membrane modification and progressive cellular degradation under higher mercury exposure. Physiological analyses indicated significant treatment effects on respiration, photosynthetic performance and pigment composition. Respiration increased under mercury exposure, indicating high metabolism associated with stress responses. Photosystem II efficiency (Fv/Fm) was significantly affected by both mercury concentration and exposure duration, while pigment analyses revealed alterations in chlorophylls and carotenoids, consistent with physiological stress. Taken together, these findings demonstrate that mercury exposure induces stress responses in Ulva australis, even in the absence of visible external damage. These outcomes support the use of U. australis as a sensitive model species for marine ecotoxicological studies.