Photosynthetic and ultrastructural responses of Ulva australis to Zn stress

This research evaluated the effect of zinc (Zn) on the ultrastructure and the photosynthetic efficiency of a common green alga. Ulva australis was grown in the laboratory for 7 days under a range of different Zn concentrations (0, 25, 50 and 100 μg L−1). Growth rate (Gr), photosynthetic efficiency (Fv/Fm and ETRmax), photosynthetic pigments, and metal accumulation were measured. Samples of 1 mm length were taken to analyse the effect of Zn on the ultrastructure using transmission electron microscopy (TEM) and cytochemical responses (TB-O and PAS) were evaluated by light microscopy (LM). There were no significant differences in the growth rate, Fv/Fm, ETRmax and the photosynthetic pigments chlorophyll a, chlorophyll b and carotenoids (p > 0.05) after 7 days of Zn exposure. However, TEM revealed cytoplasm retraction, compression of cellulose fibrils, dissembled thylakoids and electron-dense bodies suggesting ultrastructural impacts from metal exposure and accumulation. Cytological analysis demonstrated that Zn affected U. australis cells at the three concentrations tested. The main effect was cytoplasm retraction and a decrease on the amount of starch granules, following exposure at 25 μg L⁻¹ and 50 μg L⁻¹ of Zn. We conclude that concentrations of Zn assessed in U. australis in this research has a short-term cellular effect as revealed by TEM and cytological analysis, demonstrating the importance of measuring a broad suite of endpoints to better understand species responses to environmentally relevant concentrations of Zn. However, U. australis was able to physiologically tolerate adverse conditions, since there was no effect on the photosynthetic performance and growth.