Sea depth effects on the algal photosynthetic apparatus II. An electron microscopic study of the photosynthetic apparatus of Halimeda tuna (Chlorophyta, Siphonales) at −0·5 m and −6·0 m sea depths

Abstract Halimeda tuna, a sciophytic alga, shows, with regard to sea depth, and consequently to light intensity, morphological differences analogous to the modifications characteristic of land plants adapted to environments with different light intensity (sun and shade plants). The −6·0 m plants have thinner segments, larger utricles and thinner cell walls than those of the −0·5 m plants. The thickness of the layers of plastids inside the utricles is affected by light intensity: plastid distribution covers fewer layers, but a larger exposed surface in −6·0 m plants compared to −0·5 m plants. Sea depth does not seem to affect the chloroplast fine structure: the number of thylakoids, their distribution and the structure of the stroma remains unmodified. However the −0·5 m plants are characterized by a larger number of senescent chloroplasts than the − 6·0 m plants, suggesting that high light conditions may hasten ageing of both photosynthetic apparatus and thallus.