The phenomenon of photoinhibitíon, by which all organisms performing oxygenic photosynthesis decrease their photosynthetic activity when exposed to excessive irradiance, depends, at molecular level, mainly on light-induced modifícations of photosystem 2 (PS2). In the last decade a great deal of research has been focused on the molecular mechanisms responsible for these modifícations and some of the processes involved háve been clarified. Inactivation of PS2 and degradation of its components, as well as some of the mechanisms involved in the restoration of PS2 activity can be studied in vitro using preparations characterized by various degree of complexity from entire thylakoid membrane to the simplest preparation of isolated PS2 reactíon centre. Although the in vitro approach contributes essentíal informatíon to our understasiUng of the molecular process underlying photoinhibitíon, extrapolatíon to the in vivo situation is not obvious and many points demand íiirther experimental work.
Ultrastructural and physiological effects of exposure to 1 ppm and 5 ppm of cadmium (Cd) on cultured cells of Koliella antarctica, a green microalga from Antarctica, were investigated. The amount of Cd in the alga rose with the increase of the metal concentration in the growth medium and most Cd remained outside the cells, bound to the components of the cell walls. The increase of Cd in the microalga was concomitant with the decrease of other elements, mainly calcium (Ca). Exposure to 1 ppm Cd slowed culture growth by inhibiting cell division and also caused the development of some misshapen cells with chloroplast showing disordered thylakoids. However, this concentration did not substantially affect the chlorophyll (Chl) content or photosystem (PS) activity. At 5 ppm, Cd cell growth suddenly stopped and some cells lysed. After a week of Cd contamination, the cells were enlarged and severely damaged. The chloroplasts showed great ultrastructural alterations and a reduced Chl content. Cd exposure negatively affected PSII, whose activity was almost completely lost after four days. and N. La Rocca ... [et al.].