First and second line mechanisms of cadmium detoxification in the lichen photobiont Trebouxia impressa (Chlorophyta).

"First line" defence mechanisms, such as phytochelatin biosynthesis, and "second line" mechanisms, such as stress protein induction, were investigated in cadmium-exposed cells of Trebouxia impressa Ahmadjian, a green microalgal species that is a common photobiont of the lichen Physcia adscendens (Fr.) H. Olivier. When T. impressa cells were exposed to 0, 9 and 18 microM Cd for 6, 18 and 48 h, glutathione and phytochelatins efficiently protected the cells against Cd damage. By contrast, the highest Cd concentration (36 microM) at the longest exposure-time (48 h) caused marked drops in glutathione and phytochelatin content, several types of ultrastructural damage, and decreases in cell density and total chlorophyll concentration. In this case, induction of stress proteins was observed, but only long after the induction of phytochelatins. Thus, stress proteins could represent a "second line" mechanism to counteract Cd stress, activated when there is a decline in the "first line" mechanism of Cd detoxification given by phytochelatins.