Light and seat stress adaptation of the symbionts of temperate and coral reef foraminifers probed in hospite by the chlorophyll a fluorescence kinetics

Since the early 80's massive bleaching affects the reef ecosystem. It invol ves, besides corals, several other species among which large foraminifers, and it corresponds to the loss of their photosynthetic symbionts or the sym bionts' pigments. The cause is unclear, though temperature elevation and st rong irradiation have been considered to be primary factors. In this work w e investigated in two genera of coral reef foraminifers (Amphistegina lobif era and Amphisorus heimprichii) and in the temperate foraminifer Sorites va riabilis the response of photosystem II (PSII) of their symbionts in hospit e upon light stress (white light of 550 mu E m(-2) s(-1) and red light of 3 200 mu E m(-2) s(-1)) and heat stress (up to 32 degrees C), by means of the Chla fluorescence transients O-J-I-P they exhibit upon illumination. The t ransients were analysed according to the JIP-test which leads to the calcul ation of several structural and functional parameters providing a quantific ation of PSII behaviour. We observed that the various parameters undergo mo difications that differ concerning both their extent and their degree of el asticity, thus indicating that different adaptive strategies are employed i n response to stress. The most pronounced of these regulatory changes is a wide decrease of the quantum yield of electron transport. However, the exte nt of the changes, different for the three studied species, was in general smaller when the cultures were kept under low light (70 mu E m(-2) s(-1)) t han in darkness. By the applied stressors, PSII was not damaged and, except for some cells in which an expulsion of symbionts was initiated, no bleach ing was observed. This can be well correlated with the observed adaptabilit y of PSII. As a working hypothesis, it is proposed that the decrease of the capacity for electron transport activity might be among the factors trigge ring bleaching in the field.