We here present an outline of the JIP-test, a screening test based on the quantitative analysis of the chlorophyll α fluorescence rise 0-J-I-P transient exhibited by all photosynthetic organisms. The JIP-test leads to the calculation of several phenomenological and biophysical parameters quantifying the photosystem II (PSII) behaviour. The parameters express (a) the energy fluxes for absorption, trapping and electron transport, (b) the flux ratios or yields, (c) the concentration of reaction centres and (d) a performance index. The JlP-test was proved to be a very useful tool for the in vivo investigation of the adaptive behaviour of the photosynthetic apparatus to a wide variety and combination of stressors, e.g. high or low temperature, high light intensity, atmospheric C0₂ or ozone elevation etc. The successful "fingerprinting" of stressors on the behaviour/performance of the photosynthetic apparatus provides the basis for using this behaviour/performance as a bio-indicator of climate changes. As an example, a case study on light and heat stress adaptation of the symbionts of coral reef foraminifers in hospite is presented. This study tackles the question about the origin of the big environmental problem of massive bleaching of the reef ecosystem, which involves, besides corals, several other species among which large foraminifers, and corresponds to the loss of their photosynthetic symbionts and/or the symbionts' pigments. We used the JIP-test to investigate in three genera of large foraminifers the response of PSII of their symbionts in hospite upon light and heat stress (up to 32 °C). While low-light was found to offer a strong thermoprotection, heat stress in the dark was found to result in a wide decrease of the capacity for photosynthetic activity. Strong illumination induced even wider decreases of the photosynthetic activity, however highly reversible. The extent of the reversibility from the high-light stress was much bigger under low-light than in the dark. Since the symbionts' photosynthetic products are the major energy source for the symbiotic association, it is proposed that the reduction in their productivity and, concomitantly, of their delivery to the host, affects negatively the co-habitation and, hence, triggers bleaching. The results suggest that warm nights should be considered as a major factor provoking coral reef bleaching.