Distinct physiological responses to a high light and low CO2 environment revealed by fluorescence quenching in photoautotrophically grown Chlamydomonas reinhardtii.

Mechanisms for countering environmental stress are essential to photosynthetic organisms. Alteration of the photosynthetic apparatus, a mechanism for balancing the flux of light energy and carbon fixation, can be characterized by fluorescence properties. In this study, we have established a simple protocol to determine the extent of energy-dependent quenching (qE) and quenching by state transition (qT) in Chlamydomonas cells by examining their fluorescence properties under light fluctuations. We identified qE as the uncoupler-sensitive NPQ component that was rapidly relaxed upon transition to dark conditions. We characterized the qT component by determining low-temperature fluorescence spectra and analyzing a state-transition-less mutant. By these methods, we observed that similar abiotic stresses-high light conditions (where excess energy is supplied) and low CO2 conditions (where energy utilization is limited)-induced different types of NPQ. High light conditions induced mainly qE-quenching that increased gradually while low CO2 conditions induced mainly qT-quenching that peaked in 20 min and then decreased gradually. That high light and low carbon signals induced different physiological responses suggests that they triggered different genetic responses, which altered protein expression under each of the conditions.