A system for imaging variable chlorophyll fluorescence of aquatic phototrophs

We describe a modular imaging system for mapping the variable chlorophyll fluorescence of photosynthetic organisms and tissues in the aquatic environment. The system is based on an array of blue LED's (470 nm) for excitation of chlorophyll and a modulated CCD camera equipped with a long-pass filter for detection of chlorophyll fluorescence >665 nm. The exposure of the sample with blue actinic light, non-actinic measuring light, and a flash of saturating light is based on a software-controlled sequence of variable pulse-widths of the LED's. The LED and the camera modulation are controlled via a custom designed timer-unit connected to a PC. We present examples of application for mapping the distribution of photosynthetic activity in coral tissue, communities of endolithic algae within coral skeleton as well as in tissue of didemnid ascidians harboring symbiotic cyanobacteria. It is possible to study spatio-temporal patterns of variable fluorescence (and the derived quantum yield of photosystem II-related electron transport) in heterogeneous aquatic systems at sub-mm spatial resolution.