Development and Application of Variable Chlorophyll Fluorescence Techniques in Marine Ecosystems

Since its introduction in the early 1960s, in vivo chlorophyll fluorescence has been used as an index of photosynthetic biomass in marine ecosystems. In the late 1980s, however, active fluorometric techniques, originally based on the pump and probe method, were used to derive estimates of photosynthetic electron transport, the overall quantum efficiency of photosynthetic energy conversion, and the effective cross section of Photosystem II. It was quickly realized that nutrient limitation, but not acclimation to light or temperature, had a profound influence on photosynthetic energy conversion efficiency (reported as, e.g., F_v/F_m). Subsequently, variable fluorescence techniques were employed to assess physiological control of oceanic photosynthesis by nutrients that potentially limit photosynthetic electron transport. The pump and probe technique was subsequently supplanted by a fast repetition rate (FRR) fluorescence method, which greatly improved the precision and efficiency of variable fluorescence measurements at sea. The FRR method has revealed how the availability of iron, fixed inorganic nitrogen, and phosphate control photosynthetic electron transport rates throughout the world oceans. The technique has been further applied to corals, seagrasses, single cells of free-living marine phytoplankton, and anoxygenic aerobic photosynthetic bacteria. Variable fluorescence data reveal extraordinary physiological plasticity of the photosynthetic apparatus in the genetically diverse group of organisms that comprise the primary producers in contemporary oceanic ecosystems.