All plants absorb energy from the sun, typically more than the plant can consume through the process of photosynthesis. The extra energy is mostly released as heat as the plants respirate oxygen and water vapor. But a fraction of that energy is re-emitted as fluorescent light, particularly in red wavelengths. MODIS is the first instrument to observe this signal on a global scale.
Red-light fluorescence says something about the physiology of plants and the efficiency of photosynthesis, as different parts of the plant's energy-harnessing machinery are activated based on the amount of light and nutrients available. The amount of fluorescence increases when plants are under stress from a lack of iron, a critical nutrient in the sea. When water is iron-poor, plants slow their growing processes and struggle to dissipate excess solar energy that cannot be used in photosynthesis.
The fluorescence data from MODIS gives scientists a tool to see where waters are iron-enriched or iron-limited, and where plankton might flourish or not. Iron is typically picked up by winds blowing dust from deserts and other arid areas, and by river plumes and island currents.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 188.8.131.52.0