Photosynthesis measured on a global scale

A research team led by geoscientists from Brown University and the Marine Biological Laboratory has provided some crucial ground-truth for a method of measuring plant photosynthesis on a global scale from low-Earth orbit.
The researchers have shown that chlorophyll fluorescence, a faint glow produced by plant leaves as a byproduct of photosynthesis, is a strong proxy for photosynthetic activity in the canopy of a deciduous forest. That glow can be detected by orbiting satellites and could be used to monitor global photosynthetic activity in real time.

“We show that fluorescence is tightly coupled to photosynthesis, capturing both daily and seasonal fluctuations,” said Xi Yang, a postdoctoral researcher at Brown and the study’s lead author. “This is the first time anyone has linked fluorescence to photosynthesis over a long time scale in a deciduous forest and validated orbital measurements of fluorescence with ground-based measurements.”
The findings are published in the journal Geophysical Research Letters. Yang led the work as a graduate student in the Brown–Marine Biological Laboratory (MBL) graduate program, working with Brown geoscientist Jack Mustard and MBL associate scientist Jianwu (Jim) Tang.
Catching photons on the rebound
When plants photosynthesize, chlorophyll molecules in leaves absorb photons from sunlight. The plant then converts the energy from those photons into sugar and other carbohydrates using carbon dioxide absorbed from the atmosphere as a carbon source. But not all the photons absorbed by chlorophyll are for photosynthesis. Around 1 percent of them are re-emitted as lower energy photons, which creates the faint glow known as fluorescence.
The glow isn’t visible to the naked eye, but a few years ago scientists from NASA and the Japanese Aerospace Exploration Agency found that spectrometers aboard climate satellites could detect fluorescence coming from croplands and forest canopies. That raised the possibility of measuring photosynthesis on a global scale.