WASHINGTON - Scientists have launched a first global study on the health and productivity of ocean plants using a unique signal detected by US space agency NASA’s Aqua satellite.

Ocean scientists can now remotely measure the amount of fluorescent red light emitted by phytoplankton and assess how efficiently these microscopic plants turn sunlight and nutrients into food through photosynthesis. They can also study how changes in the global environment alter these processes at the centre of the ocean food web.

Using the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite, scientists have now observed “red-light fluorescence” over the open ocean. MODIS is the first instrument to observe this signal on a global scale.

All plants absorb energy from the sun, typically more than they can consume through photosynthesis. A small fraction of this extra energy is re-emitted as fluorescent light in red wavelengths.

“The amount of fluorescent light emitted is not constant; it changes with the health of the plant life in the ocean,” said Michael Behrenfeld, a biologist who specialises in marine plants at Oregon State University.

“This is the first direct measurement of the health of the phytoplankton in the ocean,” said Behrenfeld. “We have an important new tool for observing changes in phytoplankton every week, all over the planet.”

Single-celled phytoplankton fuels nearly all ocean ecosystems, serving as the most basic food source for marine animals. Phytoplankton accounts for half of all photosynthetic activity on Earth and play a key role in the balance of carbon dioxide in the atmosphere.

The health of these marine plants affects the amount of carbon dioxide the ocean can absorb from the atmosphere and how the ocean responds to a changing climate.

Scientists previously used satellite sensors to track the amount of plant life in the ocean by measuring the amount and distribution of chlorophyll.

“Chlorophyll gives us a picture of how much phytoplankton is present,” said researcher Scott Doney, a marine chemist from the Woods Hole Oceanographic Institution in Woods Hole, Massachusetts. “Fluorescence provides insight into how well they are functioning in the ecosystem.”

The findings were published in the May edition of the journal Biogeosciences.