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When Sun's Too Strong, Plankton Make Clouds
People say size doesn't matter, and that may be true for tiny plankton, those free-floating ocean plants that make up the bottom of the marine food-chain. Little plankton may be able to change the weather, and longer term climate, in ways that serve them better.

This is a satellite image of plankton Image to right: Phytoplankton can be identified by satellite through their chlorophyll (light green). This image is a composite from the Northern Hemisphere spring seasons of 1998-2004. Click on image to enlarge. Credit: NASA/GSFC and ORBIMAGE

It's almost hard to believe, but new NASA-funded research confirms an old theory that plankton can indirectly create clouds that block some of the Sun's harmful rays. The study was conducted by Dierdre Toole of the Woods Hole Oceanographic Institution (WHOI) and David Siegel of the University of California, Santa Barbara (UCSB).

The study finds that in summer when the Sun beats down on the top layer of ocean where plankton live, harmful rays in the form of ultraviolet (UV) radiation bother the little plants. UV light also gives sunburn to humans.

When plankton are bothered, or stressed by UV light, their chemistry takes over.

This is an image of plankton.Image to left: New NASA-funded research confirms an old theory that ocean-dwelling microscopic single-celled organisms called plankton can indirectly create clouds that block some of the Sun's harmful rays. Click on image to view movie. (1.9 MB) Credit: NASA

The plankton try to protect themselves by producing a chemical compound called DMSP, which some scientists believe helps strengthen the plankton's cell walls. This chemical gets broken down in the water by bacteria, and changes into another substance called DMS.

DMS then filters from the ocean into the air, where it breaks down again to form tiny dust-like particles. These tiny particles are just the right size for water to condense on, which is the beginning of how clouds are formed. So, indirectly, plankton help create more clouds, and more clouds mean that less direct light reaches the ocean surface. This relieves the stress put on plankton by the Sun's harmful UV rays.

This image shows where on the globe the Sargasso Sea is. Image to right: The Sargasso sea is an irregularly-shaped region in the Atlantic Ocean that is set apart, not by the presence of land masses, but by vast expanses of seaweed, called Sargassum, that float on its surface utilizing small balloon-like floats. Credit: NASA

DMS levels peak from June through the end of September. Surprisingly, plankton levels are at a minimum during this time. That means that the number of plankton does not affect how much DMS they produce. During the summer, the study found that a whopping 77 percent of the changes in amounts of DMS were due to exposure to UV radiation. The researchers found it amazing that a single factor could have such a big affect on this process.

"For someone studying marine biology and ecology, this type of variation is absolutely incredible," Siegel said.

The researchers were also surprised to find that the DMS molecules completely refresh themselves after only three to five days. That means the plankton may react to UV rays quickly enough to impact their own weather. Toole and Siegel were surprised by the lightning-fast rate of turnover for DMS.

Image of a map showing the location of the Sargasso sea.Image to left: The Sargasso Sea is a region of slow-moving ocean currents surrounded by rapidly-moving ocean currents, such as the Gulf Stream to its east. It is located off the coast of Bermuda. In this composite image of night-time city lights, you can see the bigger cities in the brighter areas. Click on image to enlarge. Credit: NASA, DOD

The next step for the researchers will be to see how much the added clouds from plankton actually impact climate. By figuring out how plankton react to light, scientists now have the information they need to use computer models to recreate the impacts of plankton on cloud cover. Since the white clouds can reflect sunlight back out to space, the researchers believe the plankton-made clouds may have some affect on global temperatures.

The study was funded by NASA. Studies of DMS have been funded by the National Science Foundation.

Krishna Ramanujan
Goddard Space Flight Center