14 July 2004
UV Light Turns Plankton Into Cloud Factories
by Kate Melville
Much like Dr. Evil's weather control machine, plankton may be able to change the weather, and longer term climate, in ways to benefit the tiny organism. New research confirms a theory that plankton can indirectly create clouds that block some of the Sun's harmful UV rays. The study was conducted by Dierdre Toole of the Woods Hole Oceanographic Institution (WHOI) and David Siegel of the University of California (UCSB).
The study found that 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. When they are stressed, plankton try to protect themselves by producing a compound called dimethylsulfoniopropionate (DMSP). Though no one knows for sure, some scientists believe DMSP helps strengthen the plankton's cell walls. This chemical gets broken down in the water by bacteria, and it changes into another substance called dimethylsulfide (DMS). DMS then filters from the ocean into the air, where it reacts with oxygen, to form different sulfur compounds. Sulfur in the DMS sticks together in the air and creates tiny dust-like particles. These 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 less direct light reaches the ocean surface. This relieves the stress put on plankton by the Sun's UV rays.
For some time, scientists have been studying related processes in the laboratory, but this is the first time scientists have shown how variations in light impact plankton in a natural environment. The research was done in the Sargasso Sea, near Bermuda.
Research also found that the cloud producing compound peaks in the summer in the ocean, when UV rays are high, but plankton numbers are at their lowest. "Plankton levels are at a minimum in the summer but DMS is at its peak," said Toole.
In the warmest months, the top layer of the ocean warms as well. This heating of the top 25 meters creates a contrast with cooler deeper layers. The deeper layers hold many of the nutrients that plankton need to live on. Just as how oil separates from water, the warmer upper layer creates something of a barrier from the cooler lower layers and less mixing occurs. Also, the shallow upper layer exposes the plankton to more UV light. Under conditions where there are low nutrients in the water and levels of UV light are high, plankton create more DMS.
DMS levels peak from June through the end of September. During the season, the study found that a whopping 77 percent of the changes in amounts of DMS were due to exposure to UV radiation. "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. To give an example for comparison, when carbon dioxide gets into the atmosphere where it acts as a greenhouse gas and traps heat, it may last for decades. Toole adds that the cycles that break down DMS scream along at these very fast rates, even though overall amounts over the course of the year remain pretty stable with a slow increase over summer and a gradual decline over winter.
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. "There is the potential that this cycle could slow global warming," said Siegel. "But right now we have no idea of the size of it or even what it means." The researchers add that this effect may help to slow or lessen climate change, but would in no way reverse the trend or stop it altogether.