Iron, which is necessary for the sustenance of life, is scarce in the ocean. National Science Foundation (NSF)-supported researchers found that light helps transform the mineral into a form that can be easily taken up by phytoplankton and other microorganisms. They report their findings in the September 27 issue of the journal Nature.
"This discovery helps us better understand one of the essential links in the ocean's food chain," said Donald Burland, acting director of NSF's Chemistry Division. "It may also have implications for global climate change, since living organisms are important in the absorption and release of carbon dioxide from the oceans."
Iron and other trace metals are important biochemical ingredients in the production of plankton, the most abundant organisms in seawater, which are at the bottom of the aquatic food chain. But iron is rare in surface seawater, and scientists believe it occurs almost entirely in complex molecules in which the iron is strongly bound by organic ligands presumed to be of biological origin. Bacteria produce small molecules called siderophores to help them obtain iron from their environments, and this process may contribute to the pool of tightly bound iron complexes.
"We determined that iron bound to the oceanic siderophores react to light," said chemist Alison Butler of the University of California at Santa Barbara. "This photochemical reaction helps transform the iron complexes into a form that enables marine organisms to more easily acquire the essential iron."
The sun's energy turns the molecules into more loosely bound configurations of iron and oxygen atoms, Butler explained. This enables bacteria, plankton and other microorganisms to grab and use the iron.
The team's research grew partly from early studies indicating that fertilizing the oceans with iron could stimulate the growth of plant life that consume carbon dioxide, and thereby counteract global warming. In those studies, iron dropped from ships encouraged phytoplankton to bloom profusely, but only for a short time.
"Understanding the uptake of this scarce micronutrient will help provide more insight into how these microscopic plants and bacteria cope in these oceanic environments," said oceanographer Ken Bruland of the University of California at Santa Cruz.
Butler believes the findings on how the complex iron molecules are broken down by sunlight could also contribute to research on alternate drug delivery systems, possibly providing a nanoscale vessel that reacts upon exposure to light.
The Center for Environmental Bio-Inorganic Chemistry in Princeton, N.J., established by NSF and the Department of Energy to study environmental issues at the molecular level, funded the research. This center and others established by NSF and DOE conduct interdisciplinary research aimed at understanding the natural environment and addressing global environmental challenges.