Feedback Effect on Marine Life May Affect Atmospheric Oxygen
Researchers have identified a simple feedback effect that may help control oxygen levels in the earth's atmosphere by altering the productivity of marine organisms. By adjusting the internal oceanic recycling of a key nutrient in response to changing levels of oxygen, the feedback effect would regulate the growth of ocean plants that produce oxygen as a by-product of photosynthesis.
Described in a paper published January 26 in the journal Science, the work provides new information on how complex geological and biological systems work together to maintain the planet's environment within the narrow bounds necessary to support life. It may also provide additional clues to other complex aspects of atmospheric chemistry.
Researchers prepare a box core sediment sampling device to make measurements off the coast of Peru.
"There is a feedback between the use of oxygen on the land areas and the productivity of organisms in the oceans," said Philippe Van Cappellen, assistant professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology. "We now have a direct link between inorganic and biological processes that at first seemed to be very disconnected."
Van Cappellen and coauthor Ellery D. Ingall of the University of Texas at Austin's Marine Science Institute base their conclusions on levels of the nutrient phosphorus measured in both modern and ancient sediments taken from the ocean floor.
"I first noticed that something was going on with phosphorus in sediments several years ago during a study of an ancient sediment sequence," Ingall explained. "With changes in water oxygenation, major changes were always observed in the distribution of phosphorus in the sediments."
Ingall saw similar effects in more modern sediments examined off the coast of Peru during research conducted with Richard Jahnke of the Skidaway Institute of Oceanography in Savannah, GA. In both ancient and modern sediments, the researchers measured larger amounts of phosphorus in sediments produced during conditions in which oxygen levels in the surrounding ocean water were high. Conversely, low levels of water oxygenation corresponded to reduced amounts of phosphorus stored in the sediments.
"We find that when the oceans are fully oxygenated, the sediments tend to be more effective sinks for phosphorus than when the oceanic waters are depleted in oxygen," explained Van Cappellen. "This is critical because the productivity of the oceans appears to be limited by how much bioavailable phosphorus is available."
Well-known oxygen-dependent chemical processes centered on iron oxides account for a portion of the phosphorus uptake. But the two researchers believe a biological factor -- likely bacteria living in the ocean floor sediments -- also play a significant role.
"Iron alone cannot explain what we are seeing," Van Cappellen said. "We propose that the bacteria living in the top layers of sediments can be efficient scavengers of phosphorus, though this scavenging seems to be linked to the availability of oxygen. These bacteria can switch from one type of energy cycle to another depending on whether they have enough oxygen. Microbial physiology therefore may play a role in how efficiently phosphorus is removed from the water column."
Such a microbial effect has been observed in freshwater lakes, and also in the operation of wastewater treatment plants, where high levels of oxygenation result in improved removal of phosphates, he noted.
With support from the National Science Foundation (NSF) and the American Chemical Society (ACS), Ingall and Van Cappellen plan additional field research aimed at testing their model.
Over geologic time, oxygen levels in ocean water are tied closely to the amount of oxygen in the atmosphere. This proposed feedback system would therefore counter fluctuations in atmospheric oxygen caused by changes in oxygen consumption during weathering of minerals and old organic matter exposed on land.
Because the marine plants also remove carbon dioxide during the photosynthesis process, the proposed feedback cycle could help researchers attempting to better understand other aspects of global climate change.
The model advanced by Van Cappellen and Ingall may also help explain the environmental stability that would have been required for life to thrive and evolve on the earth during the past 500 million years. Without steady levels of atmospheric oxygen, higher forms of life would have been unable to survive.
The work may also lead researchers to more closely examine the role of phosphorus in the environment. Because the nutrient exists in low concentrations in sediments, it has been difficult to measure and often ignored by scientists measuring carbon, oxygen, sulfur and iron -- whose effects have been more completely documented.
Van Cappellen notes that the proposed feedback mechanism shows the interdependence of seemingly-unrelated parts of the global system.
"You really cannot think about the evolution of the earth or the atmosphere without looking at the links to biology," he said. "You must consider how the biosphere interacts with the geosphere. I think this shows that life has developed ways of compensating for any drastic changes in its environment."
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