Vast amounts of dissolved organic matter in the ocean, once though to be inert, may play a surprising role in mitigating the greenhouse effect, according to bioengineering researchers at the University of Washington, reporting in this week's issue of Nature.
These tiny chains, or polymers, of carbon-based molecules comprise a significant portion of all the organic material in the oceans but were thought to be too small to register on the marine food chain. Instead, they appear to be spontaneously assembling into molecular networks called polymer gels that would give them a vital role in the carbon cycle, the UW researchers report in this week's (Feb. 5) issue of Nature. These microgels would provide an unexpected mechanism for dissolved organic matter to either enter the carbon cycle or abandon it and ultimately remove carbon dioxide from the atmosphere.
"Understanding processes that can potentially affect the carbon cycle is critical to understanding global warming," explains Bioengineering Professor Pedro Verdugo, who co-authored the Nature report along with graduate student Wei-Chun Chin and post-doctoral fellow Monica V. Orellana. "Gigantic amounts of CO2 are being produced through burning of fossil fuels and we should be experiencing a tremendous greenhouse effect, but some of the CO2 we're producing remains unaccounted for. Our research raises the possibility that dissolved organic matter in the oceans might be playing an unforeseen role in removing CO2 from the atmosphere."
Microscopic algae in the oceans are among the planet's most important engines of photosynthesis, the process of converting sunlight and carbon dioxide into oxygen and carbon compounds that make up the base of the food chain. During summer blooms in some coastal areas and in the polar regions, microalgae can produce jelly-like layers comprised mostly of carbon polysugars that cover several square miles. These marine photosynthesized compounds, together with organic material discharged by rivers, get dispersed in the ocean into organic particles of various sizes. Larger polymer clusters, called particulate organic matter, can be colonized by bacteria and re-enter the food chain. But the smaller polymers of dissolved organic matter have long been considered minor players in the carbon cycle.
"It hasn't been clear what is happening to these dissolved organic matter polymers," Verdugo says. "What we have shown is that they can reassemble as microgels and can re-enter the carbon cycle or abandon it. We're not sure yet how much this is happening in the oceans, but chances are it is happening quite a lot and is a very important part of the carbon cycle."
In research funded by the National Science Foundation's Office of Polar Programs, Verdugo's team analyzed dissolved organic polymers in filtered sea water from Puget Sound, the Arctic Ocean and the North Pacific Ocean. Using dynamic laser scattering spectroscopy and flow cytometry, the researchers observed the polymers spontaneously assembling into tiny hydrogels.
Once formed, Verdugo says, the microgels can be colonized by bacteria and re-enter the food chain. Alternatively, the UW research indicates that due to their negative electrostatic charge, the gels can accumulate calcium carbonate from sea water and crystallize, eventually sinking as sediment and sequestering carbon on the ocean floor. Laboratory experiments showed that a very small increase of pH in the sea water, which periodically can occur in marine environments, induced crystalline mineralization and sedimentation. In addition, researchers discovered that the hydrogels undergo a phase transition process, common to all polymer gels, whereby they expel water and collapse into dense particles that could also settle out as sediment. Either of these processes would remove carbon from the marine life cycle and could provide new clues to the apparent disappearance of carbon dioxide from the atmosphere, Verdugo explains.
"By no means have we proved that this is the case," he states. "But we now know that these processes operate and could be providing a way to stow carbon in the bottom of the oceans. Perhaps the most significant contribution of these studies is that they furnish a new approach for studying, through the lenses of polymer physics and bioengineering, how organic materials are processed in the ocean."