Amazon River Found To Be A Metabolic Hotspot
May 21, 2013

Amazon River Eats Almost All Of What The Rainforest Feeds It

April Flowers for - Your Universe Online

The Amazon rainforest is known as the lungs of the planet because it inhales carbon dioxide and exudes oxygen into the atmosphere. The plants of the forest use the carbon dioxide to promote leafy growth, which eventually falls to the ground and decomposes or washes away from the region´s plentiful rainfall.

Until recently, the belief was that most of the rainforest´s carbon floated away on the Amazon River, ending up deep in the ocean. A study from the University of Washington over a decade ago showed that rivers exhale huge amounts of carbon dioxide. However, the study left open the question of how that was possible; since bark and stems were thought to be too tough for river bacteria to digest.

A new University of Washington-led study has found the answer, proving that woody plant matter is almost completely digested by bacteria living in the Amazon River, which plays a major part in fueling the river´s breath. The findings, published in Nature Geoscience, have implications for global carbon models and for the ecology of the Amazon and other rivers around the world.

“People thought this was one of the components that just got dumped into the ocean,” said Nick Ward, a UW doctoral student in oceanography. “We´ve found that terrestrial carbon is respired and basically turned into carbon dioxide as it travels down the river.”

The main part of the woody tissue is formed from tough lignin, the second most common component of terrestrial plants. Until now, scientists believed that most of it was buried on the seafloor where it would stay for centuries or a millennia. The new study, however, shows that river bacteria break the lignin down within two weeks. The result is that just five percent of the Amazon rainforest´s carbon ever reaches the ocean.

“Rivers were once thought of as passive pipes,” said Jeffrey Richey, a UW professor of oceanography. “This shows they´re more like metabolic hotspots.”

The prior studies demonstrated how much carbon dioxide was outgassing from rivers, but scientists knew it didn´t add up. This led to speculation that there might be some unknown, short-lived carbon source that freshwater bacteria could turn into carbon dioxide.

“The fact that lignin is proving to be this metabolically active is a big surprise,” Richey said. “It´s a mechanism for the rivers´ role in the global carbon cycle — it´s the food for the river breath.”

Approximately one-fifth of the world´s freshwater is discharged from the Amazon. The river plays a large role in global processes, as well as serving as a test bed for natural river ecosystems.

For more than three decades, Richey and his collaborators have studied the Amazon River. The earlier study took place more than 500 miles upstream from the current study site situated at the mouth of the world´s largest river — a treacherous study site. The team of US and Brazilian researchers chose this location because they sought to understand the connection between the river and the ocean.

“There´s a reason that no one´s really studied in this area,” Ward said. “Pulling it off has been quite a challenge. It´s a humongous, sloppy piece of water.”

The team crossed the three river mouths using flat-bottomed boats. The river mouths are each so wide that you cannot see land from the boats, with water so rich with sediment that it looks like chocolate milk. Part of the reason this area is so hard on researchers is that the ocean is raised 30 feet by the tides, reversing the flow of freshwater at the river mouth, and winds blow at up to 35 mph.

Ward collected river water samples in all four seasons in these dangerous conditions. The original samples were compared with ones left to sit for up to a week at river temperatures. Once back in the laboratory at UW, Ward used newly developed techniques to scan the samples for some 100 compounds that covered 95 percent of all plant-based lignin. Techniques previously used could identify only 1 percent of the plant-based carbon in the water.

The study estimates that about 40 percent of the Amazon´s lignin breaks down in soils. Another 55 percent breaks down in the river system and the final five percent reaches the ocean, where it may break down or sink to the ocean floor.

“People had just assumed, ℠Well, it´s not energetically feasible for an organism to break lignin apart, so why would they?´” Ward said. “We´re thinking that as rain falls over the land it´s taking with it these lignin compounds, but it´s also taking with it the bacterial community that´s really good at eating the lignin.”