Carbon consumers

Bacteria cause net CO₂ uptake in the Amazon River plume

Global Biogeochem. Cycles doi:10.1029/2006GB002831 (2007)

Credit: © NASA

Bacteria growing in the low-salinity Amazon River plume waters, which stretch 3,000 km into the tropical Atlantic Ocean, are found to absorb significant amounts of CO₂ from the atmosphere¹. Because tropical oceans are warm, they hold less dissolved carbon than oceans elsewhere and so typically emit CO₂ to the atmosphere. But research now shows that communities of nitrogen-fixing bacteria, reliant on the nutrients contained in the Amazon river run-off, can shift the air–ocean balance so that instead of emitting CO₂ the ocean absorbs it.

Sarah Cooley from the University of Georgia and colleagues measured levels of dissolved carbon in the Amazon River plume during April and May of 2003. Combining their observations of ocean and river composition with those from previous studies and with satellite observations of the plume, they calculated the total annual CO₂ uptake by the plume waters.

They found the CO₂ uptake, driven by bacterial consumption of carbon, to be about 15 million tonnes per year — emissions equivalent to about 20,000 return flights from London to Los Angeles. Carbon sequestration comes as a surprise in a region thought to emit CO₂ to the atmosphere.