Ocean CO2 Storage Revised

The ocean is Earth's largest single sink for CO2 outside of the planet's crust itself. Simple sea creatures depend on carbon dissolved in the ocean's water for their existence, and their actions create a biological carbon “pump” that removes vast quantities of CO2 from the atmosphere. Large amounts are suspended in the water column as dissolved organic carbon (DOC), and each year the ocean's biological pump deposits some 300 million tons of carbon in the deep ocean sink. New findings have revealed that massive amounts of carbon are converted into “inedible” forms of organic carbon that remain out of circulation for thousands of years, effectively sequestering the carbon by removing it from the ocean food chain. According to Jiao Nianzhi, a microbial ecologist here at Xiamen University, the amount stored is tremendous: “It's really huge. It's comparable to all the carbon dioxide in the air.”

On average, the world's oceans absorb 2% more carbon than they emit each year, forming an important sink in the overall carbon cycle. CO2 is absorbed by the ocean in a number of ways. Some dissolves into the water column, forming carbonic acid (H2CO3) while more enters the seas through the food chain. Green, photosynthesizing plankton converts as much as 60 gigatons of carbon per year into organic carbon—roughly the same amount fixed by land plants and almost 10 times the amount emitted by human activity. But this form of carbon is only stored for a short period of time.

According to a news focus article in Science by Richard Stone: “Even more massive amounts of carbon are suspended in the water column as DOC. The oceans hold an estimated 700 billion tons of carbon as DOC—more than all land biomass put together (600 billion tons of carbon) and nearly as much as all the CO2 in the air (750 billion tons of carbon).” The carbon cycle with its various sinks and sources are shown in the IPCC diagram below.

What is more important is the conversion of immense amounts of bioavailable organic carbon into difficult-to-digest forms known as refractory DOC. The microbe driven conversion has been named the microbial carbon pump (MCP) by Jiao. Once transformed into a form less palatable to hungry marine microorganisms, the sequestered carbon can build up in the ocean's waters forming a huge reservoir of stored carbon. The process is described in a Nature Reviews Microbiology perspective, “Microbial production of recalcitrant dissolved organic matter: long-term carbon storage in the global ocean,” written by Jaio and colleagues. Their findings are described in the article's abstract:

The biological pump is a process whereby CO2 in the upper ocean is fixed by primary producers and transported to the deep ocean as sinking biogenic particles or as dissolved organic matter. The fate of most of this exported material is remineralization to CO2, which accumulates in deep waters until it is eventually ventilated again at the sea surface. However, a proportion of the fixed carbon is not mineralized but is instead stored for millennia as recalcitrant dissolved organic matter. The processes and mechanisms involved in the generation of this large carbon reservoir are poorly understood. Here, we propose the microbial carbon pump as a conceptual framework to address this important, multifaceted biogeochemical problem.

Many oceanographers credit Jiao with first recognizing the dominant role microbes play in “pumping” bioavailable carbon into a pool of relatively inert compounds “The existence of this ‘inedible’ organic carbon in the ocean has been known for quite some time. But its role in the global carbon cycle has been recognized only recently,” says Michal Koblizek, a microbiologist at the Institute of Microbiology in Trebon, Czech Republic. Furthermore, Markus Weinbauer, a microbial oceanographer at Laboratoire d'Océanographie de Villefranche in France, states that the concept “could revolutionize our view of carbon sequestration.”

About 95% of organic carbon is bound up as refractory DOC forming the largest pool of organic matter in the ocean. Studies using carbon-14 suggest that refractory compounds swirl in this microbial eddy for more than 6000 years, several times the circulation time of the ocean. This means that the ocean has a way to absorb vast amounts of CO2 without increased acidification, hiding the carbon away in the abyssal deeps. This process is shown in the diagram below.

Biological sequestration in the ocean. Source C. Bickel/Science.

From the Arctic to the tropics, microbes are busy sequestering carbon in the ocean in volumes previously unimagined. This may, in part, explain why ocean absorption of CO2 is not shrinking. Refractory DOC consists of thousands of compounds, such as complex polysaccharides and humic acids. Unfortunately, the origins of most refractory DOC remain unknown. Some is produced when light degrades organic matter near the ocean surface. Natural oil seeps contribute some, as do human oil spills—the oil spill in the Gulf of Mexico being an extreme example. Other compounds are likely forged in underwater vents or in wildfires and blown or washed into the sea. “We lack understanding of the mechanisms of its formation or variations in its magnitude and composition,” says Farooq Azam, a microbiologist at Scripps Institution of Oceanography.

The MCP “may act as one of the conveyor belts that transport and store carbon in the deep oceans,” says Chen-Tung “Arthur” Chen, an ocean carbonate chemist at National Sun Yat-sen University in Kaohsiung, Taiwan. According to the news report in Science, the MCP also appears to function in deep waters. There, bacteria adapted to the high-pressure environment may have “a special capacity” to degrade refractory DOC, says Christian Tamburini, a microbiologist at the Centre d'Océanologie de Marseille in France.

The microorganisms that are responsible for this new carbon pump are an unusual class of bacteria called aerobic anoxygenic phototrophic bacteria, or AAPBs. Jiao found that AAPBs are prone to infection and he isolated the first phage that's specific for these bacteria (a phage, short for bacteriophage, is a virus that infects bacteria). The phages identified by Jiao rip apart their hosts, spilling their contents, including organic carbon, into the water. This viral action may play a significant part in the accumulation of refractory DOC.

Epifluorescence microscopy picture of marine bacteria.

The water column holds on average 35 to 40 μmoles of carbon in the form of refractory DOC per liter. An increase of a mere 2 to 3 μmoles per liter would sequester several billion additional tons of carbon, leading some to suggest trying to use this mechanism in geoengineering schemes to remove CO2 from the atmosphere. Billions years ago the oceans held 500 times as much DOC as today, most likely generated by the MCP, Jiao says. Earth's ecosystem has changed tremendously since bacteria were our planet's only inhabitants, but the microbial sequestration potential could still be huge. According to Jiao, no chemical equilibrium would limit conversion of bioavailable DOC to refractory DOC, which in turn would not exacerbate ocean acidification.

Here is a previously unsuspected mechanism that can explain how nature keeps Earth's ecology in balance, despite the presence of human CO2 emissions. The microbial carbon pump, perhaps in concert with the “jelly pump” discovered by Lebrato and Jones in 2006, is busy compensating for the relatively small amount of carbon human activity releases each year. It is a natural regulation mechanism that science did not know existed and still does not fully understand, meaning that how Earth's ecology and climate interact must be revised. As Steven Wilhelm, a microbiologist at the University of Tennessee, notes, “We are just at the dawn of developing this understanding.”

Carbon is essential to life on Earth, and the carbon cycle helps regulate both life and climate. Nature has many hidden mechanisms that help manage carbon and CO2—mechanisms that were unknown when the commotion over anthropogenic global warming erupted decades ago and other mechanisms as yet undiscovered. Predictions that the ocean will soon lose its capacity to absorb CO2 were made in ignorance and have been shown to be wrong. What new discoveries the young and incomplete field of climate science will make in the future is anyone's guess. One thing is certain, those who would cry doom and predict the death of the ocean at human hands will have to invent new hazards to frighten the public. This is “settled science” like quicksand is solid ground.

Be safe, enjoy the interglacial and stay skeptical.