Oceanic carbon is trapped by dilution 

Researchers have overturned a decades-old hypothesis about the organic carbon storage in the oceans, a finding that could change the way scientists look at this important part of the global carbon cycle.

Dissolved organic carbon (DOC) in the planet's oceans is the second largest carbon reservoir in the biosphere and can stay in the ocean for centuries, not affected by microbial degradation. The 'dilution hypothesis' suggests that components of the DOC are not available at a concentration high enough for prokaryotes to metabolize them. But in 1968, a study showed that prokaryotes didn't grow even when offered concentrated DOC, rejecting the dilution hypothesis in favour of the idea that the DOC components are intrinsically resistant to microbial degradation.

A team led by Jesus Arrieta, who was affiliated with the Spanish National Research Council and is now with the Red Sea Research Center at the King Abdullah University of Science and Technology (KAUST), revisited the question using a new approach to concentrate the smaller molecules in the oceanic DOC and measure microbial growth. 

Contrary to the 1968 findings, the cultures consistently grew better at higher DOC concentrations, showing that dilution is the limiting factor.

Despite their initial excitement, the researchers repeated the experiments several times to ensure that their results held. “After all, we were working with a hypothesis that had been discarded about 45 years earlier,” says Arrieta.

According to Arrieta, the dilution hypothesis “changes the way we think about the preservation of one of the largest pools of organic matter in the biosphere.” For example, the possibility of enhancing carbon storage in oceanic DOC to reduce atmospheric CO2 concentrations “is probably quite limited” given the new findings.

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