Scientists have long speculated that ‘dissolved organic carbon’ (DOC) molecules accumulate within Earth’s oceans because marine bacteria cannot metabolize them. If this theory is correct, it could suggest a potential solution for safely storing extracted atmospheric carbon. However, research from Carlos Duarte, Jesus Arrieta and colleagues at KAUST suggests that these molecules only remain because they are too diluted for bacteria to use them efficiently.
The seas contain more than 70 percent of the planet’s DOC — primarily the degraded remnants of plant and animal life. But Duarte explains that this pool remains very stable over time, which belies the notion that a large proportion of dissolved organic carbon cannot be degraded by marine bacteria.
An alternative model suggests that DOC pools contains digestible compounds that are simply too dilute to provide useful nutrition, although this has proven challenging to confirm. “We previously lacked the capacity to concentrate DOC and characterize its molecular composition,” explains Arrieta.
He and his colleagues addressed this question by analyzing samples from 14 sites across the Atlantic and Pacific (see image). They consistently observed that experimentally increasing the concentration of DOC within a given sample stimulated the growth of deep-sea bacteria within that sample. Indeed, the organisms behaved exactly as would be predicted for microbes given access to increasing amounts of nutrients. “All the experiments we performed suggest a universal relationship between DOC concentration and bacterial growth rate throughout the world’s oceans,” says Duarte.
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