Although each individual cell contains minuscule quantities of hydrocarbons, the researchers estimated that the amount produced by two of the most abundant cyanobacteria in the world—Prochlorococcus and Synechococcus—is more than two million tonnes in the ocean at any one time. This indicates that these two groups alone produce between approximately 308 and 771 million tonnes of hydrocarbons per year, yet the concentration at any time in unpolluted areas of the oceans is tiny, thanks to other bacteria that break down the hydrocarbons as they are produced.

Hydrocarbons are ubiquitous in the oceans, even in areas with minimal crude oil pollution, but what hadn’t been recognized until now is the likely quantity produced continually by living oceanic organisms. Based on our laboratory studies, we believe that at least two groups of cyanobacteria are responsible for the production of massive amounts of hydrocarbons, and this supports other bacteria that break down the hydrocarbons as they are produced.

—Professor Christopher Howe, senior author

The scientists argue that the cyanobacteria are key players in an important biogeochemical cycle, which they refer to as the short-term hydrocarbon cycle. The study suggests that the amount of hydrocarbons produced by cyanobacteria dwarfs the amount of crude oil released into the seas by natural seepage or accidental oil spills.

Our study demonstrates the role cyanobacteria play in the ocean ‟hydrocarbon cycle” and reveals the massive scale of this process. The widespread distribution of cyanobacteria and hydrocarbon-degrading bacteria in freshwater, marine, and terrestrial environments suggests the hydrocarbon cycle is pervasive in many natural ecosystems.

—Lea-Smith et al.

However, the hydrocarbons produced by cyanobacteria are continually broken down by other bacteria, keeping the overall concentrations low. When an event such as an oil spill occurs, hydrocarbon-degrading bacteria are known to spring into action, with their numbers rapidly expanding, fueled by the sudden local increase in their primary source of energy.

The researchers caution that their results do not in any way diminish the enormous harm caused by oil spills. Although some microorganisms are known to break down hydrocarbons in oil spills, they cannot repair the damage done to marine life, seabirds and coastal ecosystems.

Oil spills cause widespread damage, but some parts of the marine environment recover faster than others. This cycle is like an insurance policy—the hydrocarbon-producing and hydrocarbon-degrading bacteria exist in equilibrium with each other, and the latter multiply if and when an oil spill happens. However, these bacteria cannot reverse the damage to ecosystems which oil spills cause.

—Dr. David Lea-Smith, lead author

The researchers stress the need to test if their findings are supported by direct measurements on cyanobacteria growing in the oceans. They are also interested in the possibility of harnessing the hydrocarbon production potential of cyanobacteria industrially as a possible source of fuel in the future, although such work is at a very early stage.

Resources

• Lea-Smith, D. et. al. “Contribution of cyanobacterial alkane production to the ocean hydrocarbon cycle.” PNAS doi: 10.1073/pnas.1507274112