David J. Lea-Smith, Steven J. Biller, Matthew P. Davey, Charles A. R. Cotton, Blanca M. Perez Sepulveda, Alexandra V. Turchyn, David J. Scanlan, Alison G. Smith, Sallie W. Chisholm, and Christopher J. Howe (2015) Contribution of cyanobacterial alkane production to the ocean hydrocarbon cycle, PNAS 112 (44) 13591-13596, doi:10.1073/pnas.1507274112
Background: oil spill first responders
Oil spills are devastating for marine life, but fortunately nature has a built-in disaster squad: oil-eating bacteria show up soon after spills and degrade the mess fairly quickly, sparing us the worst effects. How this happens is a bit of a mystery. Oil is difficult to digest even for bacteria, and most organisms with the enzymes necessary for eating oil can’t survive with any other food source. Yet whenever there’s a spill in parts of the ocean with no known sources of oil pollution (for example, the Exxon Valdez spill off the coast of Alaska in 1989), the oil degrading community is on the scene quickly. They appear to be present wherever we look for them, but how do they survive in the absence of their only food source?
This study shows that cyanobacteria, the smallest and most abundant class of photosynthetic algae produce significant amounts of alkanes, a chemical class comprising about 50% of crude oil. David Lea-Smith, and a group of researchers from University of Cambridge and MIT, hypothesized that these cyanobateria produce enough of the chemical to support an oil-consuming bacterial community around the world. This process cultivates a natural set of first-responders whenever a disastrous oil spill occurs.
Oil production by cyanobacteria
First, the authors analyzed the genomes of Prochlorococcus and Synechococcus, the two most abundant types of cyanobacteria and showed that they have the enzymatic machinery to produce alkanes. Next, they analyzed the alkane content of the cyanobacteria themselves. The amount of alkane per organism was incredibly tiny, less than a quarter of 1% of each cell’s weigh, and since the bacteria themselves are so small, that works out to less than a femtogram of alkanes per cell—that’s 10-15, or a millionth of a billionth of a gram! But because cyanobacteria are so abundant and the ocean is so large, this corresponds to the production of as much as 500 million tons of alkanes per year across the world’s oceans. For comparison, the Deepwater Horizon oil spill — the largest and most devastating marine oil spill in history — released about half a million tons of crude oil into the Gulf of Mexico.
Finally, the authors performed a series of incubation experiments with oil-degrading bacteria. They added either crude oil or the alkanes produced by cyanobacteria to a culture of the bacteria, while incubating other cultures without oil as a control. The bacteria in the control group without added oils died quickly, but the crude oil and alkane cultures thrived, demonstrating that they need a constant source of oil to survive (Figure 1). Cyanobacteria are the most likely source for that oil in non-spill conditions, and a back-of-the-envelope calculation based on the results of the experiments shows that the size of the microbial population that could be supported by cyanobacterial oil production matches the size of the oil-degrading community found in unpolluted natural waters.
Significance and implications
This study helps explain an apparent paradox: oil degraders need a constant supply of oil to survive, but always
seem present and ready to go when a spill does occur, even in areas with a low baseline of oil contamination. Cyanobacteria appear to provide a vital service in bridging the gap by producing enough alkanes to keep oil-degrading microbes from starving, and able to quickly break down oil spills no matter where they occur (Figure 2).While oil spills are still devastating to marine ecosystems, this system limits the persistence of one of the largest components of oil. It’s an accident of nature that makes the ocean far more resilient to catastrophes like oil spills than we might expect.
I recently completed a PhD in Marine Science at the University of South Carolina and am now a postdoc at Memorial University of Newfoundland. I research the effects of climate change on soil organic matter in boreal forests and peatlands. I spend my free time picking berries and exploring “The Rock” (Newfoundland).