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Oxygen in the Earth's atmosphere was largely created and is maintained by photosynthesis, in which plants convert sunlight into biological energy through a process that requires chlorophyll. In the oceans, SAR11 is a partner in this process, recycling organic carbon and producing the nutrients needed for the algae that produce about half of the oxygen that enters Earth's atmosphere every day.

The carbon cycle ultimately affects all plant and animal life on Earth.

However, it's now clear that SAR11 has its own mechanism to use sunlight energy that does not involve chlorophyll. Rather, it uses retinal, the same protein used by the eyes of animals and humans to detect light, and serves as a "proton pump" to energize the cell membrane. Proteorhodopsin was only discovered in 2000, but until now, it had not been found in a living organism. It's still not totally clear, Giovannoni said, how this energy producing mechanism benefits the cell.

"When we turned the lights off, there was no mechanism for the proteorhodopsin gene to produce energy, but that didn't seem to make any difference in the growth rate of SAR11," Giovannoni said. "So we know that under normal conditions this alternate form of energy production is not required. This system may be there for emergencies. But it may still be very important to ocean life, and that's what we need to find out more about."