“It has long been known that cyanobacteria can use sunlight to carry out photosynthesis,” says Jarone Pinhassi, a researcher in aquatic ecology at Linnaeus University in Kalmar. But there was previously no evidence that other bacteria, those with the photo pigment proteorhodopsin, can exploit sunlight to improve their survival.
The light-sensitive pigment proteorhodopsin was discovered in 2000 by American scientists who were studying the genes in marine bacteria. Among animals, proteorhodopsin is found in the retina, and it enables humans to see in the dark. In microorganisms, the function can vary, but it was long unclear how marina bacteria use proteorhodopsin and what they might do with the energy they capture from sunlight.
Now, a decade later, the current study presents the first direct evidence that proteorhodopsin is useful for marine bacteria. This is shown through mutation studies in a marine bacterium closely related to the species Vibrio cholerae, the bacterium that causes the disease cholera. The study shows that the photo protein supplies the bacterium with energy to survive “hard times” in anticipation of better conditions for growth. This is of great importance in the sea since the nutrition supply often varies a great deal.
There are roughly a billion bacteria per liter of sea water, and these bacteria’s activities, breaking down organic material and growing, are central for an understanding of the ocean’s turnover of carbon dioxide and the cycle of nitrogen and phosphorous. The fact that nearly half of all marine bacteria in the oceans also have proteorhodopsin shows how widespread this photo protein is, and what potential it might have to affect the survival of bacteria.
“There is an enormous capacity for bacteria to take advantage of molecular innovations that benefit them,” says Jarone Pinhassi. “And the fact that it’s good to exploit sunlight hardly surprises anyone.”