When the going gets tough, the tough may feed on light. New research indicates that the most abundant bacteria in the ocean possess a gene that could let them harness light to generate energy in the absence of food. Such light-harvesting microbes could answer a long-standing puzzle about how some bacteria thrive in nutrient-poor ocean "deserts."
Bacteria in the oceans generally get energy by munching on organic carbon. However, some bacteria mysteriously thrive in nutrient-poor parts of the ocean, where little food is available. Or is it? In 2000, a team of scientists led by marine microbiologist Edward DeLong, now at the Massachusetts Institute of Technology, isolated a gene belonging to a widespread group of marine bacteria called SAR86 (Science, 15 September 2000, p. 1869). The gene encodes a type of protein called a proteorhodopsin, which uses light energy to pump ions out of the cell. That creates an ion gradient, which generates energy for the cell as the protons flow back in.
Now researchers have found a proteorhodopsin in another common marine bacterium. A team led by marine microbiologist Stephen Giovannoni of Oregon State University in Corvallis made the surprising discovery while sequencing the genome of SAR11, a microbe that makes up about 25% of the cells in the ocean. To see whether this proteorhodopsin also allows SAR11 to convert light to energy, the team cultivated SAR11 cells in both light and darkness, determined which cells were expressing the gene, and measured their growth rates.
The findings were perplexing. All of the SAR11 cells--whether grown with the lights on or off--expressed the proteorhodopsin gene, and the researchers noted structural changes in the proteins that suggested they were actively pumping protons. But there were no differences in the cell growth rates between light and dark, the researchers report 3 November in Nature. That suggests the gene may only come into play when it's really needed, such as in nutrient-deprived parts of the ocean such as the Sargasso Sea--where SAR11 is known to account for as much as 50% of all cells, Giovannoni says.
The current study is "a big step forward," says DeLong. "It's illuminating to find [a proteorhodopsin] in one of the most ubiquitous bacteria in the ocean's surface." And now there's growing evidence that photoproteins may turn up in additional marine microbes, he adds. "All notions about ... its potential importance seem to be bearing out."