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IDEAS about how sunlight powers life on Earth may be out-of-date. Two groups
of researchers have found new sources of bacterial photosynthesis in the world’s
oceans, which could alter models of global ecology.

“Authors of photosynthesis textbooks will have to eat their words,” says
biophysicist Paul Falkowski of Rutgers University in New Brunswick, New Jersey.
“I know, because I’m one of them.”

Together, the newly found bacteria may make up more than 10 per cent of the
bacteria on the ocean surface. And while the researchers don’t think the
bacteria can “fix” atmospheric carbon dioxide, both groups say the microbes
could have more subtle effects on the global carbon cycle and ecology. “We now
know that there are all these bacteria out there grabbing energy from sunlight,”
says Falkowski. “The question is what are they doing with it?”

Photosynthesis might allow bacteria to grow on less abundant carbon sources,
for example. This could explain the mystery of how so many microbes survive on
the open ocean, an ecosystem that appears to have little food. The cells might
also use their solar energy to fix nitrogen, suggests Falkowski. “There is more
nitrogen fixation going on than we can account for,” he says. “I bet some of
these bacteria are helping.”

The main players in photosynthesis are green plants and cyanobacteria, which
capture energy from sunlight using a pigment called chlorophyll. They use the
energy to split water, trap atmospheric CO₂ and produce oxygen. But
Falkowski’s team was studying a form of photosynthesis that can’t split water’s
strong bonds—one used by microbes called a-proteobacteria.

These bacteria were thought to be confined to small niches such as deep-sea
hydrothermal vents. Last year, Falkowski’s team was using flashes of light to
search for vent bacteria in water sampled from the Pacific by a manned sub.
While waiting for the sub to return, the bored researchers on the support vessel
turned their instruments on the surrounding ocean. To their surprise, they found
the bacteria accounted for 1 to 10 per cent of photosynthesis in surface waters.
Later, they also detected the bacteria in water on the Atlantic coast.

Meanwhile, Ed DeLong of the Monterey Bay Aquarium Research Institute in Moss
Landing, California, and his team were studying bacteria that weren’t even
supposed to be photosynthetic. So little was known about their SAR86
g-proteobacteria that biologists couldn’t even grow them in a lab. “We knew they
are in surface waters around the world, but didn’t have a clue about their
physiology,” says DeLong.

The team cloned big chunks of the bacterium’s DNA to look for interesting
genes. One that coded for a photosynthetic pigment called rhodopsin stood out.
The only microbes thought to possess rhodopsin are Archaea, an entirely
different kingdom of life. Archaea use rhodopsin instead of chlorophyll in a
primitive form of photosynthesis, where sunlight drives a proton pump the cell
uses to make chemical energy.

To see if this was the role of the SAR86 rhodopsin, DeLong’s group put the
gene into the microbe’s lab cousin, Escherichia coli. Sure enough, when
light shone on these cells, they pumped protons out just like photosynthetic
Archaea. “Just months ago, no one would have thought bacteria could do that,”
says DeLong.