Photosynthesis: Take It or Leave ItOctober 17, 2001 / Posted by: Shige Abe
Cindy L. VanDover, a biologist and hydrothermal vent expert at the College of William and Mary in Williamsburg, VA. wondered if photosynthetic bacteria might live near hydrothermal vents. This was a striking speculation, considering that such vent systems lie thousands of feet deep in the ocean, well below the depth to which sunlight penetrates. The ecosystems surrounding such vents survive because of bacteria that garner energy from hydrogen sulfide, not from light.
But water emerges from hydrothermal vents at hundreds of degrees, kept from boiling only by the intense pressure. The hot water, and perhaps hot rocks, VanDover speculated, would emit infrared radiation. And some bacteria might absorb the radiation just the way algae and land plants absorb sunlight.
At a meeting, she presented the idea to Paul G. Falkowski and Zbigniew S. Kolber, oceanographers at Rutgers University in New Brunswick, NJ. Falkowski and members of his laboratory had developed a successful method of detecting photosynthesis from changes in the fluorescence of chlorophyll in the photosynthetic apparatus of marine phytoplankton. The team hits the cells with pulses of light, effectively charging up the chlorophyll. After it’s absorbed enough pulses, chlorophyll reemits the light as fluorescence. Different types of chlorophyll fluoresce at different wavelengths. Falkowski and Kolber call the patented device a Fast Rate Repetition (FRR) fluorometer.
Intrigued, Falkowski and Kolber tuned their detector to the infrared spectrum. “If there are photosynthetic bacteria in vents,” Falkowski says, “they’re probably going to be fluorescing in the infrared.”
After checking and calibrating the detector in the lab on known bacteria, they headed to sea with the deep-sea submersible Alvin.
The Reality Check
“We went out to sea, and 18 dives later, on Alvin, it was very clear that there were no detectable photosynthetic bacteria in deep-sea vents,” Falkowski says.
“But seeing as you’re sitting around in the ocean for days on end, what we did was to just look around at water samples from the surface,” Falkowski continues. “And lo and behold, in the upper ocean, there are tons of photosynthetic bacteria that had never been seen before because nobody had made a detector out in the infrared. Nobody looked there.” On ten later dives in Alvin, Falkowski and Kolber found a photosynthetic bacterial count of nearly zero throughout most of the water column, except in the surface waters, Falkowski says. “And then what you see is that ten percent of the bacteria, of all bacteria, in the upper ocean are actually these photosynthetic bacteria.” The team’s research appeared in the June 29, 2001, issue of the journal Science.
Photosynthetic microbes in the open ocean have been well known for decades. But marine biologists assumed these were mostly cyanobacteria, the organisms that make the bubbly greenish slime on the edges of stagnant ponds. Cyanobacteria contain garden-variety chlorophyll, similar to that of plants. And like plants, they make all of their own food, absorbing carbon dioxide and producing oxygen – hence the bubbles. These are autotrophs.
Other bacteria in the surface waters appeared to live by absorbing food molecules (organic carbon) produced by the photsynthesizers. That is, they were heterotrophs.
But the new bacteria were not cyanobacteria. Instead of chlorophyll, they used bacteriochlorophyll, which fluoresces in the infrared. “So you could easily distinguish between these and the normal phytoplankton in the background,” Falkowski says.
Although they do absorb atmospheric carbon dioxide and form food molecules using the sun’s energy, they don’t produce oxygen like most photosynthetic organisms do. An even bigger surprise came when Falkowski’s team grew these bacteria in the lab. It turns out they can turn photosynthesis on and off. They don’t make all of their own food. They just use photosynthesis to supplement their normal bacterial diet of dissolved organic molecules.
“It’s a very strange metabolism,” Falkowski says. “In effect, they have a very strong competitive advantage over the normal heterotrophic bacteria, in that their growth rate can be very, very high because they’re using light energy to help them refix carbon…. They are obligate photoheterotrophs. In other words, they cannot live without organic carbon, as far as we know.” Falkowski and Kolber call the bacteria AAPs, aerobic anoxygenic photoheterotrophs.
The Other Surprise
In an independent discovery coincidentally also published in June, 2001, Edward F. DeLong, Oded Beja and colleagues at the Monterey Bay Aquarium Research Institute and the University of Texas Medical School in Houston discovered an entirely different group of bacteria that also make a living by absorption of light. Instead of culturing the bacteria in the lab, they used a technique called ecogenomics to search for genes in seawater samples containing many species.
The Carbon Cycle
Neither research group claims that the new bacteria will change the overall view of the carbon flow between the atmosphere and the ocean. Both kinds of bacteria would have shown up in experiments using radioactively labeled carbon to detect biomass in surface waters, Falkowski says. “But they would have been misassigned to phytoplankton…. They are obviously part of a cycle, part of a heretofore unassessed bacterial carbon-fixation mechanism.” And they’re a big part, in numbers at least.
Each research group estimates that its bacteria make up some 10 percent of the total bacterial population in the open ocean, DeLong points out. “If we take the higher numbers, that’s 20 percent of the total number of cells in marine surface waters.”
Even at 15 to 20 percent of the total bacterial population, Falkowski says, the numbers of cells is truly astronomical. “We found them in every water body we have sampled, from the Southern Ocean to the tropics,” he says. “There are approximately 10^24 [- that’s a 1 with 24 zeros after it -] bacterial cells in the ocean, which is about two orders of magnitude more than there are stars in the known universe.”
“We’ve known for a long time that microorganisms, particularly bacteria, are really quite abundant in seawater, but what we really haven’t known is the identity of those microorganisms and what their functions are out in the environment,” DeLong says.
DeLong hopes to apply his genomics techniques in further research, he says. “Taking it back to the environment to understand the dynamics of these organisms in their natural environment is kind of a next step. One question in my mind is, ‘How much more can we learn – how many more surprises are there – by applying some of these new genomic technologies?’”
Falkowski and his colleagues are also looking at the ecology of the new bacteria. He hopes to find where, exactly, the bacteria fit into the open-ocean ecosystem. Furthermore, he hopes to nail down some outstanding questions of how their odd metabolism works – Can they use atmospheric nitrogen like some soil bacteria closely associated with the roots of certain land plants do? – and more about how they absorb carbon. Finally, Falkowski wants to know how many species of AAPs populate the ocean, and how diverse their genomes are.
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