Oceanus , 27 July 2006
Towing an underwater video microscope across the Atlantic Ocean, two
scientists found unexpected abundances of colonial cyanobacteria that fertilize
the oceans with nitrogen. The bacteria may turn out to be “a crucial component”
that stimulates the growth of plants and animals in vast ocean regions that
might otherwise be nearly barren, they reported in the June 9, 2006, issue of
the journal Science.
Just like plants on land, microscopic marine plants at the base of the food
chain require nitrogen to grow. But the source of nitrogen, especially in the
open ocean far from coastal waters fed by runoff from rivers and land,
“represents one of the most intriguing conundrums in oceanography,” Zbigniew
Kolber, a research engineer at Monterey Bay Aquarium Research Institute, wrote
in Science. Two scientists at Woods Hole
Oceanographic Institution (WHOI) “decided to have another ‘look’ at this
problem, literally,” he wrote.
Cabell Davis of the Biology Department and Dennis McGillicuddy of the Applied
Ocean Physics and Engineering Department towed a device called the Video
Plankton Recorder (VPR) behind the WHOI-operated research vessel
Knorr from the Azores almost back to
Woods Hole. It remained in the water 24 hours a day, taking digital photographs
30 times per seconds as it undulated from the surface to depths of 425 feet (130
meters).
“It was the VPR’s longest tow, more than 5,500 kilometers (nearly 3,000 nautical
miles), but slow—like driving a lawnmower across country at 12 knots,” Davis
said.
The VPR images showed unexpected widespread populations of
Trichodesmium, bacteria that were
formerly (and inaccrately) known as blue-green algae. They form tiny colonies of
golden-brown spiky balls (called “puffs”) or reddish rafts (called “tufts”). The
fragile colonies, 1 to 3 millimeters in diameter, are visible to the eye, but
are not easy to collect or count because they tend to break apart when handled.
The VPR gave scientists the ability to look observe and count
Trichodesmium in a non-invasive way.
Davis and McGillicuddy found that Trichodesmium
live not only at the surface, as scientists previously thought, but throughout
the region where light penetrates the ocean. They estimated that
Trichodesmium were two to three times
more abundant than previously believed.
Trichodesmium thrive in waters that are low in nutrients because of their
ability to use molecular nitrogen (N2) found in air and water. They incorporate
it into nitrogenous compounds that other organisms can use. (Plants that do
this, such as alfalfa, are used as “green fertilizer” to add useable nitrogen to
soil.)
“If traditional sampling has underestimated colonies in other regions of the
world, estimates of global Trichodesmium
abundance will increase dramatically, changing our perception of the importance
of this organism to the productivity of the world ocean,” Davis said.