Minute cyanobacteria more gluttonous than once thought
Sarah L. Stern
Researchers from Woods Hole Oceanographic Institution in Massachusetts
have conducted a survey of the North Atlantic to determine the population and distribution
of the colonial cyanobacteria (also known as blue-green algae) genus Trichodesmium.
The results show that, because of previous underestimations of their population,
these bacteria may be responsible for much of the nitrogen production that has been
unaccounted for in the Earth’s nitrogen cycle.
The filamentous bacteria live in the ocean’s
tropical and subtropical regions, which cover nearly half of the Earth’s surface
and contain some of the largest known ecosystems. A long-standing dilemma in oceanography
has been the lack of sufficient nitrogen to support the observed rates of photosynthesis.
The video plankton recorder — a submersible microscope —
enabled oceanographers to investigate the amount of cyanobacterial colonies in a
wide swath of the Atlantic Ocean. Courtesy of Cabell S. Davis, Woods Hole Oceanographic
The investigators used a digital microscope
called the video plankton recorder to image the bacterial colonies in situ. They
chose the device, manufactured by Seascan Inc. of Falmouth, Mass., because it works
without disturbing the bacterial colonies. The traditional method of collection
— using plankton nets — can damage or destroy the colonies, leading
to underestimations of their number.
A schematic of the video plankton recorder shows the location of several of its sensors.
Courtesy of Jayne Doucette, Woods Hole Oceanographic Institution.
The microscope was towed by the research
vessel Knorr, which crossed the Atlantic at an average speed of 6 m/s. The device
automatically oscillated vertically from the ocean’s surface to a depth of
130 m at an average velocity of 1 m/s and, when approaching the surface, moved laterally
to avoid sampling in the Knorr’s wake. The ship traversed the Gulf Stream,
seven cyclonic eddies, six anticyclonic eddies and the wake of Hurricane Fabian.
Although nondestructive, this method
left solitary bacterial filaments not residing with the rest of the colony uncounted
because they were indistinguishable from other sea creatures, resulting in an underestimation
of the total population of ~10 percent (known from previous studies using
bottle and diver collections). However, plankton nets yield even more severe underestimation.
The researchers found that Trichodesmium
colonies are more abundant in warmer waters and, for unknown reasons, within anticyclonic
eddies. They also noted that the colonies apparently were not affected by the mixing
produced in the wake of the hurricane and that their concentration in deeper waters
is much higher than previously measured. These deep colonies contribute substantially
to nitrogen fixation.
Images of various Trichodesmium colonies were collected by the video
plankton recorder. Courtesy of Cabell S. Davis.
Trichodesmium has been estimated
to produce between 80 and 110 Tg of new nitrogen annually. However, this study shows
a rate almost three to five times higher. The scientists noted that remote sensing
via satellite could reveal surface but not depth distributions. It is not definitively
known what factors control the colonies’ distribution.
Science, June 9, 2006, pp. 1517-1520.
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