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Minute cyanobacteria more gluttonous than once thought

Aug 2006
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 Institution.

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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