Saturday, 6 December 2008


Finally we have a name for the brown gunge that has been visible in broad streaks at the ocean surface since entering the first cyclonic eddy, and which has been clogging our plankton nets and slowing down the filtering of our numerous water samples - Trichodesmium. In our samples it appears as masses of loose fibres and fuzzy clumps. It occurs in nutrient poor tropical and subtropical ocean waters, and is an important marine nitrogen-fixing bacterium that is being studied extensively for its role in nutrient cycling in the ocean. It forms blooms and provides a substrate for many small oceanic organisms, such as other bacteria, diatoms, dinoflagellates, protozoa, and copepods. The species Trichodesmium erythraeum is endemic to the Indian Ocean.

Trichodesmium is commonly called "sea saw-dust" because its colonies and large brown blooms have been mistaken as sandbars by ships in the ocean (including Captain Cook who wrote the first documentation of Trichodesmium over 200 years ago). These photosynthetic cyanobacteria can be found as filaments (trichomae) comprised of 10's-100's of cells or in colonies 1-10 mm in length. The fact that these colonies can be seen by the naked eye is what gave Trichodesmium its name - the Greek word "trichoma" for hair and "desmus" for bonded = "bonded-hair," which is how Trichodesmium colonies look to the human eye. The colonies can be yellowish-brown to deep red in color due to their primary light harvesting pigment, phycoerythrin. They are buoyant and able to regulate their position in the water column due to large gas-filled vacuoles or vesicles in each individual cell. Trichodesmium blooms are surface aggregations that can be 10-1000's of km wide. They occur during periods of low wind stress and warm temperatures. Some of these blooms are so vast that they are visible from space. However, the bacteria on the surface do not generally survive for extended periods of time for various reasons, including UV damage. [Source: Wikipedia]

Figures: (a) Streaks of Trichodesmium seen from the Nansen; (b) a Trichodesmium bloom visible from space (from AIMS Research.); (c) thick Trichodesmium “gunge” from the bongo net haul; (d) Trichodesmium sp. as seen under a microscope.

Written by: Jenny Huggett (DEAT: Marine & Coastal Management, South Africa)

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