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David Bradley ISSUE #13
March 2001

Oceanic chemical weapons

Georg Pohnert   
Georg Pohnert
Plankton are right at the bottom of the marine food chain, make no mistake, but it is not just whales and fish they have to worry about. They also eat each other.

Zooplankton, the tiniest of animals, include the protozoa, minute crustaceans and larvae, feed voraciously on their photosynthesising cousins the ubiquitous phytoplankton. Georg Pohnert at the Max Planck Institute of Chemical Ecology in Jena, Germany, has discovered that phytoplankton can hit back with chemical weapons when attacked.

The diatom Thalassiosira rotula - a type of single-celled algae one of the many types of phytoplankton - was already known to produce the toxic aldehydes decadienal and decatrienal. The compounds seem to prevent the eggs of copepods, tiny crustaceans of which there are more than 14,000 known species, from hatching successfully. Copepods while finding algae rather tasty and nutritious have little success grazing on it because of the presence of these aldehydes.

    Caulerpa taxifolia
Pohnert has now discovered that such chemical defences are more widespread than previously thought. Diatoms, such as Thalassiosira, are small, single-celled algae belonging to one special group with silica cell walls, he explains, they are among the most important carbon dioxide fixing plants of all. He and his colleagues have now found that several other algae produce, not only decadienal and decatrienal, but also a whole range of related chemical weapons. These compounds block the onslaught of another tiny crustacean, the amphipodae, as well as halting fungal attack.

Pohnert reported in Angewandte Chemie (vol. 39, 4352) that these substances are only produced, and then very rapidly, when the diatom is being attacked. Pohnert used deuteration techniques to figure out which nutrients are being used to make the chemical weapons. It turns out that long chain-like fatty acid molecules provide the starting materials for the diatom and that there is a chemical trigger within the zooplankton that switches on the diatoms' enzymes for manufacturing the weapons. Zooplankton, in other words, are their own worst enemy.

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1,6,10-trien-8-ynyl acetate
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"Research into the chemical defence mechanisms of plankton is still in its infancy, nevertheless, it is important in order for us to gain a better understanding of the complex processes in marine habitats," he explains. For instance, understanding how zooplankton interact with water-borne, disease-causing microbes such as those that lead to dengue fever, has been used as a means to control the microbes.

Research into the chemical weaponry of other species too could have practical ecological applications. For instance, Pohnert's team is studying the tropical green algae Caulerpa taxifolia. This alga was accidentally introduced into the Mediterranean Sea several years ago and is rapidly colonizing the waters. Figuring out what makes it so successful might lead to ways to eradicate it from such non-native habitats. "At this stage of the project we are trying to understand the defence of Caulerpa taxifolia from a mechanistic perspective," Pohnert explains, "this is done with a focus on the interaction of Caulerpa with its predators such as sea slugs." He suggests that by identifying the way the ecosystem works at the molecular level one could look for predators of the algae that might be used to control it.