Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2000, 39 (23), 4352 - 4354
Chemical Cudgel in Plants
of single-celled algae
Plankton are at the bottom of the food chain in all natural bodies of water and are thus correspondingly important for their ecological balance. In the cycle of eating and being eaten it isn’t only fish and whales that prey on these tiny animals and plants - they also eat each other. For example, a variety of plant-eating miniature animals like to feast on single-celled phytoplankton. Their prey knows how to defend itself, however. Their defensive strategy has now been more thoroughly investigated by Jena researcher Georg Pohnert.
It has been known for some time that two compounds classified as aldehydes are used as chemical weapons by certain diatoms. They inhibit the propagation of copepods, which eat the algae. Pohnert has now discovered a substance that is closely structurally related in another type of diatom. It apparently keeps amphipodae from finishing off the diatom and fights against parasitic fungi.
Interestingly, none of the defensive substances can be detected in the diatoms when these are intact. If the algae are wounded, however, they pour out a considerable quantity of the aldehydes within a few seconds. Where do this chemical cudgel come from so suddenly? Pohnert "fed" the algae various substances that were labeled with deuterium, an isotope of hydrogen. This allowed him to determine that the algae make their weapons out of fatty acids that contain a chain of 20 carbon atoms. In cases of acute need, these C20 fatty acids are enzymatically transformed into the aldehydic weapons of defense.
This dynamic defense strategy is clever. It saves the diatoms from always having an arsenal of "loaded weapons" at the ready. The danger of self-induced poisoning is thus minimized if a harmless precursor of the defensive substance is stored. Additionally, this tactic saves energy, since the production of the aldehydes is a costly undertaking for the algae. If enemies come onto the scene, a very high local concentration of the chemical weapons can be produced lightning fast.
"Research into the chemical defense mechanisms of plankton is still in its infancy. It is important in order for us to gain a better understanding of the complex processes in this habitat," explains Pohnert.