title: β-cyclocitral, a grazer defence signal unique to the cyanobacterium Microcystis.
abstract: β-Cyclocitral is often present in eutrophic waters and is a well known source of airborne and drinking water malodor, but its production and functional ecology are unresolved. This volatile organic compound (VOC) is derived from the catalytic breakdown of β-carotene, and evidence indicates that it is produced by the activation of a specific carotene oxygenase by all species of the bloom-forming cyanobacterium Microcystis. Previous work has shown that β-cyclocitral affects grazer behavior, but the nature of this interaction and its influence on predator-prey dynamics was unresolved. The present study combined analytical and behavioral studies to evaluate this interaction by using Microcystis NRC-1 and Daphnia magna. Results showed that β-cyclocitral was undetectable in live Microcystis cells, or present only at extremely low concentrations (2.6 amol /cell). In contrast, cell rupture activated a rapid carotene oxygenase reaction, which produced high amounts (77 ± 5.5 amol β-cyclocitral/cell), corresponding to a calculated maximum intracellular concentration of 2.2 mM. The behavioral response of Daphnia magna to β-cyclocitral was evaluated in a bbe© Daphnia toximeter, where β-cyclocitral treatments induced a marked increase in swimming velocity. Acclimation took place within a few minutes, when Daphnia returned to normal swimming velocity while still exposed to β-cyclocitral. The minimum VOC concentration (odor threshold) that elicited a significant grazer response was 750 nM β-cyclocitral, some 2,900 times lower than the per capita yield of a growing Microcystis cell after activation. Under natural conditions, initial grazer-related or other mode of cell rupture would lead to the development of a robust β-cyclocitral microzone around Microcystis colonies, thus acting as both a powerful repellent and signal of poor quality food to grazers.
authors.affiliation: University of Zürich, Limnological Station, Kilchberg, Switzerland. juttner@limnol.uzh.ch
authors.forename: Friedrich
authors.initials: F
authors.lastname: Jüttner
authors.forename: Susan B
authors.initials: SB
authors.lastname: Watson
authors.forename: Eric
authors.initials: E
authors.lastname: von Elert
authors.forename: Oliver
authors.initials: O
authors.lastname: Köster
mesh_terms: D000447:Aldehydes / Q000032:analysis / Q000494:pharmacology; D000818:Animals; D002338:Carotenoids / Q000032:analysis; D003621:Daphnia / Q000187:drug effects* / Q000502:physiology; D004224:Diterpenes / Q000494:pharmacology; D004924:Escape Reaction; D005247:Feeding Behavior / Q000187:drug effects; D046931:Microcystis / Q000378:metabolism*; D009812:Odorants / Q000032:analysis; D013550:Swimming; D014801:Vitamin A / Q000031:analogs & derivatives; D050559:beta-Carotene 15,15'-Monooxygenase / Q000378:metabolism
chemical_list: D000447:Aldehydes; D004224:Diterpenes; C487773:trans-sodium crocetinate; D014801:Vitamin A; D002338:Carotenoids; C516118:beta-cyclocitral; D050559:beta-Carotene 15,15'-Monooxygenase