0939 – 5075/2009/0100 – 0049 $ 06.00 © 2009 Verlag der Zeitschrift für Naturforschung, Tübingen · http://www.znaturforsch.com · D Introduction Geosmin, a sesquiterpene tertiary alcohol that has lost an isopropyl group, is a major threat of water industry worldwide. Its extremely low threshold odour concentration of 15 ng/l (= 82 pM) (Persson, 1980) and the ease to recognize the muddy earthy odour are the primary causes of consumer complaints against drinking water (Bru- chet, 1999). Producers of geosmin in the source water are planktonic and benthic cyanobacteria and in the distribution pipes microorganisms of unknown taxonomic position. In all so far investi- gated geosmin-producing cyanobacteria, geosmin was the major sesquiterpene accompanied by an array of sesquiterpene hydrocarbons and alcohols. But very few of these sesquiterpenes which may also contribute to the musty odour bouquet of cyanobacteria have been identified. Germacrene D and Ȗ-cadinene were found as minor compo- nents among nine unidentified sesquiterpenes in Oscillatoria splendida (Tsuchiya et al., 1981). In addition, several mass spectra were published, but could not be attributed to any of the known sesquiterpenes (Tsuchiya and Matsumoto, 1988). The structure determination from mass spectro- metric signals is unreliable because the epimers described for higher plants must not also be typi- cal products of lower plants. In general epimers show only minor differences in their EI mass spec- tra, and reference compounds that would allow identification by retention time analysis are not available. This is the reason why the structure elu- cidation of sesquiterpenes is severely hampered. The isolation of cyanobacterial biomass to ob- tain sufficient material for NMR studies is rather time-consuming, and essential oils in measurable quantities have not yet been obtained for any of these geosmin-producing cyanobacteria. Here we describe sesquiterpenes of Calothrix which have not yet been described for cyanobacteria. Since the biogenesis of sesquiterpenes may be similar in actinomycetes, these data will also be a contri- bution to the knowledge of sesquiterpenes in this group of microorganisms. Sesquiterpenes of the Geosmin-Producing Cyanobacterium Calothrix PCC 7507 and their Toxicity to Invertebrates Claudia Höckelmann a , Paul G. Becher a,b , Stephan H. von Reuß c , and Friedrich Jüttner a, * a Limnological Station, Institute of Plant Biology, University of Zürich, Seestrasse 187, CH-8802 Kilchberg, Switzerland. E-mail: juttner@limnol.uzh.ch b Present address: Chemical Ecology Group, Swedish University of Agricultural Sciences (SLU), Box 102, 23053 Alnarp, Sweden c University of Hamburg, Institute of Organic Chemistry, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany * Author for correspondence and reprint requests Z. Naturforsch. 64 c, 49 – 55 (2009); received July 29/October 5, 2008 The occurrence of sesquiterpenes was investigated with the geosmin-producing cyanobac- terium Calothrix PCC 7507. The essential oil obtained by vacuum destillation was studied in more detail by GC-MS methods and superposition with authentic compounds. Geosmin was the dominating compound while the other sesquiterpenes were minor components. Sesqui- terpenes that have not been described before in cyanobacteria were isodihydroagarofuran, eremophilone and 6,11-epoxyisodaucane. Closed-loop stripping analysis revealed that most of the sesquiterpenes were found in the biomass of Calothrix, while eremophilone was main- ly observed in the medium of the axenic culture. Eremophilone showed acute toxicity (LC 50 ) against Chironomus riparius (insecta) at 29 μM and against Thamnocephalus platyurus (crus- tacea) at 22 μM. The compound was not toxic for Plectus cirratus (nematoda). 6,11-Epoxyiso- daucane and isodihydroagarofuran exhibited no toxicity to invertebrates when applied in concentrations up to 100 μM. Key words: Sesquiterpenes, Isodihydroagarofuran, Cyanobacterium, Insecticide