1. Department of Microbial Ecology; Centre for Life Sciences; University of Groningen; P.O. BOX 11103; 9700 CC; Groningen; The Netherlands
2. National Institute of Public Health and the Environment; SEC/GMO Office; P.O. Box 1; 3720 BA; Bilthoven; The Netherlands
3. Naturalis Biodiversity Center; P.O. Box 9517; 2300 RA; Leiden; The Netherlands
4. Resource Ecology Group; Droevendaalsesteeg 3a; 6708 PB; Wageningen UR; The Netherlands