1. Sub-department of Environmental Technology, Wageningen University, Bomenweg 2, 6703 HD Wageningen, The Netherlands, UNESCO-IHE, Environmental Resources, Westvest 7, 2611 AX Delft, The Netherlands, Laboratory of Microbiology, Wageningen University, Dreijenplein 10, 6703 HB Wageningen, The Netherlands, and Department of Microbiology, Radboud University Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands