Abstract
However, his genuine lifelong field of research became the phototrophic bacteria. In 1959, stimulated by H. G. Schlegel, he began to attempt the culture of the phototrophic sulfur bacteria, which at that time had not been obtained in pure culture. Within 2 years, he had developed a new culture technique for these fastidious organisms, which quickly became known worldwide as Pfennig's technique or Pfennig's medium. His further scientific path was decisively set by a sabbatical year with Cornelis B. van Niel at Pacific Grove, California. Pfennig returned to Göttingen with the aim of investigating the physiology and ecology of the bacteria that are involved in the biological sulfur cycle in the sense of van Niel from a holistic viewpoint, which naturally included their systematics. He himself described his motivations thoroughly in his autobiography [Annu Rev Microbiol 47 (1993), 1–31]. He also returned to Göttingen as a new man; he had become an open-minded, merry, stimulating partner, discussing everything. The time in California in the laboratories of van Niel and of Roger Stanier and Germaine Cohen-Bazire in Berkeley, where he was involved in the discovery of the chlorosomes, had made him find his own qualities as microbiologist, researcher and teacher. He brought back with him the open-minded American team spirit and so, in the following years in Göttingen, we brought into pure culture hundreds of strains of purple and green sulfur bacteria (phototrophs) and studied their specific growth requirements with respect to nutrients and light. Norbert Pfennig thus succeeded in isolating almost all phototrophic sulfur bacteria that had been observed in natural environments and described (from nature) since 1832 and in discovering many unknown species. Soon the non-sulfur purple bacteria were added, and there also he discovered new species.
It is therefore not surprising that Pfennig and his co-workers became known worldwide as experts on the taxonomy of phototrophic bacteria and were asked to write the respective chapters in Bergey's Manual, The Prokaryotes, etc. Well-known scientists came to his lab (amongst others, Ralph Wolfe, Marvin Bryant, Jerald Ensign and Hans van Gemerden) or sent cultures to be identified. Pfennig studied the syntrophisms of the green bacteria and, with Hanno Biebl, discovered the sulfur reducers (Desulfuromonas, etc.); they showed a completely new type of metabolism, coupling the anaerobic reduction of sulfur to the complete oxidation of ethanol or acetate. These organisms were excellently suited to the construction of syntrophic communities with phototrophic bacteria. Several guest professorships followed (in the USA and Turkey, for example). Norbert Pfennig and his doctoral student Friedrich Widdel discovered the enormous breadth of substrate utilization of the sulfate-reducing bacteria, thus proving the extraordinary importance of this metabolic group in nature, something that had long been postulated previously. Many of his publications were of relevance to systematics and so it is not surprising that the Board of Trustees of the Bergey's Manual Trust elected Norbert Pfennig as a member in 1974 where, for many years, he influenced the development of bacterial systematics.