Is this the ref. you found ??
Physolic acid is mentioned in,
Earth-Science Reviews
Volume 45, Issues 1–2, November 1998, Pages 45–60
Geomicrobiology: its significance for geology
Henry L. Ehrlich
Department of Biology, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, USA
Suggest that your library gets this journal issue, or maybe contact Prof. Ehrlich directly.