Microbial reduction and deposition of gold nanoparticles was achieved at 25 degrees C over the pH range 2.0-7.0 using the mesophilic bacterium Shewanella algae in the presence of H-2 as the electron donor. The reductive deposition of gold by the resting cells of S. algae was a fast process: 1 mM AuCl4- ions were completely reduced to elemental gold within 30 min. At a solution pH of 7, gold nanoparticles 10-20 nm in size were deposited in the periplasmic space of S. algae cells. At pH 2.8, gold nanoparticles 15-200 nm in size were deposited on the bacterial cells, and the biogenic nanoparticles exhibited a variety of shapes that included nanotriangles: in particular, single crystalline gold nanotriangles 100-200 nm in size were microbially deposited. At a solution pH of 2.0, gold nanoparticles about 20 nm in size were deposited intracellularly, and larger gold particles approximately 350 nm in size were deposited extracellularly. The solution pH was an important factor in controlling the morphology of the biogenic gold particles and the location of gold deposition. Microbial deposition of gold nanoparticles is potentially attractive as an environmentally friendly alternative to conventional methods. (C) 2007 Elsevier Ltd. All rights reserved.