Targeting the development of cell-based bioreactors for the production of metal nanoparticles, the possibility to perform the sustained synthesis of colloidal gold using Klebsormidium flaccidum green algae was studied. A first strategy relying on successive growth/reduction/reseeding recycling steps demonstrated maintained biosynthesis capability of the microalgae but limitation in metal content due to toxic effects. An alternative approach consisting of progressive gold salt addition revealed to be suitable to favor cell adaptation to larger metal concentrations and supported particle release over month periods. Biotechnol. Bioeng. 2012;109: 284–288. © 2011 Wiley Periodicals, Inc.