Dilution methods to deprive Chlamydomonas reinhardtii cultures of sulfur for subsequent hydrogen photoproduction

Abstract

Sulfur deprivation of Chlamydomonas reinhardtii cultures gradually inactivates photosynthetic O 2 evolution and leads to the establishment of anaerobiosis in the medium. Sulfur-deprived algal cultures kept under anaerobic conditions will then produce H 2 gas for 3–5 days under continuous illumination. Currently, sulfur deprivation is achieved by mechanical centrifugation of cultures grown in sulfur-replete medium, followed by extensive and costly washing. The cells are finally resuspended in sulfur-free medium. The current study investigates two procedures to deprive algal cultures of sulfur that eliminate the centrifugation step. These procedures involve sulfur deprivation by dilution of sulfur-replete cultures into either sulfur-limited medium or sulfur-free medium. We demonstrate that efficient H 2 photoproduction can be achieved on a timely basis using either procedure. However, the dilution of sulfate-replete algal cultures 1 :10 v/ v into sulfur-free medium is the most appropriate procedure. These observations serve as the basis for developing an algal H 2-production system that is cheaper, less time-consuming, and less amenable to contamination with other microorganisms than systems employing centrifugation for sulfur deprivation.