Under dark
anoxia, the unicellular green algae Chlamydomonas reinhardtii may produce
hydrogen by means of its
hydrogenase enzymes, in particular HYD1, using
reductants derived from the degradation of intercellular
carbon stores. Other
enzymes belonging to the fermentative pathways compete for the same
reductants. A complete understanding of the mechanisms determining the activation of one pathway rather than another will help us engineer Chlamydomonas for fermentative metabolite production, including
hydrogen. We examined the expression pattern of the fermentative genes PDC3,
LDH1, ADH2, PFL1, and PFR1 in response to day-night cycles, continuous light, continuous darkness, and low or high
oxygen availability, which are all conditions that vary on a regular basis in Chlamydomonas' natural environment. We found that all genes except PFL1 show daily fluctuations in expression, and that PFR1 differentiated itself from the others in that it is clearly responsive to low
oxygen, where as PDC3,
LDH1, and ADH2 are primarily under diurnal regulation. Our results provide evidence that there exist at least three different regulatory mechanisms within the fermentative pathways and suggest that the fermentative pathways are not redundant but rather that availability of a variety of pathways allows for a differential metabolic response to different environmental conditions.