No Light, No Germination: Excitation of the Rhodospirillum centenum Photosynthetic Apparatus Is Necessary and Sufficient for Cyst Germination

Rhodospirillum centenum is a Gram-negative alphaproteobacterium that is capable of differentiating into dormant cysts that are metabolically inactive and desiccation resistant. Like spores synthesized by many Gram-positive species, dormant R. centenum cysts germinate in response to an environmental signal, indicating that conditions favor survival and proliferation. Factors that induce germination are called germinants and are often both niche and species specific. In this study, we have identified photosynthesis as a niche-specific germinant for R. centenum cyst germination. Specifically, excitation of wild-type cysts suspended in a nutrient-free buffer with far-red light at >750 nm results in rapid germination. This is in stark contrast to mutant strains deficient in photosynthesis that fail to germinate upon exposure to far-red light under all assayed conditions. We also show that photosynthesis-induced germination occurs in a carbon- and nitrogen-free buffer even in strains that are deficient in carbon or nitrogen fixation. These results demonstrate that photosynthesis not only is necessary for germination but is itself sufficient for the germination of R. centenum cysts.