Changes in hydrogen production and polymer accumulation upon sulfur-deprivation in purple photosynthetic bacteria

The work investigated physiological conditions directing cellular metabolism toward either H 2-production or storage polymer accumulation in purple photosynthetic bacteria. Hydrogen-producing cultures of the purple anoxygenic photosynthetic bacterium Rhodospirillum rubrum were resuspended in media lacking sulfur (S) nutrients. S-deprived cultures displayed lack of growth, cessation of bacteriochlorophyll and protein accumulation, and inhibition of H 2 evolution. Cell volume increased substantially and large amounts of polymer were found to accumulate extracellularly. Poly-β-hydroxybutyrate (PHB) content increased about 3.5-fold within 24 h of S-deprivation. Most cells remained viable after 100 h of S-deprivation and cultures were capable of resuming growth and H 2-production when supplemented with sulfate. Transcript levels, protein amount, and activity of the nitrogenase enzyme, which are responsible for H 2-production, decreased with a halftime of about 15 h upon S-deprivation. In addition, the nitrogenase NifH subunits were modified by ADP-ribosylation, indicating post-translational inactivation. Comparative aconitase activity measurements of control and S-deprived cells failed to indicate a general stress to Fe–S proteins, as aconitase, a Fe–S protein in the citric acid cycle sensitive to oxidative stress, maintained activity throughout the course of the S-deprivation. In contrast to nifH transcriptional down-regulation, expression of cysK (encoding cysteine synthase) was upregulated in response to S-deprivation. The described physiology is not specific to R. rubrum, as Rhodobacter sphaeroides and Rhodopseudomonas palustris exhibited a similar response to S-deprivation. It is suggested that manipulation of the supply of S-nutrients may serve as a tool for the alternative production of H 2 or PHB in purple photosynthetic bacteria, thus affording opportunities to design photobiological systems that serve in both energy conversion and storage processes.