Chemical triggering cyanobacterial glycogen accumulation: methyl viologen treatment increased Synechocystis sp. PCC 6803 glycogen storage via enhancing levels of gene transcript and substrates in glycogen synthesis.

Two-stage cultivation is effective for glycogen production by cyanobacteria. Cells were firstly grown under adequate nitrate supply (BG11) to increase biomass, and subsequently transferred to nitrogen deprivation (-N) to stimulate glycogen accumulation. However, the two-stage method is time consuming and requires extensive energy. Thus, one-stage cultivation that enables both cell growth and glycogen accumulation is advantageous. Such one-stage method could be achieved using a chemical triggering glycogen storage. However, there is a limited study of such chemicals. Here, nine compounds previously reported to affect cyanobacterial cellular functions were examined in Synechocystis sp. PCC 6803. 2-Phenylethanol, phenoxyethanol, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, and methyl viologen can stimulate glycogen accumulation. The oxidative stress agent, methyl viologen significantly increased glycogen levels up to 57 and 69% (w/w DW) under BG11 and -N cultivation, respectively. One-stage cultivation where methyl viologen was directly added to pre-grown culture, enhanced glycogen storage to 53% (w/w DW), compared to 10% (w/w DW) glycogen level of the control cells without methyl viologen. Methyl viologen treatment reduced the contents of total proteins (including phycobiliproteins) but caused the increased transcript levels of glycogen synthetic genes and the elevated levels of metabolite substrates for glycogen synthesis. Metabolomic results suggested that upon methyl viologen treatment, proteins were degraded to amino acids, some of which could be used as carbon source for glycogen synthesis. Results of oxygen evolution and metabolomic analysis suggested that photosynthesis and carbon fixation were not completely inhibited upon methyl viologen treatment, and these two processes may partially generate upstream metabolites required for glycogen synthesis.