Chlorosis is a process that describes the depigmentation of cyanobacterial cells triggered by different environmental influences like the deprivation for combined nitrogen. This phenomenon is known from plants especially in autumn but can also be seen in cyanobacteria. Boresch (1910) was the first to describe the chlorosis as a change in the colour of the cyanobacterial culture due to nitrogen depletion. This state ensures long-term survival due to low-level photosynthesis in chlorotic cells of the strain Synechococcus PCC 7942 (Görl et al 1998, Sauer et al 2001). We want to understand the molecular mechanisms of this remarkable ability to survive conditions of extreme starvation. How do the cells sense the onset of starvation, and how do they respond to this stressful condition?

The chlorosis response is governed by a complex and poorly understood regulatory network, which converges at the expression of the nblA gene, the triggering factor for phycobiliprotein degradation. We used a precise dosage of L-methionine-sulfoximine (MSX) which inhibits the glutamine synthetase mimicking the metabolic situation of nitrogen starvation. Addition of nitrate to such MSX-inhibited cells eliminates the associated redox-stress by enabling electron flow towards nitrate/nitrite reduction and thereby, prevents the induction of nblA expression and the associated chlorosis response. This work further established a cryptic role of nitrate/nitrite reductases as electron sinks to balance conditions of over-reduction (Klotz et al 2015).

Polyhydroxybutyrate (PHB) is a common carbon storage polymer among heterotrophic bacteria as well as for the model organism Synechocystis sp. PCC 6803 upon the limitation of macronutrients. We have previously reported a mutation in the gene sll0783 that impairs PHB accumulation in this cyanobacterium (Schlebusch & Forchhammer 2010). Metabolome analysis revealed a difference in sorbitol levels between the wild type and the mutant, indicating a more oxidizing intracellular environment than in the wild type. We confirmed this by directly measuring the NADPH/NADP ratio and by altering the intracellular redox state of wild type and sll0783 mutant and we were able to physiologically complement the mutant phenotype by making the intracellular environment more reducing. The NADPH pool is an important factor for regulation of PHB biosynthesis and metabolism, which is also of interest for potential biotechnological applications (Hauf et al 2013).