Using osmotic stress to stabilize mannitol production in Synechocystis sp. PCC6803

Background Mannitol is a C(6) polyol that is used in the food and medical sector as a sweetener and antioxidant, respectively. The sustainable production of mannitol, especially via the direct conversion of CO(2)by photosynthetic cyanobacteria, has become increasingly appealing. However, previous work aiming to achieve mannitol production in the marineSynechococcussp. PCC7002 via heterologous expression of mannitol-1-phosphate-5-dehydrogenase (mtlD) and mannitol-1-phosphatase (m1p, in short: a 'mannitol cassette'), proved to be genetically unstable. In this study, we aim to overcome this genetic instability by conceiving a strategy to stabilize mannitol production usingSynechocystissp. PCC6803 as a model cyanobacterium. Results Here, we explore the stabilizing effect that mannitol production may have on cells faced with osmotic stress, in the freshwater cyanobacteriumSynechocystissp. PCC6803. We first validated that mannitol can function as a compatible solute inSynechocystissp. PCC6803, and in derivative strains in which the ability to produce one or both of the native compatible solutes was impaired. Wild-typeSynechocystis, complemented with a mannitol cassette, indeed showed increased salt tolerance, which was even more evident inSynechocystisstrains in which the ability to synthesize the endogenous compatible solutes was impaired. Next we tested the genetic stability of all these strains with respect to their mannitol productivity, with and without salt stress, during prolonged turbidostat cultivations. The obtained results show that mannitol production under salt stress conditions in theSynechocystisstrain that cannot synthesize its endogenous compatible solutes is remarkably stable, while the control strain completely loses this ability in only 6 days. DNA sequencing results of the control groups that lost the ability to synthesize mannitol revealed that multiple types of mutation occurred in themtlDgene that can explain the disruption of mannitol production. Conclusions Mannitol production in freshwaterSynechocsytissp. PCC6803 confers it with increased salt tolerance. Under this strategy, genetically instability which was the major challenge for mannitol production in cyanobacteria is tackled. This paper marks the first report of utilization of the response to salt stress as a factor that can increase the stability of mannitol production in a cyanobacterial cell factory.