Abstract
Cyanobacteria are prokaryotes that can assimilate inorganic carbon via oxygenic photosynthesis, which results in the formation of organic compounds essentially from CO2, water, and light. Increasing concerns regarding the increase in atmospheric CO2 due to fossil energy usage fueled the idea of a photosynthesis-driven and CO2-neutral, i.e., cyanobacteria-based biotechnology. The ability of various cyanobacteria to tolerate high and/or fluctuating salinities attenuates the requirement of freshwater for their cultivation, which makes these organisms even more interesting regarding a sustainable utilization of natural resources. However, those applications require a detailed knowledge of the processes involved in salt acclimation. Here, we review the current state of our knowledge on the regulation of compatible solute accumulation in cyanobacteria. The model organism Synechocystis sp. PCC 6803 responds to increasing salinities mainly by the accumulation of glucosylglycerol (GG) and sucrose. After exposure toward increased salt concentrations, the accumulation of the main compatible solute GG is achieved by de novo synthesis. The key target of regulation is the enzyme GG-phosphate synthase (GgpS) and involves transcriptional, posttranscriptional, and biochemical mechanisms. Recently, the GG-degrading enzyme GG hydrolase A (GghA) was identified, which is particularly important for GG degradation during exposure to decreasing salinities. The inversely ion-regulated activities of GgpS and GghA could represent the main model for effectively tuning GG steady state levels according to external salinities. Similar to GG, the intracellular amount of sucrose is also salt-regulated and seems to be determined by the balance of sucrose synthesis via sucrose-phosphate synthase (Sps) and its degradation via invertase (Inv). In addition to their role as stress protectants, both compatible solutes also represent promising targets for biotechnology. Hence, the increasing knowledge on the regulation of compatible solute accumulation not only improves our understanding of the stress physiology of cyanobacteria but will also support their future biotechnological applications.
Highlights
Cyanobacteria are the only prokaryotes performing oxygenic photosynthesis, a process which uses light energy to assimilate CO2 into carbohydrates and biomass, thereby releasing oxygen as byproduct from water splitting
It is commonly accepted that oxygenic photosynthesis has evolved within the group of cyanobacteria
By acquiring the genes for the synthesis of other compatible solutes and concomitant conquest of marine and hypersaline habitats, the genes for sucrose metabolism were lost in some groups
Summary
Cyanobacteria are the only prokaryotes performing oxygenic photosynthesis, a process which uses light energy to assimilate CO2 into carbohydrates and biomass, thereby releasing oxygen as byproduct from water splitting. In contrast to large-scale industrial fermenters for heterotrophic bacteria, appropriate light delivery represents one of the major challenges for cyanobacterial mass cultivation This is due to the fact that shading effects occur proportional with cell growth, which in turn limit biomass and often product formation (for a review on photobiorectors, see Posten, 2009). The stress tolerance of the used cyanobacterial strains comes to the fore considering the indispensable effects of fluctuating temperatures and changing light conditions when exposed to natural sunlight (Singh et al, 2017) This counts for the anticipated cultivation in salty media, i.e., naturally available brackish or seawater. The tolerance of the producer strains to salt is one of the most important challenges
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