Comparative studies on the photoproduction of nonhydrogenous resources by cyanobacteria

Salt-Regulated Accumulation of the Compatible Solutes Sucrose and Glucosylglycerol in Cyanobacteria and Its Biotechnological Potential

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.

The glucosylglycerol-degrading enzyme GghA is involved in acclimation to fluctuating salinities by the cyanobacterium Synechocystis sp. strain PCC 6803

The ggpS gene, which encodes the key enzyme for the synthesis of the compatible solute glucosylglycerol (GG), has a promoter region that overlaps with the upstream-located gene slr1670 in the cyanobacterium Synechocystissp. PCC 6803. Like ggpS, the slr1670 gene is salt-induced and encodes a putative glucosylhydrolase. A mutant strain with a slr1670 deletion was generated and found to be unable to adapt the internal GG concentrations in response to changes in external salinities. Whereas cells of the wild-type reduced the internal pool of GG when exposed to gradual and abrupt hypo-osmotic treatments, or when the compatible solute trehalose was added to the growth medium, the internal GG pool of ∆slr1670 mutant cells remained unchanged. These findings indicated that the protein Slr1670 is involved in GG breakdown. The biochemical activity of this GG-hydrolase enzyme was verified using recombinant Slr1670 protein, which split GG into glucose and glycerol. These results validate that Slr1670, which was named GghA, acts as a GG hydrolase. GghA is involved in GG turnover in fluctuating salinities, and similar proteins are found in the genomes of other GG-synthesizing cyanobacteria.

Biosynthesis, biotechnological production, and applications of glucosylglycerols

Glucosylglycerols (GGs) are known as compatible solutes accumulated by some bacteria including cyanobacteria as well as higher plants for their adaptations to salt or desiccation stresses. Since being identified in Japanese sake, their physiological effects and potential applications on human health cares have been explored in the following 15 years. Several different synthesis methods have been successively developed for the production of GGs. However, the efficiency of GG synthesis, especially biological synthesis, is still low. With the recent advances in genome sequencing and synthetic biology tools, systematical screening of enzyme candidates and metabolic engineering approaches is necessary for improving GG synthesis efficiency. In this review, we will summarize GG structure information, protective effects on human skin and digestive system as well as industrial enzymes, together with their synthesis by chemical, enzymatic, and biological in vivo approaches in detail, and provide some prospects on improving GG production.

Heterosides--compatible solutes occurring in prokaryotic and eukaryotic phototrophs

The acclimation to osmotic and/or salt stress conditions induces an integrated response at different cellular levels. One acclimation strategy relies on the massive accumulation of low molecular mass compounds, so-called compatible solutes, to balance osmotic gradients and to directly protect critical macromolecules. Heterosides are compounds composed of a sugar and a polyol moiety that represent one chemical class of compatible solutes with interesting features. Well-investigated examples are glucosylglycerol, which is found in many cyanobacteria, and galactosylglycerols (floridoside and isofloridoside), which are accumulated by eukaryotic algae under salt stress conditions. Here, we review knowledge on physiology, biochemistry and genetics of heteroside accumulation in pro- and eukaryotic photoautotrophic organisms.

Salt acclimation of cyanobacteria and their application in biotechnology

The long evolutionary history and photo-autotrophic lifestyle of cyanobacteria has allowed them to colonize almost all photic habitats on Earth, including environments with high or fluctuating salinity. Their basal salt acclimation strategy includes two principal reactions, the active export of ions and the accumulation of compatible solutes. Cyanobacterial salt acclimation has been characterized in much detail using selected model cyanobacteria, but their salt sensing and regulatory mechanisms are less well understood. Here, we briefly review recent advances in the identification of salt acclimation processes and the essential genes/proteins involved in acclimation to high salt. This knowledge is of increasing importance because the necessary mass cultivation of cyanobacteria for future use in biotechnology will be performed in sea water. In addition, cyanobacterial salt resistance genes also can be applied to improve the salt tolerance of salt sensitive organisms, such as crop plants.