Current knowledge and recent advances in understanding metabolism of the model cyanobacterium Synechocystis sp. PCC 6803

Cyanobacteria are key organisms in the global ecosystem, useful models for studying metabolic and physiological processes conserved in photosynthetic organisms, and potential renewable platforms for production of chemicals. Characterising cyanobacterial metabolism and physiology is key to understanding their role in the environment and unlocking their potential for biotechnology applications. Many aspects of cyanobacterial biology differ from heterotrophic bacteria. For example, most cyanobacteria incorporate a series of internal thylakoid membranes where both oxygenic photosynthesis and respiration occur, while CO2 fixation takes place in specialised compartments termed carboxysomes. In this review, we provide a comprehensive summary of our knowledge on cyanobacterial physiology and the pathways in Synechocystis sp. PCC 6803 (Synechocystis) involved in biosynthesis of sugar-based metabolites, amino acids, nucleotides, lipids, cofactors, vitamins, isoprenoids, pigments and cell wall components, in addition to the proteins involved in metabolite transport. While some pathways are conserved between model cyanobacteria, such as Synechocystis, and model heterotrophic bacteria like Escherichia coli, many enzymes and/or pathways involved in the biosynthesis of key metabolites in cyanobacteria have not been completely characterised. These include pathways required for biosynthesis of chorismate and membrane lipids, nucleotides, several amino acids, vitamins and cofactors, and isoprenoids such as plastoquinone, carotenoids, and tocopherols. Moreover, our understanding of photorespiration, lipopolysaccharide assembly and transport, and degradation of lipids, sucrose, most vitamins and amino acids, and heme, is incomplete. We discuss tools that may aid characterisation of cyanobacterial metabolism, notably CyanoSource, a barcoded library of targeted Synechocystis mutants, which will significantly accelerate characterisation of individual proteins

Sink regulation of photosynthesis in sugarcane

Thesis (Ph.D.) - University of KwaZulu-Natal, Westville, 2007.The C4 plant, sugarcane (Saccharum spp. hybrids), accumulates sucrose to high concentrations and, as a result, has been the focus of extensive research into the biochemistry and physiology of sucrose accumulation. Despite this, the relationship between source leaf photosynthetic rates and sucrose accumulation in the culm has not been well documented. The observations that photosynthetic activity declines during culm maturation in commercial cultivars and that high-sucrose accumulating ancestral genoptypes photosynthesize at rates two-thirds of those of low-sucrose ancestral Saccharum species indicate that source-sink communication may play a pivotal role in determining sucrose yield. The relationship between source and sink tissues in sugarcane was investigated using a supply-demand paradigm, an approach novel in the study of the crop. The demand for photosynthate from the primary culm growth sink was shown to be closely linked to photosynthetic rates, sucrose export and the eventual physiological decline of source leaves. Results from initial field experiments revealed that leaf assimilation rates were negatively correlated with leaf hexose concentrations, but not those of sucrose. Further manipulation of leaf sugar status, through sugar-feeding and cold-girdling techniques, demonstrated the regulatory role of leaf sugar concentrations on photosynthetic activity, thus revealing sucrose, and particularly hexose, as key signal molecules in the modulation of the amount of photosynthate available for export to the sink. Gene expression profiling, by means of array technologies, indicated that changes in leaf sugar status and photosynthetic rates result in concurrent modifications in the expression of several genes involved in fundamental metabolic pathways, including photosynthesis, carbohydrate metabolism, stress response and sugar-signaling. Notable amongst these, was the identification of a potential trehalose 6-phosphate (T6P) sugar-signaling mechanism, thus implicating the trehalose pathway as a central regulatory system in the communication of sink carbon requirements to the source leaf. This study demonstrated that maturation of the culm results in a decreased demand for sucrose, which invokes a sugar-mediated feedback signal to decrease leaf photosynthetic supply processes. However, sugarcane leaves appear to retain the capacity to increase the supply of assimilate to culm tissues under conditions of increased assimilate demand. Uncoupling of the signaling pathways that mediate negative feedback between source and sink tissues may result in improved leaf assimilation rates and, consequently, lead to increased sugarcane sucrose yields

Engineering highly productive cyanobacteria towards carbon negative emissions technologies

Cyanobacteria are a diverse and ecologically important group of photosynthetic prokaryotes that contribute significantly to the global carbon cycle through the capture of CO2 as biomass. Cyanobacterial biotechnology could play a key role in a sustainable bioeconomy through negative emissions technologies (NETs), such as carbon sequestration or bioproduction. However, the primary issues of low productivities and high infrastructure costs currently limit the commercialisation of such applications. The isolation of several fast-growing strains and recent advancements in molecular biology tools now offer promising new avenues for improving yields, including metabolic engineering approaches guided by high-throughput screening and metabolic models. Furthermore, emerging research on engineering coculture communities could help to develop more robust culturing systems to support broader NET applications

Editorial: Structure and function of chloroplasts, Volume III

Chloroplasts are endosymbiotic organelles derived from cyanobacteria. They have a double envelope membrane, including the outer envelope and the inner envelope. A complex membrane system, thylakoids, exists inside the chloroplast. It is the site of the light-dependent reactions of photosynthesis. The stroma is the main site of the carbon fixation reactions. Although photosynthesis is a very complicated process with many proteins involved, there are many other important processes that occur in chloroplasts, including the regulation of photosynthesis, the biogenesis and maintenance of the structures, carbohydrate, lipid, tetrapyrrole, amino acid, and isoprenoid metabolism, production of some phytohormones, production of specialized metabolites, regulation of redox, and interactions with other parts of the cell (Sabater, 2018). During evolution, most of the cyanobacterial genes were lost and many of them were transferred into the nuclear genome. A majority of chloroplast proteins are nuclear-encoded and possess an N-terminal transit peptide which helps the protein to be targeted into chloroplasts. Chloroplasts have their own highly reduced genome which works coordinately with the nuclear genome for the biogenesis and function of chloroplasts (Liebers et al., 2022). This Research Topic presents studies covering different aspects of chloroplast function, including photosynthesis, biogenesis, structure, and maintenance. These works push the frontiers of chloroplast research further in the field of plant biology

Variable Neighbourhood Search: A Case Study for a Highly-Constrained Workforce Scheduling Problem

This paper describes a Variable Neighbourhood Search (VNS) combined with simulated annealing to tackle a highly constrained workforce scheduling problem at British Telecommunications plc (BT). A refined greedy algorithm is firstly designed to create an initial solution which meets all hard constraints and satisfies some of the soft constraints. The VNS is then used to swap out less promising combinations, continually moving towards a more optimal solution until meeting finishing requirements. The results are promising when compared to the stand- alone greedy algorithm. We believe there is scope for this to be extended in several ways, i.e. into a more complex variation of VNS to further improve results, to be applied to further data sets and workforce scheduling problem scenarios, and to have input parameters to the algorithm selectively optimized to discover what kind of improvements in efficiency and fitness are possible. There is also scope for this to be used in similar combinatorial optimization problems