Improving polyglucan production in cyanobacteria and microalgae via cultivation design and metabolic engineering

Abstract

Photosynthetic microorganisms, such as cyanobacteria and microalgae, are currently being investigated as alternative biomass resources for bioethanol production, owing to their benefits, including high-photosynthetic activity and whole-year cultivation without utilization of arable land. Polyglucans comprise the major carbohydrate content of these organisms. Polyglucans can be utilized as a carbon source for microbial fermentation. Although polyglucan production has so far been promoted by nutrient limitation, it must be further enhanced to accommodate market demand. This review focuses on the recent progress in the production of α-polyglucans such asglycogen and starch in cyanobacteria and green microalgae via cultivation design, including modifying the nutrient supply and replacing the growth medium. The control and manipulation of polyglucan metabolism necessitates the elucidation of the polyglucan production mechanism. We reviewed gene expression and metabolite accumulation profiles of cyanobacteria and green microalgae during nutrient limitation-stimulated α-polyglucan accumulation. We also focus on the enhancement in cyanobacterial glycogen production via the genetic engineering of glycolysis, CO2 concentration mechanism, and photosynthetic light-harvesting protein based on the polyglucan accumulation mechanism. The combined strategies of cultivation design and genetic engineering should be considered for further enhancement of polyglucan productivity for bioethanol production.

Enhancement of microalgae anaerobic digestion by thermo-alkaline pretreatment with lime (CaO)

“…Moreover, arabinose release was only detected in that case. Such a significant sugar release could be attributed to the cell wall damage, since the cell wall of the studied microalgae species is constituted by these monomeric sugars [31,32].…”

Production of polyhydroxybutyrates and carbohydrates in a mixed cyanobacterial culture: Effect of nutrients limitation and photoperiods

“…On the other hand, glycogen is a water-soluble α-polyglycan which constitutes the primary and most suitable substrate for biofuel generation, mainly in anaerobic fermentation, anaerobic digestion and bio-hydrogen technologies [6,7]. The use of cyanobacteria to produce glycogen represents an advantage in comparison with other higher plants or green algae producing carbohydrates [8], due mainly to their lack of a hard cellulosic cell wall, which usually requires of additional pretreatment and further costly conversion processes to extract the product [9,10].…”

“…Most of the studies related to the production of PHBs and glycogen from cyanobacteria are based on pure or genetically modified cultures [6,[11][12][13][14]. Such cultures involve the use of sterile medium substrates in expensive and highly controlled processes, which yield products still too expensive to compete with petroleum-based ones.…”

Improving carbohydrate production of Chlorella sorokiniana NIES‐2168 through semi‐continuous process coupled with mixotrophic cultivation

“…Due to the growing concerns about the sustainability of petroleum resources and the increasing energy demands, many countries are making considerable efforts on developing alternative energy resources, such as biomass, to satisfy the future demands . Biomass is the organic matter derived from living organisms, which have great potential as the renewable energy resources, such as bio‐alcohols and biodiesel . Microalgae, photosynthetic microorganisms, could be one of the most promising biomass feedstock due to their high photo‐conversion efficiency reflecting by their high CO 2 fixation rate and high growth rate .…”