The increase in atmospheric carbon dioxide is linked to climate changes; hence there is an urgent need to reduce the accumulation of CO2 in the atmosphere. The utilization of CO2 as a raw material in the synthesis of chemicals and liquid energy carriers offers a way to mitigate the increasing CO2 buildup. This review covers six important CO2 transformations namely: chemical transformations, photochemical reductions, chemical and electrochemical reductions, biological conversions, reforming and inorganic transformations. Furthermore, the vast research area of carbon capture and storage is reviewed briefly. This review is intended as an introduction to CO2, its synthetic reactions and their possible role in future CO2 mitigation schemes that has to match the scale of man-made CO2 in the atmosphere, which rapidly approaches 1 teraton.

The teraton challenge. A review of fixation and transformation of carbon dioxide

The in vitro antibacterial activity of dietary spice and medicinal herb extracts.

The therapeutic journey of benzimidazoles: a review.

The production of poly-(γ-glutamic acid) from microorganisms and its various applications

The lack of tax incentives for biomethane use requires the optimization of both biogas production and upgrading in order to allow the full exploitation of this renewable energy source. The large number of biomethane contaminants present in biogas (CO2, H2S, H2O, N2, O2, methyl siloxanes, halocarbons) has resulted in complex sequences of upgrading processes based on conventional physical/chemical technologies capable of providing CH4 purities of 88–98 % and H2S, halocarbons and methyl siloxane removals >99 %. Unfortunately, the high consumption of energy and chemicals limits nowadays the environmental and economic sustainability of conventional biogas upgrading technologies. In this context, biotechnologies can offer a low cost and environmentally friendly alternative to physical/chemical biogas upgrading. Thus, biotechnologies such as H2-based chemoautrophic CO2 bioconversion to CH4, microalgae-based CO2 fixation, enzymatic CO2 dissolution, fermentative CO2 reduction and digestion with in situ CO2 desorption have consistently shown CO2 removals of 80–100 % and CH4 purities of 88–100 %, while allowing the conversion of CO2 into valuable bio-products and even a simultaneous H2S removal. Likewise, H2S removals >99 % are typically reported in aerobic and anoxic biotrickling filters, algal-bacterial photobioreactors and digesters under microaerophilic conditions. Even, methyl siloxanes and halocarbons are potentially subject to aerobic and anaerobic biodegradation. However, despite these promising results, most biotechnologies still require further optimization and scale-up in order to compete with their physical/chemical counterparts. This review critically presents and discusses the state of the art of biogas upgrading technologies with special emphasis on biotechnologies for CO2, H2S, siloxane and halocarbon removal.

A review on the state-of-the-art of physical/chemical and biological technologies for biogas upgrading

A new benzoxazole, UK-1, was isolated from the mycelial cake of an actinomycete strain 517-02. Based on morphological, cultural and physiological characteristics, strain 517-02 was seemed to be a close relative of Streptomyces morookaense. UK-1 showed potent cytotoxic activity against B16, HeLa and P388 cells and did not show any antimicrobial activity.

https://www.jstage.jst.go.jp/article/antibiotics1968/46/7/46_7_1089/_pdf

UK-1, a novel cytotoxic metabolite from Streptomyces sp. 517-02. I: Taxonomy, fermentation, isolation, physico-chemical and biological properties

The addition of levan as a so-called "acceptor" accelerated the rate of polymerization of levan catalyzed by levansucrase [EC 2.4.1.10]. However, this effect was seen only under conditions of low ionic strength. Under conditions of high ionic strength, only levan with an average degree of polymerization (DP) of 120 was synthesized with a reasonable yield. Incorporation of [14C]fructose residues into levan of high molecular weight (DP 1,200) added as an "acceptor" at high ionic strength was slight. It is suggested that the DP of levan synthesized is generally regulated by ionic strength, and that enzymic synthesis of levan occurs in the absence of an "acceptor."

Synthesis of levan by levansucrase. Some factors affecting the rate of synthesis and degree of polymerization of levan.

The mechanism of polygodial antifungal action was studied using Saccharomyces cerevisiae. It inhibited the incorporation of radioactive precursors into macromolecules in whole cells. However, no specificity in the inhibition of precursor incorporation was observed among various species of macromolecules, i.e., DNA, RNA, protein, and polysaccharide. It also inhibited the exogenous but not endogenous respiration of the cells. Polygodial greatly enhanced the leakage of cellular constituents from the treated cells, and the examination under an electron microscope revealed structural damage of the cell membranes of treated cells. These results support the conclusion that polygodial acts primarily by damaging the permeability barrier of the yeast cells.

Mode of Action of Polygodial, an Antifungal Sesquiterpene Dialdehyde

The levan synthesized by Bacillus subtilis levansucrase in the presence of alcohols was of only high molecular weight, while in solutions of high ionic strength only low molecular weight (MW) levan was produced. The addition of low MW levan to the enzyme reaction mixture at low ionic strength stimulated synthesis of a high MW levan, but the levan added was not incorporated into this high MW levan. Methylation analysis revealed that low MW levans contained glucose, which was isolated as 2,3,46-tetra-O-methyl alditol acetate showing that the glucose units existed as terminal residues. The molecular weight of levan estimated on the basis of glucose content coincided with that determined by the gel filtration method. Methylation analysis also revealed that the number of fructose residues of the linear fraction linked by leads to 6(F)2 leads to type bonds was 22 for levan with a molecular weight of (8.4(-22)) x 10(3), while it was 11 for that of 2,000 x 10(3). The number of (formula: see text) type branched residues increased with increase in the molecular weight of the levan synthesized.

The Molecular Structure of Low and High Molecular Weight Levans Synthesized by Levansucrase

Sesquiterpene dialdehydes, isolated originally as insect antifeedants from East African Warburgia trees, have been shown to exhibit strong antimicrobial activity. Their chemical reactivities and biological activities were investigated in comparison with those of related compounds. There was a good correlation between the antifungal activity and the papain inhibitory activity of these compounds. Both activities appear to result from their highly specific reactivity with sulfhydryl groups. Consideration of the structure–activity relationships led to the proposal of a structure unit, the enal–aldehyde moiety, that is essential to biological activity.

Susumu Oi

Papers

UK-1, a novel cytotoxic metabolite from Streptomyces sp. 517-02. I: Taxonomy, fermentation, isolation, physico-chemical and biological properties

Synthesis of levan by levansucrase. Some factors affecting the rate of synthesis and degree of polymerization of levan.

Mode of Action of Polygodial, an Antifungal Sesquiterpene Dialdehyde

The Molecular Structure of Low and High Molecular Weight Levans Synthesized by Levansucrase

Structure–Activity Relationship of the Warburgia Sesquiterpene Dialdehydes