Occurrence, classification, and biological function of hydrogenases: an overview.

Biological hydrogen production; fundamentals and limiting processes

Cyanobacteria are photoautotrophic prokaryotes which include a large variety of species of widespread occurrence and with diverse morphological, physiological and biochemical properties. Many cyanobacteria are known to be able to synthesise outermost slimy investments and to release polysaccharidic material into the culture medium during cell growth. These released polysaccharides (RPSs), being easily recoverable from the culture medium, are attracting much interest in view of their possible uses in several industrial applications. In this paper, an overview of the current knowledge on both RPS-producing cyanobacterial strains (including the possible roles of the exopolysaccharides) and chemical characteristics of the cyanobacterial RPSs is given, with particular emphasis on RPS properties and possible industrial applications. On the whole, cyanobacterial RPSs are characterised by a great variety in both number (from two to 10) and type of constitutive monosaccharides (various arrangements of acidic and neutral sugars). Most polymers show an anionic nature due to the presence of uronic acids and/or other charged groups such as pyruvyl or sulfate. Polypeptide moieties as well as acetyl substituents have also sometimes been found, causing additional structural complexity. All the cyanobacterial RPSs so far tested showed a pseudoplastic behaviour, but with marked differences in both viscosity values and shear thinning. In terms of RPS production, the responses of cyanobacteria to changes of culture conditions appear strain-dependent. RPS productivities shown by some cyanobacteria are well comparable with those reported for other photosynthetic microorganisms proposed for polysaccharide production, but very low in comparison with those of heterotrophic microorganisms. Nevertheless, cyanobacteria may be regarded as a very abundant source of structurally diverse polysaccharides, some of which may possess unique properties for special applications, not fulfilled by the polymers currently available. However, much work has still to be done to bridge the wide gap existing between data on the biology of the RPS-producer strains and information concerning technological and other useful properties of the cyanobacterial RPS.

Exocellular polysaccharides from cyanobacteria and their possible applications

Cyanobacteria (blue-green algae) are photosynthetic prokaryotes used as food by humans. They have also been recognized as an excellent source of vitamins and proteins and as such are found in health food stores throughout the world. They are also reported to be a source of fine chemicals, renewable fuel and bioactive compounds. This potential is being realized as data from research in the areas of the physiology and chemistry of these organisms are gathered and the knowledge of cyanobacterial genetics and genetic engineering increased. Their role as antiviral, anti-tumour, antibacterial, anti-HIV and a food additive have been well established. The production of cyanobacteria in artificial and natural environments has been fully exploited. In this review the use of cyanobacteria and microalgae, production processes and biosynthesis of pigments, colorants and certain bioactive compounds are discussed in detail. The genetic manipulation of cyanobacteria and microalgae to improve their quality are also described at length.

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Bioactive compounds from cyanobacteria and microalgae: an overview.

Cyanobacteria may possess several enzymes that are directly involved in dihydrogen metabolism: nitrogenase(s) catalyzing the production of hydrogen concomitantly with the reduction of dinitrogen to ammonia, an uptake hydrogenase (encoded by hupSL) catalyzing the consumption of hydrogen produced by the nitrogenase, and a bidirectional hydrogenase (encoded by hoxFUYH) which has the capacity to both take up and produce hydrogen. This review summarizes our knowledge about cyanobacterial hydrogenases, focusing on recent progress since the first molecular information was published in 1995. It presents the molecular knowledge about cyanobacterial hupSL and hoxFUYH, their corresponding gene products, and their accessory genes before finishing with an applied aspect—the use of cyanobacteria in a biological, renewable production of the future energy carrier molecular hydrogen. In addition to scientific publications, information from three cyanobacterial genomes, the unicellular Synechocystis strain PCC 6803 and the filamentous heterocystous Anabaena strain PCC 7120 and Nostoc punctiforme (PCC 73102/ATCC 29133) is included.

https://mmbr.asm.org/content/mmbr/66/1/1.full.pdf

Hydrogenases and Hydrogen Metabolism of Cyanobacteria

Although cyanobacteria are oxygenic phototrophic organisms, they often thrive in environments that become periodically anoxic. This is particularly the case in the dark when photosynthetic oxygen evolution does not take place. Whereas cyanobacteria generally utilize endogenous storage carbohydrate by aerobic respiration, they must use alternative ways for energy generation under dark anoxic conditions. This aspect of metabolism of cyanobacteria has received little attention but nevertheless in recent years a steadily increasing number of publications have reported the capacity of fermentation in cyanobacteria. This review summarizes these reports and gives a critical consideration of the energetics of dark fermentation in a number of species. There are a variety of different fermentation pathways in cyanobacteria. These include homo- and heterolactic acid fermentation, mixed acid fermentation and homoacetate fermentation. Products of fermentation include CO2, H2, formate, acetate, lactate and ethanol. In all species investigated, fermentation is constitutive. All enzymes of the fermentative pathways are present in photoautotrophically grown cells. Many cyanobacteria are also capable of using elemental sulfur as electron acceptor. In most cases it seems unlikely that sulfur respiration occurs. The main advantage of sulfur reduction seems to be the higher yield of ATP which can be achieved during fermentation. Besides oxygen and elemental sulfur no other electron acceptors for chemotrophic metabolism are known so far in cyanobacteria. Calculations show that the yield of ATP during fermentation, although it is low relative to aerobic respiration, exceeds the amount that is likely to be required for maintenance, which appears to be very low in these cyanobacteria. The possibility of a limited amount of biosynthesis during anaerobic dark metabolism is discussed.

https://pure.uva.nl/ws/files/3238369/2576_25311y.pdf

Fermentation in cyanobacteria

The cyanobacterium Microcystis PCC7806 fermented endogenously stored glycogen to ethanol, acetate, CO2, and H2 when incubated anaerobically in the dark. The switch from photoautotrophic to fermentative metabolism did not require de novo protein synthesis, and fermentation started immediately after cells had been transferred to dark anoxic conditions. From the molar ratios of the products and from enzyme activities in cell-free extracts, it was concluded that glucose derived from glycogen was degraded via the Embden-Meyerhof-Parnas pathway. In addition, CoA-dependent pyruvate:ferredoxin oxidoreductase, alcohol dehydrogenase, acetate kinase, and hydrogenase were present. The specific activities of these enzymes were sufficiently high to account for the rates of product formation by cell suspensions.

Fermentation in the unicellular cyanobacterium Microcystis PCC7806.

The cyanobacterium Microcystis PCC7806, grown under continuous light, fermented endogenously stored glycogen to equimolar amounts of acetate and ethanol when incubated anaerobically in the dark. In addition, H2, CO2 and some l-lactate were produced. This fermentation pattern differed from that displayed by cells which had been grown under an alternating light/dark (16/8 h) cycle. Such cells produced much more ethanol than acetate, while no lactate was formed. These differences could not be related to the levels of key enzymes of fermentation, which were identical in the two cultures. The cultures grown under continuous light contained twice as much glycogen as the light/dark-grown cells and the former metabolized it at a rate approximately 3 times as fast as the latter culture. Fermentation in the culture grown under continuous light showed low carbon recovery (59–80%) and high oxidation/reduction balance (approximately 1.5). On the basis of calculations of ATP yield it was concluded that this culture was...

A comparison of fermentation in the cyanobacterium Microcystis PCC7806 grown under a light/dark cycle and continuous light

Microbial mats are characterized by strong diel fluctuations in oxygen concentration which to a large extent can be attributed to the physiology of the cyanobacteria (Revsbech et al. 1983). In the light these organisms carry out oxygenic photosynthesis resulting in oxygen supersaturation of the mat. In the dark the primary mode of energy generation is respiration of endogenous glycogen (Smith 1982). However, in well-established microbial mats diffusion of oxygen in the mat will normally be insufficient to cover the demands and, as a result, the mat will turn anoxic. Evidently, the cyanobacteria have to switch to another way of energy generation in order to survive. Organisms isolated from such habitats were investigated for their mechanisms of anaerobic dark energy generation and turned out to be capable of fermentation. Forinstance, Oscillatoria limnetica, which thrives in the sulfide-rich hypolimnion of Solar Lake (Sinai) displays a homolactic fermentation using endogenous storage glucan as the substrate (Oren and Shilo 1979). Alternatively, this organism is capable of anaerobic respiration using elemental sulfur as the electron acceptor. Another mat-building cyanobacterium, Oscillatoria limosa, carries out a heterolactic and homoacetic fermentation simultaneously (Heyer et al. 1989). In the presence of elemental sulfur this organism stops producing hydrogen and sulfide is produced instead. Oscillatoria terebriformis was isolated from a hot-spring microbial mat and produces lactate form exogenous glucose or fructose (Richardson and Castenholz 1987).

Anaerobic dark energy generation in the mat-building cyanobacterium Microcoleus chthonoplastes

The cyanobacterium Microcystis PCC7806 was found to possess an NAD-dependent lactate dehydrogenase (EC 1.1.1.27) which catalyzes the reduction of pyruvate to l-lactate. The enzyme required fructose 1,6-bisphosphate for activity and displayed positive cooperativity towards pyruvate. Lactate was not formed during fermentation by cell suspensions, possibly due to low intracellular concentrations of fructose 1,6-bisphosphate and/or pyruvate.

https://pure.uva.nl/ws/files/2882721/945_10977y.pdf

Roy Moezelaar

Papers

Fermentation in cyanobacteria

Fermentation in the unicellular cyanobacterium Microcystis PCC7806.

A comparison of fermentation in the cyanobacterium Microcystis PCC7806 grown under a light/dark cycle and continuous light

Anaerobic dark energy generation in the mat-building cyanobacterium Microcoleus chthonoplastes

Lactate dehydrogenase in the cyanobacterium Microcystis PCC7806