Bioactive Chemicals In Cyanobacteria

Cyanobacteria, sometimes still referred to as blue-green algae, have increasingly been shown to be producers of a diverse array of toxic, or otherwise biologically active, compounds with potential applications in biomedicine, as well as implications for environmental health.4-8

Cyanobacteria are a diverse group of photosynthetic, prokaryotic organisms found in freshwater and marine environments. The origin of these organisms dates back 3 or 4 billion years9 and their cell structure closely resembles that of other Gram negative bacteria, but as a rule they live photoautotrophically. Like higher plants they possess chlorophyll a and the water soluble red and blue phycobiliproteins as well as photosystem I and II producing oxygen, which is released to the atmosphere.

They are truly prokaryotic organisms having no nuclear membranes, internal organelles, and histone proteins associated with chromosomes. They are capable of using carbon dioxide as their sole carbon source employing the reductive pentose phosphate pathway or Calvin cycle.10

This group includes various edible and toxic species. Including about 2000 strains cyanobacteria are distributed all over the world. Some of them show a remarkable ecological diversity. Because of widespread eutrophication of lakes, ponds, and some parts of oceans cyanobacteria often form blooms, which lead to water hygiene problems.11-13 They may cause unpleasant tastes and odors through excretion of volatile compounds.14 Furthermore, animal poisonings and risks to human health are described and many of them which cause toxic water toxins produced have been characterized.15-18 Possibly the synthesis of highly active toxins is a defense option of cyanobacteria against attack by other organisms like bacteria, fungi, zooplankton, and eukaryotic microalgae. Carmichael (1994) found that cyanobacterial toxins can be extremely harmful to zooplankters that feed on these cyanobacteria and may be lethal, or they may reduce the number of offspring.

In addition to the toxins a lot of active substances with antibacterial, antiviral, fungicide, enzyme inhibiting, immunosuppressive, cytotoxic, and algicide activity have been isolated from cyanobacterial biomass, or in some cases from the medium of laboratory cultures.19-26 Producing active biocide components could be an important selective advantage. For example, in the 1970s a pronounced reduction of Gram-positive bacteria was observed in lakes during the occurrence of cyanobacterial blooms.27 The production of antibacterial substances could be the reason for this phenomenon. In cyanobacterial blooms often only one species may account for >95% of the population. Though this has been interpreted as a result of competition between species, the dominance of one species could potentially hint at the formation of metabolites with cyanobactericidal activity. In addition, antibacterial, fungicidal and antiviral effective compounds formed by cyanobacteria could contribute to an improvement of the water quality in aquatic environment.28

Cyanobacteria have the appeal of being a raw unprocessed food, rich in carotenoid, chlorophyll, phycocyanin, amino acid, minerals, and many other bioactive components. The nutrient content depends on the location and environment in which the algae are grown. The environment includes altitude, temperature, and sun exposure, which can greatly affect the lipid and pigment content in algae.29

Cyanobacteria have been identified as one of the most promising group of organisms from which novel and biochemically active natural products are isolated. Cyanobacteria such as Microcystis, Anabaena, Nostoc, and Oscillatoria produce a great variety of secondary metabolites. The only comparable group of microorganisms

Other

Alkaloids

Other

Alkaloids

Amides

Pyrroles

Amino acids

Amides

Pyrroles

Amino acids

Macrolides—

Esters

Macrolides—

Esters

Burja, A.M., Banaigs, B., Abou-Mansour, E., Burgess, J.G., and Wright, P.C., Tetrahedron., 57, 9347-9377, 2001.)

is Actinomycetes, which has yielded a large number of metabolites and include traditional microbial drug producers like Actinomycetes and Hyphomycetes, which have been the focus of pharmaceutical research for decades. As the rate of discovery of new compounds from these microorganisms is decreasing, it is time to turn to cyanobacteria and exploit their potential. This is of paramount importance to fight increasingly resistant pathogens and newly emergent diseases (Hayashi et al., 1996). Many agricultural and industrial materials have been obtained from cyanobacteria at laboratory, pilot, and commercial scales including: biomass,30-33 restriction nucleases,34 antifungal, antineoplastic,35,36 antimicrobial,37 antileukemia38 andherb-icidal compounds.39 Some pigments have been produced from cyanobacteria40,41 and other products include: amino acids,42 and fertilizers.43

Bioactive molecules from cyanobacteria exhibit toxic effects against eukaryotes and antibiotic against prokaryotes. It is also reported that antibiotic effects are caused by distinct substances different from the cyanotoxins.44-46

Screening of cyanobacteria for antibiotics and pharmaceutically active compounds has received ever increasing attention for some time. The bioactive molecules isolated show a broad spectrum of biological activities including toxins, antibiotics, fungicides, and algaecides.47

Because cyanobacteria are largely unexplored, they represent a rich opportunity for discovery; the expected rate of rediscovery is far lower than for other better studied groups of organisms.48 Cyanobacteria produce a wide variety of toxins and other bioactive compounds, these may be divided into the following chemical classes: 40% lipopeptides, 5.6% amino acids, 4.2% fatty acids, 4.2% macrolides, and 9% amides (Figure 12.1). Cyanobacterial lipopeptides include compounds that may be categorized as cytotoxic (41%), antitumor (13%), antiviral (4%), antibiotics (12%), and the remaining 18% activities include antimalarial, antimycotics, multidrug resistance reversers, antifeedant, herbicides, and immunosuppressive agents (Figure 12.2).49 Cyanobacteria have a cholesterol-lowering effect in animals and humans. The level of the total cholesterol, LDL and VLDL cholesterol in rat serum was reduced when a high cholesterol diet was supplemented with cyanobacteria. Furthermore, it was found that adopohepatosis, caused by a high cholesterol diet, was "cured" by a diet

Mrt a^tiwih/

Anticancer

Antivii

Other

Antifungal

Mrt a^tiwih/

Anticancer

Antivii

Other

Cytotoxic

Antibiotic

FIGURE 12.2 Reported biological activities of marine cyanobacterial compounds.49 (From Burja, A.M., Banaigs, E.B., Abou-Mansour, Burgess, J.G., and Wright, P.C., Tetrahedron, 57, 9347-9377, 2001.)

Cytotoxic

Antibiotic

FIGURE 12.2 Reported biological activities of marine cyanobacterial compounds.49 (From Burja, A.M., Banaigs, E.B., Abou-Mansour, Burgess, J.G., and Wright, P.C., Tetrahedron, 57, 9347-9377, 2001.)

supplemented with algae owing to the activity of lipoprotein lipase, an enzyme for metabolism of triglyceride rich lipoproteins.50 Aphanizomenonflos-aquae also shows a hypocholesterolemic effect, owing to its chlorophyll content, which stimulates the liver function and decreases blood cholesterol level.51 Aphanizomenon flos-aquae inhibit the activity of a maltase and sucrase in the digestive tract of rats.52 Valencia et al. presented evidence that Aphanizomenonflos-aquae accelerate recovery from mild traumatic brain injury.53

Many studies have assessed the antibacterial activity of some cyanobacteria and their extracts.54-62,28 For example, Mian et al. (2003) investigated 22 terrestrial and freshwater cyanobacteria for antimicrobial activity. Of these, 54.5% of all extracts showed activity against Gram-positive bacteria, while 9.1% possessed antifungal activity against Candida albicans. However, no extract was obtained to be active against Gram-negative bacteria.

The above results demonstrated that terrestrial and freshwater cyanobacteria are still a promising source of new bioactive nature products.

Soltani et al. (2005) isolated 76 cyanobacteria strains from Iranian paddy fields. 22.4% of them (17 cyanobacteria) exhibited antimicrobial effects. The cyanobacteria with positive antimicrobial activity included members of the families Stigonemata-ceae, Nostocaceae, Oscillatoriaceae, and Chrococcaceae. Growth of Bacillus subtilis PTCC 1204 and Staphylococcus epidermidis PTCC 1114 was inhibited by 12 and 14 strains of cyanobacteria, respectively. In addition, eight cyanobacteria inhibited the growth of Escherichia coli PTCC 1047, and two species inhibited the growth of Salmonella typhi PTCC 1108.

Asthana et al. (2006)63 demonstrated the effectiveness of 15-30 ^l/mL of purified bioactive molecule (active principle) from Fischerella sp., isolated from Neem (Azadirachta indica) tree bark, against Mycobacterium tuberculosis, Enterobacter aerogenes, Staphylococcus aureus, Pseudomonas aeruginosa, Salmonella typhii, multidrug resistant (MDR) strains of E. coli. Mundt et al. (2001) screened antibacterial activities of lipophilic and hydrophilic extracts from cultured cyanobacteria, or "bloom" material, isolated from German lakes and the Baltic sea against some bacteria. In the above study antibacterial activity was found in the more lipophilic extracts and the aqueous extracts were ineffective.

In a recent study, Pahayokolide A was purified from Lyngbya sp. strain 152 biomass at yields of approximately 1.35% (w/v) of the crude extract. A lower yield (0.44%) was obtained when the strain was cultured without Na2CO3 supplemented medium. Samples of the purified Pahayokolide A isolated from the Na2CO3-supplemented cultures were used for toxicological and pharmacological evaluation. Pahayokolide A inhibited the growth of Gram-positive bacteria, Bacillus megaterium and Bacillus cereus, and the yeast, Saccharomyces cerevisiae as well as the green algae, Ulothrix Ev-17 and Chlamydomonas Ev-29. In addition, hormogonia development was inhibited within a zone around the Pahayokolide A treatment of a lawn of the cyanobacterium, Nostoc Ev-1, although growth of the organism was not otherwise affected.58 Further examples of antimicrobial activity of cyanobacteria are listed in Table 12.1.

In another study, a total of 44 lipophilic and hydrophilic extracts obtained from 22 samples of cultured terrestrial and freshwater cyanobacteria were investigated for their biological activities. Of these, 54.5% of all extracts showed activity against Gram positive bacteria, while 9.1% possessed antifungal activity against Candida albicans; however, no extract was active against Gram-negative bacteria.54

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