Biotechnological and industrial significance of cyanobacterial secondary metabolites

In vitro modulation of intracellular receptor signaling and cytotoxicity induced by extracts of cyanobacteria, complex water blooms and their fractions

The biological activity of cyanobacteria and their chemical components have been widely studied due to their blooms in eutrophic waters worldwide. The primary goal of this study was to determine if individual cyanobacterial species and mixtures of cyanobacteria collected from the environment contain compounds with the potential for interaction with signaling pathways of the aryl hydrocarbon receptor (AhR), androgen receptor (AR), estrogen receptor (ER), glucocorticoid receptor (GR) and retinoid acid receptor (RAR). Cytotoxicity and specific toxic potencies of products of freshwater cyanobacteria were determined by use of in vitro reporter gene trans-activation assays. The testing included samples prepared from five selected single cyanobacterial species cultivated in laboratory and five complex cyanobacterial biomasses collected from blooms in surface waters in the Czech Republic. The results demonstrate estrogenic potencies of extracts of cyanobacterial biomasses. Among the laboratory single species, the extract of Planktothrix agardhii (intracellular metabolites) had a potency of estrogenic equivalents (EEQ) of 3.8 ng 17β-estradiol/g dw. The estimates of EEQs of samples prepared from complex cyanobacterial biomasses collected from freshwaters in the Czech Republic ranged from 19 to 2200 ng 17β-estradiol/g dw. Several samples prepared from the environmental cyanobacterial biomasses potentiated the androgenic potency of dihydrotestosterone. There was no dioxin-like, glucocorticoid or anti/retinoic activity observed for any of the extracts studied. Extracts of natural complex cyanobacterial biomasses exhibited greater and more frequent presence of compounds with specific modes of action, mainly estrogenic, and also greater cytotoxicity than extracts of single cyanobacterial species. The demonstrated estrogenic potency of the compounds present in complex cyanobacterial biomasses is of environmental relevance, and could potentially contribute to endocrine disruptive effects in aquatic ecosystems in case of great bloom densities.

Cylindrospermopsin and microcystin-LR alter the growth, development and peroxidase enzyme activity of white mustard (Sinapis alba L.) seedlings, a comparative analysis

This work focuses on the comparative analysis of the effects of two cyanobacterial toxins of different chemical structure cylindrospermopsin (CYN) and microcystin-LR (MC-LR) on the white mustard (Sinapis alba L.) seedlings. Both cyanotoxins reduced significantly the fresh mass and the length of cotyledons, hypocotyls and main roots of seedlings in a concentration dependent manner. For various mustard organs the 50% inhibitory concentration values (IC50) of growth were between 3-5 μg ml(-1) for MC-LR and between 5-10 μg ml-1 for CYN, respectively. Cyanotoxins altered the development of cotyledons, the accumulation of photosynthetically active pigments and anthocyanins. Low MC-LR concentrations (0.01 and 0.1 μg ml(-1)) stimulated anthocyanin formation in the cotyledons but higher than 1 μg ml(-1) MC-LR concentrations strongly inhibited it. The CYN treated chlorotic cotyledons were violet coloured in consequence of high level of anthocyanins, while MC-LR induced chlorosis was accompanied by the appearance of necrotic patches. Necrosis and increases of peroxidase enzyme activity (POD) are general stress responses but these alterations were characteristic only for MC-LR treated mustard plants. These findings provide experimental evidences of developmental alterations induced by protein synthesis and protein phosphatase inhibitory cyanotoxins (CYN and MC-LR) in a model dicotyledonous plant.

Cylindrospermopsin inhibits growth and modulates protease activity in the aquatic plants Lemna minor L. and Wolffia arrhiza (L.) Horkel

The toxic effects of cylindrospermopsin (cyanobacterial toxin) on animals have been examined extensively, but little research has focused on their effects on plants. In this study cylindrospermopsin (CYN) caused alterations of growth, soluble protein content and protease enzyme activity were studied on two aquatic plants Lemna minor and Wolffia arrhiza in short-term (5 days) experiments. For the treatments we used CYN containing crude extracts of Aphanizomenon ovalisporum (BGSD-423) and purified CYN as well. The maximal inhibitory effects on fresh weight of L. minor and W. arrhiza caused by crude extract were 60% and 54%, respectively, while the maximum inhibitory effects were 30% and 43% in the case of purified CYN at 20 μg ml(-1) CYN content of culture medium. In CYN-treated plants the concentration of soluble protein showed mild increases, especially in W. arrhiza. Protease isoenzyme activity gels showed significant alterations of enzyme activities under the influence of CYN. Several isoenzymes were far more active and new ones appeared in CYN-treated plants. Treatments with cyanobacterial crude extract caused stronger effects than the purified cyanobacterial toxins used in equivalent CYN concentrations.

ANTIMICROBIAL AND ANTIHYPERLIPIDEMIC ACTIVITIES OF ISOLATED QUERCETIN FROM ANABAENA AEQUALIS(1)

This study aimed to isolate quercetin (for the first time) from Anabaena aequalis Borge, which inhabits soil surface of Wadi El-Alaqui Protectorate located in Aswan city, Egypt. The isolated compound showed significant antibacterial activity against the gram-positive bacteria Sarcina maxima and Micrococcus kristinae, the gram-negative bacterium Klebsiella pneumoniae, as well as against the filamentous fungus Aspergillus flavus. The isolated compound was identified as quercetin using the structure elucidation based on UV, electrospray ionization mass spectrometry (ESIMS), (1) H and (13) C NMR, proton-proton correlation spectroscopy ((1) H-(1) H COSY), distortionless enhancement by polarization transfer (DEPT), heteronuclear single quantum coherence (HSQC), and heteronuclear multiple bond correlations spectrum (HMBC). Medium lethal dose (LD50 ) of the isolated compound and its side effects against hyperlipidemia induced by ethanol intake in albino rats were carried out. No deaths were reported in rats within 72 h, which suggests that the isolated compound plays a beneficial role as an antihyperlipidemic agent in the treatment of alcohol-induced hepatic tissue damage, which can be described as one of the therapeutic values.

Cyanobacteria-mediated phenylpropanoids and phytohormones in rice (Oryza sativa) enhance plant growth and stress tolerance

Phenylpropanoids, flavonoids and plant growth regulators in rice (Oryza sativa) variety (UPR 1823) inoculated with different cyanobacterial strains namely Anabaena oryzae, Anabaena doliolum, Phormidium fragile, Calothrix geitonos, Hapalosiphon intricatus, Aulosira fertilissima, Tolypothrix tenuis, Oscillatoria acuta and Plectonema boryanum were quantified using HPLC in pot conditions after 15 and 30 days. Qualitative analysis of the induced compounds using reverse phase HPLC and further confirmation with LC-MS/MS showed consistent accumulation of phenolic acids (gallic, gentisic, caffeic, chlorogenic and ferulic acids), flavonoids (rutin and quercetin) and phytohormones (indole acetic acid and indole butyric acid) in rice leaves. Plant growth promotion (shoot, root length and biomass) was positively correlated with total protein and chlorophyll content of leaves. Enzyme activity of peroxidase and phenylalanine ammonia lyase and total phenolic content was fairly high in rice leaves inoculated with O. acuta and P. boryanum after 30 days. Differential systemic accumulation of phenylpropanoids in plant leaves led us to conclude that cyanobacterial inoculation correlates positively with plant growth promotion and stress tolerance in rice. Furthermore, the study helped in deciphering possible mechanisms underlying plant growth promotion and stress tolerance in rice following cyanobacterial inoculation and indicated the less explored avenue of cyanobacterial colonization in stress tolerance against abiotic stress.

The role of allelopathy in agricultural pest management

Allelopathy is a naturally occurring ecological phenomenon of interference among organisms that may be employed for managing weeds, insect pests and diseases in field crops. In field crops, allelopathy can be used following rotation, using cover crops, mulching and plant extracts for natural pest management. Application of allelopathic plant extracts can effectively control weeds and insect pests. However, mixtures of allelopathic water extracts are more effective than the application of single-plant extract in this regard. Combined application of allelopathic extract and reduced herbicide dose (up to half the standard dose) give as much weed control as the standard herbicide dose in several field crops. Lower doses of herbicides may help to reduce the development of herbicide resistance in weed ecotypes. Allelopathy thus offers an attractive environmentally friendly alternative to pesticides in agricultural pest management. In this review, application of allelopathy for natural pest management, particularly in small-farm intensive agricultural systems, is discussed.

Detection of reactive oxygen species (ROS) by the oxidant-sensing probe 2',7'-dichlorodihydrofluorescein diacetate in the cyanobacterium Anabaena variabilis PCC 7937

The generation of reactive oxygen species (ROS) under simulated solar radiation (UV-B: 0.30Wm(-2), UV-A: 25.70Wm(-2) and PAR: 118.06Wm(-2)) was studied in the cyanobacterium Anabaena variabilis PCC 7937 using the oxidant-sensing fluorescent probe 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA). DCFH-DA is a nonpolar dye, converted into the polar derivative DCFH by cellular esterases that are nonfluorescent but switched to highly fluorescent DCF when oxidized by intracellular ROS and other peroxides. The images obtained from the fluorescence microscope after 12h of irradiation showed green fluorescence from cells covered with 295, 320 or 395nm cut-off filters, indicating the generation of ROS in all treatments. However, the green/red fluorescence ratio obtained from fluorescence microscopic analysis showed the highest generation of ROS after UV-B radiation in comparison to PAR or UV-A radiation. Production of ROS was also measured by a spectrofluorophotometer and results obtained supported the results of fluorescence microscopy. Low levels of ROS were detected at the start (0h) of the experiment showing that they are generated even during normal metabolism. This study also showed that UV-B radiation causes the fragmentation of the cyanobacterial filaments which could be due to the observed oxidative stress. This is the first report for the detection of intracellular ROS in a cyanobacterium by fluorescence microscopy using DCFH-DA and thereby suggesting the applicability of this method in the study of in vivo generation of ROS.

Photoprotective compounds from marine organisms

The substantial loss in the stratospheric ozone layer and consequent increase in solar ultraviolet radiation on the earth's surface have augmented the interest in searching for natural photoprotective compounds in organisms of marine as well as freshwater ecosystems. A number of photoprotective compounds such as mycosporine-like amino acids (MAAs), scytonemin, carotenoids and several other UV-absorbing substances of unknown chemical structure have been identified from different organisms. MAAs form the most common class of UV-absorbing compounds known to occur widely in various marine organisms; however, several compounds having UV-screening properties still need to be identified. The synthesis of scytonemin, a predominant UV-A-photoprotective pigment, is exclusively reported in cyanobacteria. Carotenoids are important components of the photosynthetic apparatus that serve both light-harvesting and photoprotective functions, either by direct quenching of the singlet oxygen or other toxic reactive oxygen species or by dissipating the excess energy in the photosynthetic apparatus. The production of photoprotective compounds is affected by several environmental factors such as different wavelengths of UVR, desiccation, nutrients, salt concentration, light as well as dark period, and still there is controversy about the biosynthesis of various photoprotective compounds. Recent studies have focused on marine organisms as a source of natural bioactive molecules having a photoprotective role, their biosynthesis and commercial application. However, there is a need for extensive work to explore the photoprotective role of various UV-absorbing compounds from marine habitats so that a range of biotechnological and pharmaceutical applications can be found.

Production of metabolites as bacterial responses to the marine environment

Bacteria in marine environments are often under extreme conditions of e.g., pressure, temperature, salinity, and depletion of micronutrients, with survival and proliferation often depending on the ability to produce biologically active compounds. Some marine bacteria produce biosurfactants, which help to transport hydrophobic low water soluble substrates by increasing their bioavailability. However, other functions related to heavy metal binding, quorum sensing and biofilm formation have been described. In the case of metal ions, bacteria developed a strategy involving the release of binding agents to increase their bioavailability. In the particular case of the Fe³⁺ ion, which is almost insoluble in water, bacteria secrete siderophores that form soluble complexes with the ion, allowing the cells to uptake the iron required for cell functioning. Adaptive changes in the lipid composition of marine bacteria have been observed in response to environmental variations in pressure, temperature and salinity. Some fatty acids, including docosahexaenoic and eicosapentaenoic acids, have only been reported in prokaryotes in deep-sea bacteria. Cell membrane permeability can also be adapted to extreme environmental conditions by the production of hopanoids, which are pentacyclic triterpenoids that have a function similar to cholesterol in eukaryotes. Bacteria can also produce molecules that prevent the attachment, growth and/or survival of challenging organisms in competitive environments. The production of these compounds is particularly important in surface attached strains and in those in biofilms. The wide array of compounds produced by marine bacteria as an adaptive response to demanding conditions makes them suitable candidates for screening of compounds with commercially interesting biological functions. Biosurfactants produced by marine bacteria may be helpful to increase mass transfer in different industrial processes and in the bioremediation of hydrocarbon-contaminated sites. Siderophores are necessary e.g., in the treatment of diseases with metal ion imbalance, while antifouling compounds could be used to treat man-made surfaces that are used in marine environments. New classes of antibiotics could efficiently combat bacteria resistant to the existing antibiotics. The present work aims to provide a comprehensive review of the metabolites produced by marine bacteria in order to cope with intrusive environments, and to illustrate how such metabolites can be advantageously used in several relevant areas, from bioremediation to health and pharmaceutical sectors.