J Appl Phycol (2011) 23:1059–1081 DOI 10.1007/s10811-010-9626-3
Sustainability and cyanobacteria (blue-green algae): facts and challenges Naveen K. Sharma & Sri Prakash Tiwari & Keshwanand Tripathi & Ashwani K. Rai
Received: 4 November 2009 / Revised and accepted: 22 September 2010 / Published online: 18 November 2010 # Springer Science+Business Media B.V. 2010
Abstract Cyanobacteria (blue-green algae) are widely distributed Gram-negative oxygenic photosynthetic prokaryotes with a long evolutionary history. They have potential applications such as nutrition (food supplements and fine chemicals), in agriculture (as biofertilizer and in reclamation of saline USAR soils) and in wastewater treatment (production of exopolysaccharides and flocculants). In addition, they also produce wide variety of chemicals not needed for their normal growth (secondary metabolites) which show powerful biological activities such as strong antiviral, antibacterial, antifungal, antimalarial, antitumoral and anti-inflammatory activities useful for therapeutic purposes. In recent years, cyanobacteria have gained interest for producing biofuels (both biomass and H2 production). Because of their simple growth needs, it is potentially cost-effective to exploit cyanobacteria for the production of recombinant compounds of medicinal and commercial value. Recent advances in culture, screening and genetic engineering techniques have opened new ways to exploit the potential of cyanobacteria. This review analyses the sustainability of cyanobacteria to solve global problems such as food, energy and environmental degradation. It emphasizes the need to adopt multidisciplinary N. K. Sharma Department of Botany, Postgraduate College, Ghazipur, Uttar Pradesh 233001, India S. P. Tiwari Department of Applied Microbiology, VBS Poorvanchal University, Jaunpur, Uttar Pradesh 221001, India K. Tripathi : A. K. Rai (*) Department of Botany, Banaras Hindu University, Varanasi, Uttar Pradesh 221005, India e-mail:
[email protected]
approaches and a multi-product production (biorefinery) strategy to harness the maximum benefit of cyanobacteria. Keywords Agriculture . Aquaculture . Bioactive compounds . Cyanobacteria . Energy . Human health . Nutraceuticals . Secondary metabolites . Toxins
Introduction Issues related to environment, food and energy have presented serious challenge to the stability of nation-states (Tilman et al. 2009). Increasing global population, dwindling agriculture and industrial production, and inequitable distribution of resources and technologies have further aggravated the problem. They are entwined in such a way that it becomes almost impossible to grade their severity. The burden placed by increasing population on environment and especially on agricultural productivity is phenomenal. To feed such a massive population, it becomes imperative to find new ways and means to increase the production giving due consideration to biosphere’s ability to regenerate resources and provide ecological services. Cyanobacteria (blue-green algae) are Gram-negative oxygenic photosynthetic autotrophs, and are amongst the most successful and oldest life forms present (Schopf 2000; Gademan and Portman 2008). Globally, they are important primary producers and play significant roles in biogeochemical cycles of nitrogen, carbon and oxygen (30% of the annual oxygen production on earth) (Karl et al. 2002; DeRuyter and Fromme 2008). They are the organisms responsible for bringing oxygen on the earth therefore played a key role in the evolution of life. Some cyanobacteria perform unique biological process (combine N2fixation with oxygenic photosynthesis) and can be a model
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to study important biological activities or capabilities (Herrero and Flores 2008). In the early atmosphere the bulk of oxygen came from oxygenic photosynthesis by cyanobacteria (Schopf 2000). Geological records indicate that this transforming event occurred at least some 2.4–2.3 Ga ago or even earlier (Knoll 2008). However, the question of when oxygenic photosynthesis evolved still remains a contentious issue (Knoll 2008). The stromatolites of fossilised oxygen producing cyanobacteria have been reported from 2.8 Ga ago (Olson 2006) and Knoll (2008) considered cyanobacteria as the principal primary producers throughout the Proterozoic Eon (2.5–0.5 Ga). The cyanobacteria consist of a heterogeneous assemblage of oxygen evolving photosynthetic prokaryotes, and are composed of about 150 genera and 2,000 species (Pulz and Gross 2004) including unicellular, colonial, filamentous to branched filamentous forms (Thajuddin and Subramanian 2005). They are divided into five subsections (Rippka et al. 1979; Castenholz 2001). Cyanobacteria are widely distributed in habitats ranging from aquatic to terrestrial environments as well as extreme habitats such as hot springs, hypersaline waters, deserts, and Polar Regions (Whitton and Potts 2000). During their long evolutionary history, cyanobacteria have undergone several structural and functional modifications responsible for their versatile physiology and wide ecological tolerance. Their abilities to tolerate high temperature, UV radiation, desiccation, water and saline stresses contribute to their competitive success in a wide range of environments (Gröniger et al. 2000; Whitton and Potts 2000; Herrero and Flores 2008). They can photosynthesise at low photon densities and use bicarbonate ion for photosynthesis at high pH (Shapiro 1972). Many species fix atmospheric N2 in usable (soluble) form. Also, they can use diverse sulphur sources (Wolk 1973), and show efficient phosphate acquisition mechanisms (Rai and Sharma 2006). Other factors include their resistance to predation pressure and ability to regulate cellular buoyancy. Ray and Bagachi (2001) reported that some cyanobacteria produce allelopathic compounds that may assist in their dominance in the phytoplanktonic community. Cyanobacteria have mostly been studied for their prokaryotic organisation, mechanism of photosynthesis, especially the structure of photosynthetic complexes (PSI and II) (DeRuyter and Fromme 2008), N2-fixing ability of certain forms, genetic make-up and structural aspects such as gas vacuoles and akinetes (spores). Credit for origin of eukaryotic plant life over the planet also goes to cyanobacteria, as the chloroplast of eukaryotic cell is derived from a cyanobacterial ancestor (Delwiche and Palmer 1997; Tomitani et al. 2006). The economic importance of cyanobacteria primarily lies in their agronomic importance as biofertilizers due to N2-
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fixation that helps them to grow successfully in habitats where little or no combined N is available. In recent times, their ability to produce structurally novel and biologically active natural products has been recognised (Ordog et al. 2004; Gademan and Portman 2008; Sielaff et al. 2008; Wase and Wright 2008; Rosenberg et al. 2008; Tan 2010). They produce a wide variety of chemically unique secondary metabolites that include toxins, hormones, iron chelators, antibiotics, antifungal, antitumor, inflammatory and anti-inflammatory compounds. Chemically, these compounds may be peptides, alkaloids and indole alkaloids, polyketide and terpenes (Gademan and Portman 2008). Many of these compounds display significant pharmaceutical potential. Use of cyanobacteria as a food supplement has a long history (Gantar and Svircev 2008). Spirulina (now known as Arthrospira) had been collected and used by Aztec population (Pulz and Gross 2004). Even today, malnutrition, especially due to a protein-poor diet is widespread in many parts of the world. The use of cyanobacteria as a nonconventional source of food and protein seems promising (Pulz and Gross 2004; Gantar and Svircev 2008; Rosenberg et al. 2008). Also, they may be used as a source of natural chemicals to substitute synthetic cosmetics (Burja et al. 2001; Singh et al. 2005) and conventional energy resources (Deng and Coleman 1999; Dutta et al. 2005). Cyanobacteria are a highly diverse group, largely unexplored and untapped therefore, present an opportunity for discovery of novel biochemicals and their use by humankind (Skulberg 2000; Rosenberg et al. 2008). Certain cyanobacteria, known as extremophyles, inhabit extreme environments, e.g. Spirulina (alkalophilic), Mastigocladus laminosus (thermophilic), Aphanothece halophytica (halophilic) etc. Because of their extreme requirements, mass cultures of extremophyles is likely to be free from microbial contamination thus, avoiding a serious problem in outdoor cultures (Pulz and Gross 2004). Extremophilic cyanobacteria represent a potential source of biotechnologically important molecules and enzymes.
Toxic secondary metabolites (Cyanotoxins) Cyanobacteria occupy nearly all types of aquatic bodies (fresh, brackish and marine ecosystems), with possible exception of acidic waters. Under favourable growth conditions, cyanobacteria often dominate aquatic bodies resulting in cyanobacterial blooms. In the last 20 years, excessive nutrient inputs from agricultural fertiliser run off, and/or domestic and industrial effluents, as well as climatic change have greatly enhanced the incidence of cyanobacterial blooms globally (Paerl and Huisman 2009). Many of the bloom-forming cyanobacteria produce toxic secondary metabolites (cyanotoxins), mostly active
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against metazoans, and present a potential risk to public health (Paerl et al. 2001). Toxic cyanobacteria are widely distributed and have been reported from every continent. Nearly 50–75% of the cyanobacterial blooms are toxic, but out of 40 toxic cyanobacterial genera, only 14 have been characterised for their toxicity (Skulberg et al. 1993; Ouellette and Wilhelm 2003). With the global increase in the incidence of harmful cyanobacterial blooms (cyanoHAB), cyanotoxin research has expanded. In 1986, only six countries were involved in active research programmes on cyanotoxins, today almost all major nations have institute and/or university-based research programmes on cyanotoxins (Jaiswal et al. 2008). Cyanotoxins constitute a diverse group of chemical substances categorised variously depending upon their toxicity mechanism, organs affected (hepatotoxins, neurotoxins, dermatotoxins and cytotoxins) and chemical structure (cyclic peptides, alkaloids and lipopolysaccharides (LPS)). Based on bioassay, a cyanotoxin may be cytotoxic or biotoxic (Sivonen and Jones 1999). Cytotoxins (e.g. acutiphycins, indocarbazoles, mirabilene, isonitriles, para-
cyclophanes, scytophycins, tentazoles, tolytoxin, toyocamycins and tubercidin) are lethal to cultured mammalian cell lines but not to organs/systems wheras biotoxins affect the organs and/or tissue systems. Due to their lethality and ubiquity, biotoxins have received more attention; however, cytotoxins are equally important owing to their antimicrobial and antitumor activities (Jaiswal et al. 2008) and potentially can be exploited for therapeutic use (Gademan and Portman 2008). Table 1 lists common cyanobacterial biotoxins and their characteristics. Production of cyanotoxins varies with species, strains, culture conditions, age and other environmental factors (Ouellette and Wilhelm 2003). A toxic cyanobacterium may or may not produce toxins. A cyanobacterium may produce several toxins simultaneously, but only one or two usually dominate (Ouellette and Wilhelm 2003; Codd et al. 2005). Cyanotoxin production varies with growth phase (higher toxin content at late-log phase), environmental factors such as solar radiation (increased toxicity with increasing irradiance), water temperature (lower toxin concentration at sub- and super-optimal temperature), pH (greater at sub-
Table 1 Attributes of some well-known cyanobacterial biotoxins (Hitzfeld et al. 2000; Jaiswal et al. 2008) LD50 (μg kg-1 i.p. Structure mouse)
Mode of action
Organisms
Microcystins
50≥1,200
Hepta-cyclic peptides
Affects cytoskeleton of hepatocytes, tumour promotion by inhibiting protein phosphatase 1 and 2A
Microcystis aeruginosa, Planktothrix sp., Nostoc sp., Anabaena sp., Anabaenopsis sp., Synechocystis sp., Cyanobium bacillare, Arthrospira fusiformis, Limnothrix redekei, Phormidium formosum, Hapalosiphon hibernicus, Radiocystis feernandoi, Fischerella sp. strain CENA161
Nodularins
50–2,000
Penta-cyclic peptides
Affects cytoskeleton of hepatocytes by inhibiting protein phosphatase 1 and 2A Block protein synthesis
Nodularia spumigena
Cyanotoxins/Type
Hepatotoxins
Cylindrospermopsin 2,000
Alkaloid
Cylindrospermopsis raciborskii, Aphanizomenon ovalisporum, Aphanizomenon flos-aquae, Umezakia natans, Raphidiopsis curvata, Anabaena japonica, Anabaena bergii
Neurotoxins Anatoxins A
200–250
Alkaloid, secondary amine
Inhibits the release of Anabaena spp., Planktothrix spp., acetylcholine by neurons Aphanizomenon spp., Cylindrospermum sp., (post-synaptic depolarization) Raphidiopsis mediterranea, Microcystis sp.
Anatoxins A(s)
20
Alkaloid
Inhibits acetylcholinesterase
Anabaena lemmermanni, Anabaena flos-aquae, Aphanizomenon flos-aquae
Homoanatoxin A
–
Alkaloids
Inhibit post-synaptic depolarization
Oscillatoria Formosa, Raphidiopsis mediterranea
Saxitoxins (PSP)
10
Corbamate alkaloid
Inhibits nerve conduction by blocking Na channels
Anabaena circinalis, Aphanizomenon flos-aquae, Cylindrospermum raciborskii, Lyngbya wollei
–
Lipopolysaccharides Skin and mucosa
Dermatotoxins LPS endotoxins
i.p. intra-peritoneal administration of toxin
All cyanobacteria; Lyngbya sp., Schizothrix sp., Planktothrix sp.
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and super-optimal pH), nitrogen and carbon (their limitation causes 5–6-fold decrease), iron concentration and N/P ratio (see for example, Veize et al. 2002; Jiang et al. 2008; Jaiswal et al. 2008; Li et al. 2009). The ecological and physiological significance of cyanotoxin production is not yet clear (Paerl et al. 2001). They are supposed to act as deterrent against zooplankton grazing, in metal ion chelation, intraspecific signalling and in allelopathatic interactions (Dittmann et al. 2001; Rohrlack et al. 2001; Pflugmacher 2002; Prince et al. 2008). Cyanotoxins are either membrane-bound or occur free within the cells. They are released into the medium (natural ecosystems) with the ageing and death of the cyanobacterial population and/or during water treatment processes wherein, they undergo photo- and bacterial degradation. Consequently, a significant fraction of the released cyanotoxin becomes unavailable. They are also adsorbed on soil surfaces depending upon environmental factors, soil property and total organic content of the soil (Tsuji et al. 2006; Edwards et al. 2008). Cyanotoxins have adverse effect on other aquatic biota. Use of cyanobacteria infested waters for irrigation could be a threat to the yield and quality of crops (Zurawell et al. 2005; Bibo et al. 2008). Primary symptoms associated with cyanotoxin-exposed plants include oxidative damage and growth inhibition (Chen et al. 2004; Järvenpää et al. 2007; Pflugmacher et al. 2007; Saqrane et al. 2008). In animals (including humans), exposure to cyanotoxins is possible through ingestion of contaminated food and water, inhalation of the aerosolized toxins or living cells, dermal contact with toxic cyanobacteria and the toxins released in milieu (Funari and Testai 2008). It was found that sensitivity to inhaled toxins is higher than to ingestion, and differences in the route of exposure manifest different clinical symptoms (Kirkpatrick et al. 2004). Ingestion of cyanotoxin-contaminated products has far greater health significance than the exposure through water consumption or recreational activities (Funari and Testai 2008). Cyclic peptides (microcystins and nodularins) Cyclic peptides (esp., microcystins) are the most widely distributed form of cyanotoxins. They are relatively small (MW 800–1,100 Da) in comparison to other cell oligopeptides and polypeptides, and are synthesised nonribosomally by a peptide synthetase (Tillett et al. 2000; Kaebernick et al. 2002). Hydrophilic in nature, cyclic peptides enter cells through membrane transporters. They are extremely potent tumour promoters inhibiting protein phosphatases (PP) 1 and 2A (Nishiwaki-Matsushima et al. 1992) and the liver is the main organ affected by microcystins and nodularin.
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Microcystins (MCs) are amongst the most studied class of cyanotoxins. They are cyclic heptapeptide described as cyclo (- D -Alap 1 -Xaa 2 -D-MeAsp 3 -Yaa 4 Adda5-D-Glu6-Mdha7-). The amino acids Xaa2 and Yaa4 are highly variable, and are responsible for a number of MC variants. For example, MC-LR has amino acids leucine (L) at Xaa2 and arginine (R) at Yaa4 positions (Gulledge et al. 2002). To date, more than 70 variants of MCs are known (Jayaraj et al. 2006). Toxicity of the MCs results exclusively due to Adda region [β-amino acid (2S,3S,8S,9S)-3-amino-9-methoxy-2,6,8-trimethyl-10phenyldecy-4(E), 6(E)-dienoic acid], which is covalently bound to proteins (Hitzfeld et al. 2000; Gulledge et al. 2002). Reports of liver cancer due to consumption of microcystins containing drinking water have increased substantially recently (Ouellette and Wilhelm 2003; Codd et al. 2005). Krakstad et al. (2006) reported that the PPinhibiting toxins induce morphological alteration compatible with apoptosis in different cell types. Ding et al. (1999) found that MC-LR has strong mutagenicity and induced DNA damage in rat hepatocytes. MCs also enhanced bone marrow micronucleated polychromatic erythrocytes in mice. The World Health Organization has set a threshold value of 1 μg L−1 for MCs in drinking waters. In June 2006, MC-LR was classified as a possible human carcinogen (group 2B) (Grosse et al. 2006). Chronic consumption of MCs present in tap water (low dose) could be a substantial risk factor for liver and colorectal cancer (Hernández et al. 2009). Nodularin (NODLN) is a cyclic pentapeptide composed of Adda and D-erythro-β-methylaspartic acid as well as Nmethyldehydrobutyrine. Saito et al. (2001) reported a modified structure of nodularins (NODLN-Har) from Nodularia PCC 7804 containing homoarginine instead of arginine. Nodularin acts similar to microcystins except that it does not bind covalently to PP1 or PP2A (Bagu et al. 1997). Alkaloids Alkaloids constitute a broad group of heterocyclic nitrogenous compounds of low to moderate molecular weight (
