**3.3. Antioxidants**

**3. Cyanobacterial secondary metabolites by function**

A wide variety of toxic metabolites (**Table 2**) are produced by cyanobacteria that have a negative effect on target species in their surrounding areas and are referred to as cyanotoxins [2]. These toxins are found during cyanobacterial blooms on stagnant surface water bodies. Cyanobacteria that bloom include the unicellular *Microcystis* and the filamentous *Anabaena,*

Cyanotoxins have a diverse range of chemical structures including ribosomal peptides and NRPs, polyketides alkaloids and lipopolysaccharides. These toxins can be classified according to their biological effect; neurotoxins targeting the nervous system, hepatotoxins targeting the liver, cytotoxins targeting cells, dermatoxins targeting the skin or endotoxins, which are irritants [15]. The most prevalent and potent hepatotoxins are the cyclic peptides microcystins, which are produced through NRPS in *Microcystis, Anabaena, Planktothrix* and *Nostoc* [15]. An example of a non-protein amino acid neurotoxin is β-N-methylamino-L-alanine, which can be produced by a variety of cyanobacteria [21]. It was originally isolated from cycad seeds in Guam and many investigations have implicated this neurotoxin in neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS) and Parkinsonism dementia complex (PDC) [22]. Other neurotoxins include saxitoxin (paralytic shellfish poisons) and the

**Cyanotoxin Biological effects Cyanobacteria**

phosphatases (types 1 and 2A)

phosphatases (types 1 and 2A)

channels, causing neuronal communication

β-*N*-methylamino-L-alanine Neurotoxin, damages motor neurons Many species including; *Anabaena,* 

Lipopolysaccharides Endotoxin, irritant *Microcystis, Anabaena, Spirulina,* 

*Microcystis, Anabaenopsis, Nostoc*

*Aphanizomenon*, *Anabaena*, *Lyngbya*

*Cylindrospermum, Planktothrix,* 

*Nodularia*

*Oscillatoria*

*Nostoc,* [24]

*Oscillatoria*

*Lyngbya majuscula*

*Lyngbya majuscula*

Microcystin Hepatotoxin, inhibits eukaryotic protein

Nodularin Hepatotoxin, inhibits eukaryotic protein

Saxitoxin Neurotoxin, binds to voltage-gated Na+

Lyngbyatoxin Cytotoxin, binds to protein kinase C,

Aplysiatoxins Cytotoxin, binds to protein kinase C,

**Table 2.** Cyanotoxins and their biological effect.

blockage

Anatoxin-a Neurotoxin, binds to nicotinic acetylcholine

receptors irreversibly

tumour promoting

tumour promoting

**3.1. Toxic metabolites**

28 Secondary Metabolites - Sources and Applications

and *Nostoc* [20].

anatoxins [15].

Unavoidably ROS are produced by cyanobacteria during photosynthesis and respiration. Abiotic factors that produce these species include UVR, osmotic perturbations, desiccation and heat. Hydrogen peroxides (H2 O2 ), superoxides (O2 •−) and hydroxyl radicals (OH•) which damage biomolecules within cells are all examples of ROS [30].

Cyanobacteria require multiple approaches to prevent inhibitory effects of stressful environments. They can prevent the production of ROS by energy dissipation in the photosynthetic

roles within the cells [32]. They are water soluble proteins that are brightly coloured due to the covalently attached linear tetrapyrrole prosthetic groups called bilins, which gives rise to cyanobacteria prominent colour. They, along with linker protein are able to form giant supra-

Secondary Metabolites in Cyanobacteria http://dx.doi.org/10.5772/intechopen.75648 31

Sustainability in industry is increasingly important due to global warming and the depletion of fossil fuels. A considerable amount of research has been conducted to find new sources of industrially important compounds to reduce the carbon footprint and increase sustainability. Cyanobacteria has received much interest in becoming a promising alternative due to their diversity, simple growth needs and simple genetic background, which are easily manipulated

Some strains of cyanobacteria are already being used in industry, examples include the edible *Arthrospira* (*Spirulina*) and *Nostoc*, which have been used as a food source for thousands of

*Spirulina* has been well researched for its application within industry. It is used as a health food due to its extensive source of proteins, polyunsaturated fatty acids (γ-linoleic acid, GLA),

A challenge remains in assessing and understanding the ability of cyanobacteria to produce target metabolites in sufficient quantities to be of use under standard and repeatable conditions. This will be easier moving into the future as 'omic' studies enable improved under-

Natural products have been used to treat disease for thousands of years and are a useful source of bioactive compounds used in the pharmaceutical industry as leading compounds in drug discovery. They can be used as templates for synthesis of new drugs to treat complex diseases. Cyanobacteria have been widely researched for their applications in this field. They have found to possess a wide range of potential antimicrobial, anticancer, antiviral and anti-

Chemotherapies currently used in the treatment of cancer cause serious side effects; naturally derived alternatives give opportunities for synthesising new highly potent drugs with fewer side effects [15, 42]. Cytotoxic metabolites produced by cyanobacteria usually target tubulin or actin filaments in eukaryotic cells, which make them promising anticancer agents. Dolastatins found within *Leptolyngbya* and *Simploca* sp. are synthesised by NRPS-PKS enzymes and are able to disrupt microtubule formation. Other cyanobacterial metabolites act as proteases inhibitors such as the lyngbyastatins, which are cyclic depsipeptide derivatives, which are

inflammatory activities [37]. Some known bioactives are listed below (**Table 4**) [11].

antioxidants (phycocyanin and carotenoids) and vitamins [36].

standing on metabolite pathways using a whole systems approach.

**4. Potential of cyanobacterial secondary metabolites in industrial** 

molecular structures known as phycobilisomes [33].

**biotechnology**

to form cell factories [34].

**4.1. Pharmaceuticals and cosmetics**

years [35].

**Table 3.** Example of MAAs [27, 28].

apparatus. One mechanism is the non-photochemical quenching (NPQ) of excitation energy *via* photosystem II using the carotenoid zeaxanthin. They also produce enzymatic antioxidants such as; superoxide dismutases (SOD), catalases and peroxidases) as well as nonenzymatic antioxidants such as; carotenoids, phycobiliproteins, tocopherols and ascorbic acid [31].

Carotenoids absorbs light in the region of 400–500 nm and have several roles including sunscreening, singlet oxygen quenching, releasing excessive light as heat through the xanthophyll cycle and radical scavenging [30].

Another group of antioxidants are the phycobiliproteins (PBP). These are present only in cyanobacteria and are primarily used as major light harvesting antennae but also have antioxidant roles within the cells [32]. They are water soluble proteins that are brightly coloured due to the covalently attached linear tetrapyrrole prosthetic groups called bilins, which gives rise to cyanobacteria prominent colour. They, along with linker protein are able to form giant supramolecular structures known as phycobilisomes [33].
