**3. Cyanobacterial secondary metabolites by function**

#### **3.1. Toxic metabolites**

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,* and *Nostoc* [20].

Although dangerous to animals, fish and humans, these toxins have potential uses as biocides (algaecides, fungicides, herbicides and insecticides) and pharmaceuticals (antimicrobial, anti-

Mycosporine-like amino acids (MAAs) are a group of about 30 colourless, water soluble, low molecular weight molecules found primarily within the cytosol of cells and sometimes found glycosylated on the outer cell membrane such as in *Nostoc commune* [25]. MAAs have strong absorption in the UV region between 310 and 365 nm [2] with high molar extinction coefficients (ε = 28,100–50,000 l·mol−1·cm−1) providing photoprotection with the ability to disperse

They consist of cyclohexenone or cyclohexenimine chromophores conjugated to nitrogen substituents from amino acids or imino alcohols. The variety in absorption is due to the differing

There are two biosynthetic routes involved in the production of MAAs. The first is the shikimate pathway (biosynthesis of aromatic acids) [29], by first forming deoxy-D-arabinoheptulosonate-7-phosphate (DAHP) from phosphoenolpyruvate (PEP) and erythrose-4-phosphate (E4P) using DAHP synthase. DAHP is then converted to 3-dehydroquinate and subsequent transformation into 4-deoxygadusol (4-DG). The primary MAA mycosporine-glycine is then formed from the reaction of 4-DG with glycine, which can then be converted into a secondary MAA by addition of other amino acids such as serine (to produce shinorine) and threonine (to produce porphyra-334) [25]. The other pathway involved is the pentose-phosphate pathway, which also produces the intermediate 4-DG from sedoheptulose-7-phosphate *via* 2-*epi*-5-*epi*-

Another photoprotective metabolite produced by cyanobacteria alone is scytonemin (**Figure 1 (11)**), this is located in the extracellular polysaccharide sheath of cyanobacteria [19]. With a molecular weight of 544 Da it is a hydrophobic alkaloid comprising of idolic and phenolic substituents usually linked by a carbon–carbon double bond. It has an absorption maximum at 380 nm [2, 26]. Scytonemin has an extinction coefficient of 136,000 l·mol−1 cm−1 at 384 nm, which makes it an excellent photo-protective compound. It is biosynthesized in response to UV-A and has two major forms, an oxidised state (brown) and reduced state (red).

Unavoidably ROS are produced by cyanobacteria during photosynthesis and respiration. Abiotic factors that produce these species include UVR, osmotic perturbations, desiccation

), superoxides (O2

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

•−) and hydroxyl radicals (OH•) which

Secondary Metabolites in Cyanobacteria http://dx.doi.org/10.5772/intechopen.75648 29

O2

damage biomolecules within cells are all examples of ROS [30].

**3.2. Photoprotective metabolites: Mycosporine-like amino acids (MAAs) and** 

cancer, antiviral and immunosuppressant) [15, 23].

energy without producing reactive oxygen species (ROS) [26].

nitrogen substituents and side groups [27, 28] (**Table 3**).

**scytonemin**

violiolone [19].

**3.3. Antioxidants**

and heat. Hydrogen peroxides (H2

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 anatoxins [15].


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

Although dangerous to animals, fish and humans, these toxins have potential uses as biocides (algaecides, fungicides, herbicides and insecticides) and pharmaceuticals (antimicrobial, anticancer, antiviral and immunosuppressant) [15, 23].
