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

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

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

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*violiolone [19].

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).
