Mycosporine-like amino acids (MAAs) are small secondary metabolites produced by organisms that live in environments with high volumes of sunlight, usually marine environments. So far there are up to 20 known MAAs identified. They are commonly described as “microbial sunscreen” but their function is not limited to sun protection.
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Background
MAAs are widespread in the microbial world and have been reported in many microorganisms including heterotrophic bacteria, cyanobacteria, microalgae, macroalgae, ascomycetous and basidiomycetous fungi, as well as some multi-cellular organisms. Most research done on MAAs is on their light absorbing and radiation protecting properties. The first thorough description of MAAs was done in cyanobacteria living in a high UV radiation environment. The major unifying characteristic among all MAAs is light absorption. All MAAs absorb UV light that can be destructive to biological molecules (DNA, Proteins, etc.). Though most MAA research is done on their photo-protective capabilities, they are also multifunctional secondary metabolites that have many cellular functions. MAAs are effective antioxidant molecules and are able to stabilize free radicals within their ring structure. In addition to protecting cells from mutation via UV radiation and free radicals, MAAs are able to boost cellular tolerance to desiccation, salt stress, and heat stress.
Chemistry
Mycosporine–like amino acids are rather small molecules (<400Da). The structures of over 30 Mycosporine-like amino acids have been resolved and all contain a central cyclohexenone or cyclohexenimine ring and a wide variety of substitutions. The ring structure is thought to absorb UV light and accommodate free radicals. All MAAs absorb ultraviolet light, typically between 310 and 340 nm. It is this light absorbing property that allows MAAs to protect cells from harmful UV radiation. Biosynthetic pathways of specific MAAs depend on the specific MAA and the organism that is producing it. These biosynthetic pathways often share common enzymes and intermediates with other major biosynthetic pathways. An example of this is the shikimate pathway that is classically used to create phenylalanine; many intermediates and enzymes from this pathway are utilized in MAA synthesis.
Protection from UV radiation
Ultraviolet UV-A and UV-B radiation is harmful to living systems. An important tool used to deal with UV exposure is the biosynthesis of small-molecule sunscreens. Mycosporine-like amino acids (MAAs) have been implicated in UV radiation protection. The genetic basis for this implication comes from the observed induction of MAA synthesis when organisms are exposed to UV radiation (between 280-400 nm). Among many different organisms, this observation has occurred in aquatic yeasts, cyanobacteria, marine dinoflagellates, and some Antarctic diatoms. When MAAs absorb UV light the energy is dissipated as heat. UV-B photoreceptors have been identified in cyanobacteria as the molecules responsible for the UV light induced responses, including synthesis of MAAs.
Protection from oxidative damage
Some MAAs also protect cells from reactive oxygen species (i.e. singlet oxygen, superoxide anions, hydroperoxyl radicals, and hydroxyl radicals). Reactive oxygen species can be created during photosynthesis; further supporting the idea that MAAs provide protection from UV light. Mycosporine-glycine is a MAA that provides antioxidant protection even before Oxidative stress response genes and antioxidant enzymes are induced. MAA-glycine (mycosporine-glycine) is able to quench singlet oxygen and hydroxyl radicals very quickly and efficiently. Some oceanic microbial ecosystems are exposed to high concentrations of oxygen and intense light; these conditions are likely to generate high levels of reactive oxygen species. In these ecosystems, MAA-rich cyanobacteria may be providing antioxidant activity.
Accessory pigments in photosynthesis
MAAs are able to absorb UV light. A study published in 1976 demonstrated that an increase in MAA content was associated with an increase in photosynthetic respiration. Further studies done in marine cyanobacteria showed that the MAAs synthesized in response to UV-B correlated with an increase in photosynthetic pigments. Though not absolute proof, these findings do implicate MAAs as accessory pigments to photosynthesis.
Photoreceptors
They eyes for the mantis shrimp contain four different kinds of mycosporine-like amino acids as filters, which combined with two different visual pigments assist the eye to detect six different bands of ultraviolet light. Three of the filter MAAs are identified with porphyra-334, mycosporine-gly, and gadusol.
Salt stress
Osmotic stress is defined as difficulty maintaining proper fluids in the cell within a hypertonic or hypotonic environment. MAAs accumulate within a cell’s cytoplasm and contribute to the osmotic pressure within a cell, thus relieving pressure from salt stress in a hypertonic environment. As evidence of this, MAAs are seldom found in large quantities in cyanobacteria living in freshwater environments. However, in saline and hypertonic environments, cyanobacteria often contain high concentrations of MAAs . The same phenomenon was noted for some halotolerant fungi. But, the concentration of MAAs within cyanobacteria living in hyper-saline environments is far from the amount required to balance the salinity. Therefore, additional osmotic solutes must be present as well.
Desiccation stress
Desiccation (drought) stress is defined as conditions where water becomes the growth limiting factor. MAAs have been reportedly found in high concentrations in many microorganisms exposed to drought stress. Particularly cyanobacteria species that are exposed to desiccation, UV radiation and oxidation stress have been shown to possess MAA’s in an extracellular matrix. However it has been shown that MAAs do not provide sufficient protection against high doses of UV radiation.
Thermal stress
Thermal (heat) stress is defined as temperatures lethal or inhibitory towards growth. MAA concentrations have been shown to be up-regulated when an organism is under thermal stress. Multipurpose MAAs could also be compatible solutes under freezing conditions, because a high incidence of MAA producing organisms have been reported in cold aquatic environments.