UV-B-induced synthesis of photoprotective pigments and extracellular polysaccharides in the terrestrial cyanobacterium Nostoc commune

Abstract

Liquid cultures of the terrestrial cyanobacterium Nostoc commune derived from field material were treated with artificial UV-B and UV-A irradiation. We studied the induction of various pigments which are thought to provide protection against damaging UV-B irradiation. First, UV-B irradiation induced an increase in carotenoids, especially echinenone and myxoxanthophyll, but did not influence production of chlorophyll a. Second, an increase of an extracellular, water-soluble UV-A/B-absorbing mycosporine occurred, which was associated with extracellular glycan synthesis. Finally, synthesis of scytonemin, a lipid-soluble, extracellular pigment known to function as a UV-A sunscreen, was observed. After long-time exposure, the UV-B effect on carotenoid and scytonemin synthesis ceased whereas the mycosporine content remained constantly high. The UV-B sunscreen mycosporine is exclusively induced by UV-B (<315 nm). The UV-A sunscreen scytonemin is induced only slightly by UV-B (<315 nm), very strongly by near UV-A (350 to 400 nm), and not at all by far UV-A (320 to 350 nm). These results may indicate that the syntheses of these UV sunscreens are triggered by different UV photoreceptors.The terrestrial nitrogen-fixing cyanobacterium Nostoc commune Vaucher flourishes in extremely cold and dry habitats which are characterized by intense solar radiation, extreme temperature differences, and regular periods of desiccation (34, 42; for a review, see reference 7). N. commune, in its natural habitat, forms macroscopic colonies with filaments embedded in gelatinous glycan. In the past, most studies concentrated on the extraordinary drought resistance of N. commune (33; for a review, see reference 29), but only few investigated its UV tolerance (35,41).Mechanisms counteracting UV-B damage have been demonstrated in plants and cyanobacteria. Besides repair of UVinduced damages of DNA by excision repair and photoreactivation (10, 26) and accumulation of detoxifying enzymes and carotenoids (24,25), an important mechanism to prevent UV photodamage is the synthesis of UV-absorbing compounds. Several studies provide evidence that epidermally located phenylpropanoids, especially flavonoid derivatives, protect higher plants by absorbing harmful UV radiation (22, 37). Mycosporine amino acids (MAAs) are thought to fulfill a comparable purpose in lower organisms (14, 21). MAAs are water-soluble, substituted cyclohexenes which are linked to amino acids and iminoalcohols and have absorption maxima between 310 and 360 nm. Scytonemin, which has an in vivo absorption maximum at 370 nm and is located in the cyanobacterial sheath, has been proposed to serve as a UV-A sunscreen (13). It is a yellowbrown, lipid-soluble dimeric pigment of terrestrial cyanobacteria with a molecular mass of 544 Da and a structure based on indolic and phenolic subunits (30).A UV-A/B-absorbing pigment with absorption maxima at 312 and 335 nm was found in N. commune colonies exposed to high solar radiation (35). Recently, its chemical structure has been shown to be an oligo...

Identification of a fourth family of lycopene cyclases in photosynthetic bacteria

“…Cyanobacteria may need two cyclases to regulate production of mono-and dicyclic carotenoids independently. Myxoxanthophyll has been shown to accumulate in cyanobacteria in response to UV radiation while ␤-carotene levels remain constant (59,60). Independent regulation of these genes may also explain the phenotypes of the cruP and cruA mutants.…”

Effects of UV Radiation on Wetland Periphyton: Algae, Bacteria, and Extracellular Polysaccharides

“…It is possible that periphytic EPS in our study system was relatively more resistant to UV degradation, or that any UV-induced degradation of EPS was offset by increased periphytic biomass or EPS production. For example, UV exposure cues the cyanobacterium Nostoc commune to produce additional EPS, possibly to serve as a photoprotective compound (Ehling-Schulz et al 1997). It should also be noted that the Paint Creek wetland experienced a heavy rain event and a subsequent increase in water level <12 h prior to the day 3 1 sampling.…”

“…In another epipelon study, EPS was reduced in +W plots as compared to -UV plots, but only on a few sample days (Roux et al 2002). In contrast, glycan-rich EPS production increased significantly in the cyanobacterium Nostoc commune under laboratory exposures to UV-B but not UV-A (Ehling-Schulz et al 1997).…”

Effects of light intensity on components and topographical structures of extracellular polysaccharides from the cyanobacteria Nostoc sp.

“…Several recent studies on the influence of light intensity (Friedman et al, 1991;Moreno et al, 1998;Otero and Vincenzini, 2003;Trabelsi et al, 2009;Yu et al, 2010;Mota et al, 2013) and wavelengths (Ehling-Schulz et al, 1997;Chen et al, 2009) on EPS yield have been reported. Different light regimens (continuous light and light-dark cycles) do not significantly affect the monosaccharidic composition of EPS (Vincenzini et al, 1993;De Philippis and Vincenzini, 1998).…”