The effect of ultraviolet radiation on photosynthesis and ultraviolet-absorbing substances in the endemic Arctic macroalga Devaleraea ramentacea (Rhodophyta)

In field studies conducted at the Kongsfjord (Spitsbergen), the effect of f iltered natural radiation conditions (solar without ultraviolet [UV]-A + UV -B, solar without UV-B, solar) on photosynthesis and the metabolism of UV-a bsorbing mycosporine-like amino acids (MAAs) in the marine red alga Devaler aea ramentacea have been studied. While solar treatment without UV-A+UV-B d id not affect photosynthesis during the course of a day, solar without UV-B and the full solar spectrum led to a strong inhibition. However, after off set of the various radiation conditions, all algae fully recovered. Isolate s collected from different depths were exposed in the laboratory to artific ial fluence rates of photosynthetic active radiation (PAR), PAR + UV-A, and PAR + UV-A + UV-B. The photosynthetic capacity was affected in accordance with the original sampling depth, i.e. shallow-water isolates were more res istant than algae from deeper waters, indicating that D. ramentacea is able to acclimate to changes in irradiance. Seven different UV-absorbing MAAs w ere detected in this alga, namely mycosporine-glycine, shinorine, porphyra- 334, palythine, asterina-330, palythinol, and palythene. The total amount o f MAAs continuously decreased with increasing collecting depth when sampled in mid June, and algae taken in late August from the same depths contained on average 30-45% higher MAA concentrations, indicating a seasonal effect as well. The presence of increasing MAA contents with decreasing depth corr elated with a more insensitive photosynthetic rapacity under both UV-A and UV-B treatments. Populations of D. ramentacea collected from 1 m depth, wit h one fully exposed to solar radiation and the other growing protected as u nderstorey vegetation underneath the kelp Laminaria in saccharina, exhibite d quantitatively different MAA compositions in the apices, The exposed seaw eeds contained 2.5-fold higher MAA values compared with the more shaded alg ae. Moreover, the exposed isolates showed a strong tissue gradient in MAAs, pigments, and proteins, The green apices contained 5-fold higher MAA conte nts than the red bases. Transplantation of D, ramentacea from 2 m depth to the surface induced the formation and accumulation of MAAs after 1 week exp osure to the full solar spectrum. Control samples which were treated with t he solar spectrum without UV-A+B or with solar without UV-B showed unchange d MAA contents, indicating a strong UV-B effect on MAA metabolism. Ail data a ell supported the suggested physiological function of MAAs as natural UV sunscreens in macroalgae.