UV-absorbing mycosporine-like compounds in planktonic and benthic organisms from a high-mountain lake

Abstract

We investigated the occurrence, concentration and composition of mycosporines (mycosporine-like amino acids, MAAs) in planktonic organisms and epilithic cyanobacteria from a clear water high-mountain lake situated in the Central Alps, Austria. Two bi-substituted MAAs were identified by HPLC in extracts made of 1996 plankton samples with 90 % aqueous methanol, i.e. asterina-330 (lambda(max) = 330 mm) and shinorine (lambda(max) = 334 nm). Extracts with 20 % aqueous methanol for 2 h at 45 degrees C revealed the additional presence of another MAA tentatively identified as palythine (lambda(max) = 320 nm) in the 1998 planktonic assemblage. In the upper 3 m of the water column the total concentration of MAAs decreased exponentially with depth, but the maxims for both absolute and chlorophyll-a specific concentrations were observed close to the bottom at 8.5 m depth. This was explained by the accumulation of MAAs in the copepod Cyclops abyssorum tatricus that stays in deep water, during daytime. The copepodite III stage contained the 3 MAAs found in phytoplankton but also the mono-substituted compound, mycosporine-glycine (mycosporine-gly: lambda(max) = 310 nm). The concentration of MAAs in C. abyssorum tatricus was highest for shinorine (1.45 % of the dry weight) and lowest for mycosporine-gly (0.02 % of the dry weight). Epilithic cyanobacteria had a more diverse MAA spectrum than plankton, and produced not only asterina-330 and shinorine but also palythinol (lambda(max) = 332 nm), my cosporine-gly and two unidentified compounds with lambda(max) = 330 and 340 nm. The composition and also the relative abundance of the cyanobacterial MAAs changed with depth. Mycosporine-gly was found at the lakeshore where Gloeocapsa sp. dominates, but it was absent at 0.5 and 2.5 m depth dominated by Schizothrix sp. and Tolipothrix sp., respectively. We could not detect any MAAs in the cysts of the red snow alga Chlamydomonas nivalis, which develops on top of the winter cover shortly before ice-melt. These results expand to alpine lakes the range of ecosystems in which these compounds may play a significant biological role.