Distribution of mycosporine-like amino acids and photoprotective carotenoids among freshwater phytoplankton assemblages

Phytoplankton have evolved different strategies to minimize the potential damage caused by solar ultraviolet radiation (UVR), including the synthesis of UV-absorbing compounds that act as sunscreens and carotenoids that provide protection against photooxidative stress. The concen- tration and qualitative composition of mycosporine-like amino acids (MAAs) and photoprotective carotenoids (PPCs) were investigated in natural phytoplankton assemblages of 26 lakes located below and above the treeline in the Alps and the Pyrenees. Among these lakes, exposure to UV varied because of differences in the incident spectral dose, water column transparency, stratification, and maximum depth. Seven MAAs were identified, but the most abundant were shinorine (λmax = 334 nm) and palythine (λmax = 320 nm). The highest concentrations of MAAs (up to 9.6 µg (µg chl a) -1 ) and PPCs (up to 0.6 µg (µg chl a) -1 ) were found in the clearest and shallowest lakes, while phyto- plankton from lakes with low UV transparency generally presented low values. However, phyto- plankton of some clear lakes located at high altitude did not have high concentrations of these com- pounds. Consequently, underwater downwelling UVR, UV water transparency, or lake altitude explained only a low percentage (< 26%) of the variability among lakes in MAA and PPC concentra- tion. Within the water column, the concentration of MAAs decreased in most cases with depth, sug- gesting their photoprotective role. Our results indicate that MAAs and PPCs are widespread among lake phytoplankton assemblages and suggest that other environmental factors besides UV exposure are important in regulating their synthesis.

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