The increase of solar UV radiation at the Earth’s surface is based on the decrease in stratospheric ozone, correlated with a shift in the quality of the radiation due to a shift to shorter wavelengths and therefore an increase in the part of the radiation which is the most harmful. The changed radiation does not only affect the global climate but also individual organisms and ecosystems. One group, the marine macroalgae inhabit the shallow waters along the shorelines all over the world, and most of them being sessile are exposed to the increased solar UV radiation. UV radiation does not only exert damaging effects on the algae like loss in the photosynthetic capacity or damage of DNA but can induce the synthesis of UV-absorbing substances such as mycosporine-like amino acids (MAAs). The MAAs are not evenly distributed among the various macroalgae but show certain distribution patterns, e.g., according to the depth distribution of the algae; that is why they are thought to function as screening substances protecting the organisms against UV radiation. The photoinhibition of an alga can be determined via the measurement of the oxygen evolution or by measuring the PAM (pulse amplitude modulated) fluorescence. Both methods can be performed in the field and are used by various groups of researchers to allow measurements on site to obtain information on the harmful effect of the UV radiation. Surface-adapted algae show a better acclimation to light stress than algae from the subtidal which can be one factor causing the zonation of the macroalgae along the depth gradient.
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