Phlorotannins as UV-protective substances in early developmental stages of brown algae

Abstract

Communities of macro- and microalgae present a dominant group of coastal benthic ecosystems. Macroalgae exhibit a complex developmental cycle involving microscopic life stages which are fully exposed to environmental factors such as UV radiation (UV) and water temperatures displaying the most sensitive stage of the entire macroalgal life-cycle. In the last 50 years, the ozone content in the atmosphere has significantly diminished due to growing emissions of synthetic chlorofluorocarbon molecules. At the same time, global mean temperature increased due to the so-called green house effect. These global environmental changes may affect ecosystems to a hitherto unknown extent. The aims of the present study therefore involve the depiction of acclimation to UVR and PAR in the protective and metabolic mechanisms of brown algal juvenile life stages of the species Alaria esculenta, Laminaria digitata, Saccharina latissima and Saccorhiza dermatodea as well as the species' adaptive potential to enhanced temperatures. Including a comparative approach of field and laboratory experiments, the present study integrates various radiation regimes and intensities, temperature experiments and exposure times. For a broader understanding of effects, a variety of methods investigating e.g. intra- and extracellular levels of phlorotannins by the Folin-Ciocalteu method, fatty acid determination by gas chromatography, determination of reactive oxygen species and electron microscopy was applied to several juveniles of various Arctic macroalgae from Spitsbergen (Kongsfjorden). Results reveal that the sensitivity to visible and UVR of various macroalgal species is determined by their radiation tolerance and the protective potential of their parental tissue. As still only few data are available on interactive effects of PAR, enhanced UVR and temperatures, predictions of future consequences for algal recruitment and survival as well as on the community level remain difficult. While enhanced UVR in combination with low PAR in the laboratory was not affecting algal propagules, field experiments under high PAR showed detrimental effects leading to a decrease of spore germination. Most studies conducted in the past have focused on UVR effects while the present study reveals that high PAR was always underestimated and might play a more pronounced role than usually expected. Ozone depletion might be of greater importance for smaller organisms such as spores and bacteria which are physically less protected from UVR damage, especially in marine environments where concentrations of chromophoric dissolved organic matter are low and UVR transparency is high. Stratospheric ozone depletion over the Arctic in combination with rising temperatures is therefore expected to have a strong impact on the survival of macroalgal zoospores and finally the vertical zonation of Arctic and cold temperate macroalgal species around Spitsbergen.