Effects of UV radiation on five marine microphytobenthic Wadden sea diatoms, isolated from the Solthörn tidal flat (Lower Saxony, southern North Sea) – Part II: changes in carbohydrate, amino acid and fatty acid composition

Abstract

Benthic diatoms inhabiting intertidal flats face highly variable environmental conditions, due to changing water levels and exposure during low tide. The present study is the second part of a more extensive study of the adaptive potential of these species in response to varying UV radiations in the Solthörn tidal flat (Lower Saxony, southern North Sea). Five isolates (Achnanthes exigua, Amphora exigua, Cocconeis peltoides, Diploneis littoralis and Navicula digitoradiata), which were found in this area in high cell numbers in summer 2008, were used in semi-continuous cultures to study the physiological effects of UV-radiation (PAR [photosynthetically active radiation], PAR+UV-A, PAR+UV-B, PAR+UV-B+UV-A). For short- and long-term exposures (6 h, 30 days), the composition of intercellular carbohydrates, amino and fatty acids were analysed in exponential-phase cultures grown at a salinity of 30 in a 12 : 12 h light : dark cycle at 20 °C. Although all tested species showed distinct differences in their initial carbohydrate, amino and fatty acid compositions and in their responses to the different UV treatments, general response patterns could be identified. Overall physiological responses to short- and long-term UV treatments included the accumulation of proline as well as an increase in total carbohydrates and lipids, whereas significant differences in the composition of carbohydrates, amino and fatty acids occurred after long-term exposure to the UV treatments (P < 0.05). While UV-A exposure led to higher accumulations of phenylalanine, aspartic acid and saturated fatty acids, the response to UV-B long-term exposure included increases of galactose, mannose and unsaturated fatty acids in the cells. In both UV experiments there was a noteworthy accumulation of the amino acid tryptophan in most species. The combined UV-A+UV-B experiment showed a significant (P < 0.05) increase of aspartic acid, phenylalanine, galactose and saturated fatty acids in a majority of species. Overall, the results indicated significant differences in the physiological responses of the five diatom taxa during UV exposure, which suggests species-specific acclimation strategies that may explain the growth insensitivity towards at least short-term UV.