Compatible solutes in three marine intertidal microphytobenthic Wadden Sea diatoms exposed to different salinities

Abstract

Diatoms inhabiting intertidal flats are subject to strongly changing salinities due to exposure to rain and desiccation at low tide. In order to determine the physiological responses of marine benthic diatoms to salinity changes, cultures of Navicula phyllepta, Achnanthes delicatula subsp. hauckiana and Nitzschia constricta, isolated from the Solthörn tidal flat (lower Saxony, southern North Sea), were used to study the effect on growth rates of different salinities (0.5, 10, 20, 30, 40, 50). During short (1, 3, 10, 60 min) and long exposures (30 days), the composition of free amino acids and the accumulation of polyols, saccharides, quaternary ammonium compounds and β-dimethylsulphoniopropionate (DMSP) were determined. The growth rates of N. phyllepta were not affected by salinity in the tested range, synthesizing and accumulating proline as the only response to salinity increases in the growth medium. The growth responses of A. delicatula subsp. hauckiana were significantly weaker at the lowest and highest salinities in comparison to the control treatment (at a salinity of 30), producing proline, glycine betaine, glycerol and the quaternary ammonium compound homarine. In contrast, N. constricta showed the highest growth rate in the low-salinity treatment and the lowest proline concentrations in the high-salinity assay in comparison to the other species. In addition, N. constricta was the only species that synthesized the tertiary sulphonium compound DMSP. Furthermore, results from the salinity treatments showed particular similarities in compound accumulation. While quaternary ammonium compounds as well as DMSP were synthesized during high-salinity treatments, polyols and carbohydrates were found predominantly in the low-salinity experiments. Enzyme activities specific to proline metabolism in A. delicatula and N. constricta indicated that the high salinity-induced free proline accumulation may be due to a stimulation of synthesis via the ornithine pathway, whereas in N. phyllepta the glutamate pathway may contribute significantly to the free proline accumulation. In conclusion, significant differences in osmolyte compositions were found in the three diatom species during exposure to different salinities, suggesting specific intracellular acclimation processes that provide possible explanations of the species’ insensitivity towards environmental short-time salinity variations.