Effects of varying pH on the growth and physiology of five marine microphytobenthic diatoms isolated from the Solthörn tidal flat (southern North Sea, Germany)

Abstract:

Diatoms inhabiting intertidal flats are subject to strongly changing environmental conditions, which have a great influence on the oxygen, sulphide and pH gradients in the sediments. In previous studies, variations in pH in the range from 6.5 to 8.5 had only minor effects on the growth of benthic diatoms. In order to determine the physiological responses of marine benthic diatoms to pH changes, cultures of Amphora graeffii, Navicula gregaria, Navicula phyllepta, Nitzschia epithemoides and Pinnularia ambigua (=Biremis ambigua) isolated from the Solthörn tidal flat (lower Saxony, southern North Sea) were used to study the effect on growth rates and biochemical compositions at different pH values (7.25, 7.5, 7.75, 8.0, 8.25, 8.5 and 8.75). During short- (6 h) and long-term exposures (30 d), the composition of free amino acids and the accumulation of polyols, saccharides, quaternary ammonium compounds and β-dimethylsulphoniopropionate (DMSP) were determined. While growth rates of the five tested diatom taxa during short-term exposure to experimental pH were only hardly affected, long-term exposure to experimental pH led to decreasing growth rates in all tested species. Significant differences were found in physiological responses to pH treatments of the species tested. Growth in terms of chlorophyll a and protein contents showed decreasing values also after short-term exposure to experimental pH. Furthermore, at experimental short-term pH treatments, species accumulated higher levels of proline, glycerol, galactose and mannose, while only three out of the five tested taxa also accumulated high amounts of the quaternary ammonium compounds homarine and glycine betaine as well as DMSP. The physiological responses to long-term exposure to experimental pH showed distinct differences in the accumulation of proline, serine, taurine, glycerol and other compatible solutes. In conclusion, significant differences in osmolyte compositions were found in the five diatom species exposed to different pHs, suggesting specific intracellular acclimation processes. These responses provide a possible explanation for the insensitivity to short-term pH variations.

Key Words::

• Amino acids
• Carbohydrates
• Compatible solutes
• DMSP
• Marine benthic diatoms
• pH acclimatisation
• Proline
• Quaternary ammonium compounds
• Tidal flats

ACKNOWLEDGEMENTS

The author is grateful to Prof. Dr. Gerd Liebezeit for the opportunity to conduct this work in his working group and his long-lasting scientific encouragement.