Uptake and excretion of dimethylsulphoniopropionate is driven by salinity changes in the marine benthic diatomCylindrotheca closterium

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Abstract
Long-term ( ⩽ 14 d) and short-term ( ⩽ 9 h) effects of salinity on the dimethylsulphoniopropionate (DMSP) content of the marine benthic diatom Cylindrotheca closterium were investigated. When grown in batch cultures at various salinities, intracellular DMSP increased from 13.8 to 823 μmol g − 1 protein at salinities of 11 and 44 psu, respectively, but increasing salinity further did not result in a higher content. Instead, the concentration of the amino acid proline increased from 17.5 to 149.6 μmol g − 1 protein when salinity increased from 33 to 55, indicating that this compound is more important than DMSP as an osmoprotectant at higher salinities. In addition, another, hitherto unidentified, compound increased strongly in response to salinity. During salinity up- and down-shock experiments, changes in the DMSP content of C. closterium corresponded with the changes in salinity. DMSP was excreted into the medium after salinity down-shock, while salinity up-shock induced a slow synthesis of DMSP. However, C. closterium was also able to take up DMSP from the medium; the amount depended on the salinity difference applied. This is the first report of uptake of DMSP by an alga. The results demonstrate that synthesis of DMSP is too slow to be important for short-term acclimation and that its contribution to the intracellular osmotic potential of C. closterium is probably small. However, the observation that uptake of DMSP was faster after salinity up-shock from 11 to 33 than from 11 to 22, indicates that DMSP does have an osmoprotective function in C. closterium. Uptake of DMSP and other organic osmolytes from the environment may serve as a mechanism of osmoacclimation for diatoms living under highly fluctuating salinities in intertidal sediments.