Seasonal variation of diarylheptanoids in Zostera marina (Zosteraceae) from the Baltic Sea, impact of drying on diarylheptanoids and phenolics, and first report of 3-keto-steroids

Seven diarylheptanoids and three 3-keto-steroids were detected in the acetone extract obtained from Zostera marina L. collected at the coast of the German Baltic Sea. The identified diarylheptanoids encompassed tricyclic diarylheptanoids, tetracyclic diarylheptanoids, and dimers of cyclic diarylheptanoids. During the whole growing season, tetracyclic diarylheptanoids were occurring in higher concentrations than other subclasses of diarylheptanoids. Concentrations of tricyclic diarylheptanoids were higher than the ones of dimeric cyclic diarylheptanoids, except in winter. All detected diarylheptanoids showed similar seasonal trends: the contents displayed an increasing tendency from September to a peak in autumn, and then dropped until the end of winter. In spring and summer diarylheptanoid contents were relatively low and showed fluctuating patterns. Additionally, we investigated how the gradual drying of the plants affected the contents of phenolic compounds and diarylheptanoids. To this end, daily samples of air-drying plants were taken; starting from fresh, living green leaves to completely dry, brown leaves. The results revealed that phenolic compounds were gradually decomposing, whereas the amounts of diarylheptanoids were increasing during the drying process. Three 3-keto-steroids, which have already been described from other sources, are described for the first time as metabolites of Z. marina