Dear Johannes, I don't know how much it was studied, but let me mention, just in case, the well-known Proposition 18.3 of S. Mac Lane, Categorical algebra, Bull. Amer. Math. Soc. 71 (1965), 40–106, even though there is no "closed" mentioned there (also there is no "symmetric", but defining tensored category in that paper, Mac Lane also means "symmetric"). It says: If D is a tensored category, so is the category of modules over a commutative D-algebra A (I wrote A instead of Greek Lambda) - and the fact (also well-known of course) that A could be a differential graded commutative algebra is visible in Section 17 (Take D = DG(K-Mod) in Mac Lane's notation). Best regards, George [For admin and other information see: http://www.mta.ca/~cat-dist/ ]