"Troszcz się o środowisko" and "Troszczcie się o środowisko" is correct and should also be accepted, although we rather use the verb "troszczyć się" in context of taking care of given living beings or members of family.
"o" takes Locative ("o środowisku") when it means "about", which I guess statistically is most of the cases. But sometimes there are other meanings of "o", which need Accusative, and that's one of such situations.
Because "dbasz" (singular) and "dbacie" (plural) is indicative mood (tryb oznamujący). And the English phrase is in imperative mood (tryb rozkazujący), which in Polish is, respectively, "dbaj" (singular) and "dbajcie" (plural) - see also here: http://sgjp.pl/leksemy/#250217/dba%C4%87