I don't really know what the riff is played on, or in which tuning,
but I suspect it's something like a 5-string acoustic bass. Something
worth a good discussion I think.. 
I just did two versions: One for an acoustic guitar (which sounds
quite nice) and one for a..

  5-string bass

G -----------------------------------------------------------
D -----------------------------------------------------------
A --------2--------------------2-----------2-----------0h2---
E ------0---0----5-----------0---0----5------3--0--2h3-------
B --3h5-------3--3-----3-3h5-------3--3----------------------
 
  Acoustic

E -------------------------------------------------
B -------------------------------------------------
G -------------------------------------------------
D -------------------------------------------------
A --------2-----0-----------2-----0--2---------0h2-
D --0h2-2---2-0-0---0-0h2-2---2-0-0----5-2-4h5-----