Atomic carbon in APM 08279+5255 at z=3.91

We present a detection of [CI] P-3(1)-P-3(0) emission in the lensed quasar APM 08279+5255 at z = 3.91 using the IRAM Plateau de Bure interferometer. The [CI] line velocity and width are similar to the values of previously detected high-J CO and HCN lines in this source, suggesting that the emission from all of these species arises from the same region. The apparent luminosity of the [CI] line is L'(CI) (3.1 +/- 0.4) x 10(10) K km s(-1) pc(2), which implies a neutral carbon mass M-CI (4.4 +/- 0.6)m(-1) x 10(7) M-circle dot, where m is the lensing magnification factor. The [CI] line luminosity is consistent with the large molecular gas mass inferred from the nuclear CO line luminosity (similar to 10(11)m(-1) M-circle dot). We also present an upper limit on the H2O 1(10)-1(01) line luminosity in APM 08279+5255 of L'(H2O) < 1.8 x 10(10) K km s(-1) pc(2) (3 sigma).