  The thermoelectric effect in a two-dimensional (2D) massless Dirac fermion
(DF) system at the quantum limit is discussed to verify the prediction of
high-performance thermopower in an organic conductor \alpha-(BEDT-TTF)2I3.
Because of relatively large Zeeman splitting in \alpha-(BEDT-TTF)2I3, the
boundless increase of thermopower at high magnetic fields, predicted without
the Zeeman effect, is hardly expected, whereas there appears to be a broad
local maximum. This is characteristic of 2D DF systems with Zeeman splitting
and is recognized in the previous experiment. In contrast to 3D Dirac/Weyl
semimetals with robust gapless features, it might be difficult to realize
high-performance thermopower in real 2D DF systems under high magnetic fields.
