  We present the analysis of the first NuSTAR observations ($\sim 220$ ks),
simultaneous with the last SUZAKU observations ($\sim 50$ ks), of the active
galactic nucleus of the bright Seyfert 1 galaxy Mrk 509. The time-averaged
spectrum in the $1-79$ keV X-ray band is dominated by a power-law continuum
($\Gamma\sim 1.8-1.9$), a strong soft excess around 1 keV, and signatures of
X-ray reflection in the form of Fe K emission ($\sim 6.4$ keV), an Fe K
absorption edge ($\sim 7.1$ keV), and a Compton hump due to electron scattering
($\sim 20-30$ keV). We show that these data can be described by two very
different prescriptions for the soft excess: a warm ($kT\sim 0.5-1$ keV) and
optically thick ($\tau\sim10-20$) Comptonizing corona, or a relativistically
blurred ionized reflection spectrum from the inner regions of the accretion
disk. While these two scenarios cannot be distinguished based on their fit
statistics, we argue that the parameters required by the warm corona model are
physically incompatible with the conditions of standard coronae. Detailed
photoionization calculations show that even in the most favorable conditions,
the warm corona should produce strong absorption in the observed spectrum. On
the other hand, while the relativistic reflection model provides a satisfactory
description of the data, it also requires extreme parameters, such as maximum
black hole spin, a very low and compact hot corona, and a very high density for
the inner accretion disk. Deeper observations of this source are thus necessary
to confirm the presence of relativistic reflection, and to further understand
the nature of its soft excess.
