  The metallicity and gas density dependence of interstellar depletions, the
dust-to-gas (D/G), and dust-to-metal (D/M) ratios have important implications
for how accurately we can trace the chemical enrichment of the universe; either
by using FIR dust emission as a tracer of the ISM; or by using spectroscopy of
damped Lyman-alpha systems (DLAs) to measure chemical abundances over a wide
range of redshifts. We collect and compare large samples of depletion
measurements in the Milky Way (MW), LMC (Z=0.5 Zsun), and SMC (Z=0.2 Zsun). The
relation between the depletions of different elements do not strongly vary
between the three galaxies, implying that abundance ratios should trace
depletions accurately down to 20% solar metallicity. From the depletions, we
derive D/G and D/M. The D/G increases with density, consistent with the more
efficient accretion of gas-phase metals onto dust grains in the denser ISM. For
log N(H) > 21 cm^-2, the depletion of metallicity tracers (S, Zn) exceeds -0.5
dex, even at 20% solar metallicity. The gas fraction of metals increases from
the MW to the LMC (factor 3) and SMC (factor 6), compensating the reduction in
total heavy element abundances and resulting in those three galaxies having the
same neutral gas-phase metallicities. The D/G derived from depletions are a
factor of 2 (LMC) and 5 (SMC) higher than the D/G derived from FIR, 21 cm, and
CO emission, likely due to the combined uncertainties on the dust FIR opacity
and on the depletion of carbon and oxygen.
