  We calculated the radial profiles of the azimuthally averaged midplane gas
volume density $\rho_g$ for 11 high surface brightness (HSB) spiral galaxies, 7
low surface brightness (LSB) galaxies and 3 S0 galaxies assuming their gaseous
layers to be in the equilibrium state in the plane of marginally stable stellar
discs. We compared the surface star formation rate ($\Sigma_{SFR}$) and star
formation efficiency ($SFE=\Sigma_{SFR}/\Sigma_{gas}$) with $\rho_g$ and
stellar surface density $\Sigma_s$ assuming the latter to be proportional to
disc surface brightness. Both HSB and LSB galaxies follow a single sequence
$\Sigma_{SFR}-\rho_{g}$ and $SFE-\Sigma_s$ or $SFE-\rho_s$. It means that the
conditions of star formation are similar in the outer discs of normal spiral
galaxies and in the inner regions of LSB galaxies if their stellar discs have
similar densities. The relationship between $SFE$ and $\rho_s$ is close to the
law $SFE$ $\sim \rho_s^{1/2}$ expected in the theoretical model of
self-regulated star formation proposed by Ostriker et al.\cite{OstrikerAll10}.
The alternative explanation is to propose that $SFE$ is proportional to a
frequency of vertical oscillation of gas clouds around the disc midplane. In
the most rarified regions of LSB galaxies the efficiency of star formation is
nearly independent on gas and stellar disc densities being higher in the mean
than it is expected from the extrapolation of the power law fit for HSB sample
galaxies. Evidently in these regions with extremely low $\rho_{g}$ SFE depends
on local density fluctuations rather than on the azimuthally averaged disc
parameters.
