  We show that planets around M-dwarfs with $M_\star \lesssim 0.2 M_\odot$ may
not receive enough photons in the photosynthetically active range of
$400$-$750$ nm to sustain Earth-like biospheres. As a result of the lower
biological productivity, it is likely that biotic molecular oxygen would not
build up to detectable levels in the atmospheres of habitable planets orbiting
low-mass stars, consistent with prior work by Lehmer et al. (2018). We also
estimate the minimum flaring rate for sustaining biospheres with Earth-like
productivity and permitting the build-up of atmospheric oxygen, and find that
the overwhelming majority of M-dwarfs are unlikely to exceed this threshold.
