  Recently, the Kepler Space Telescope has detected several planets in orbit
around a close binary star system. These so-called circumbinary planets will
experience non-trivial spatial and temporal distributions of radiative flux on
their surfaces, with features not seen in their single-star orbiting
counterparts. Earthlike circumbinary planets inhabited by photosynthetic
organisms will be forced to adapt to these unusual flux patterns.
  We map the flux received by putative Earthlike planets (as a function of
surface latitude/longitude and time) orbiting the binary star systems Kepler-16
and Kepler-47, two star systems which already boast circumbinary exoplanet
detections. The longitudinal and latitudinal distribution of flux is sensitive
to the centre of mass motion of the binary, and the relative orbital phases of
the binary and planet. Total eclipses of the secondary by the primary, as well
as partial eclipses of the primary by the secondary add an extra forcing term
to the system. We also find that the patterns of darkness on the surface are
equally unique. Beyond the planet's polar circles, the surface spends a
significantly longer time in darkness than latitudes around the equator, due to
the stars' motions delaying the first sunrise of spring (or hastening the last
sunset of autumn). In the case of Kepler-47, we also find a weak longitudinal
dependence for darkness, but this effect tends to average out if considered
over many orbits.
  In the light of these flux and darkness patterns, we consider and discuss the
prospects and challenges for photosynthetic organisms, using terrestrial
analogues as a guide.
