  Stars with about 45 to 80% the mass of the Sun, so-called K dwarf stars, have
previously been proposed as optimal host stars in the search for habitable
extrasolar worlds. These stars are abundant, have stable luminosities over
billions of years longer than Sun-like stars, and offer favorable space
environmental conditions. So far, the theoretical and experimental focus on
exoplanet habitability has been on even less massive, though potentially less
hospitable red dwarf stars. Here we present the first experimental data on the
responses of photosynthetic organisms to a simulated K dwarf spectrum. We find
that garden cress Lepidium sativum under K-dwarf radiation exhibits comparable
growth and photosynthetic efficiency as under solar illumination on Earth. The
cyanobacterium Chroococcidiopsis sp. CCMEE 029 exhibits significantly higher
photosynthetic efficiency and culture growth under K dwarf radiation compared
to solar conditions. Our findings of the affirmative responses of these two
photosynthetic organisms to K dwarf radiation suggest that exoplanets in the
habitable zones around such stars deserve high priority in the search for
extrasolar life.
