  One of the possible signs of life on distant habitable exoplanets is the
red-edge, which is a rise in the reflectivity of planets between visible and
near-infrared (NIR) wavelengths. Previous studies suggested the possibility
that the red-edge position for habitable exoplanets around M-dwarfs may be
shifted to a longer wavelength than that for Earth. We investigated plausible
red-edge position in terms of the light environment during the course of the
evolution of phototrophs. We show that phototrophs on M-dwarf habitable
exoplanets may use visible light when they first evolve in the ocean and when
they first colonize the land. The adaptive evolution of oxygenic photosynthesis
may eventually also use NIR radiation, by one of two photochemical reaction
centers, with the other center continuing to use visible light. These
"two-color" reaction centers can absorb more photons, but they will encounter
difficulty in adapting to drastically changing light conditions at the boundary
between land and water. NIR photosynthesis can be more productive on land,
though its evolution would be preceded by the Earth-type vegetation. Thus, the
red-edge position caused by photosynthetic organisms on habitable M-dwarf
exoplanets could initially be similar to that on Earth and later move to a
longer wavelength.
