  Previous work on possible surface reflectance biosignatures for Earth-like
planets has typically focused on analogues to spectral features produced by
photosynthetic organisms on Earth, such as the vegetation red edge. Although
oxygenic photosynthesis, facilitated by pigments evolved to capture photons, is
the dominant metabolism on our planet, pigmentation has evolved for multiple
purposes to adapt organisms to their environment. We present an
interdisciplinary study of the diversity and detectability of nonphotosynthetic
pigments as biosignatures, which includes a description of environments that
host nonphotosynthetic biologically pigmented surfaces, and a lab-based
experimental analysis of the spectral and broadband color diversity of
pigmented organisms on Earth. We test the utility of broadband color to
distinguish between Earth-like planets with significant coverage of
nonphotosynthetic pigments and those with photosynthetic or nonbiological
surfaces, using both 1-D and 3-D spectral models. We demonstrate that, given
sufficient surface coverage, nonphotosynthetic pigments could significantly
impact the disk-averaged spectrum of a planet. However, we find that due to the
possible diversity of organisms and environments, and the confounding effects
of the atmosphere and clouds, determination of substantial coverage by
biologically produced pigments would be difficult with broadband colors alone
and would likely require spectrally resolved data.
