  On the Earth, photosynthetic organisms are responsible for the production of
virtually all of the oxygen in the atmosphere. On the land, vegetation reflects
in the visible, leading to a red edge that developed about 450 Myr ago and has
been proposed as a biosignature for life on extrasolar planets. However, in
many regions of the Earth, and particularly where surface conditions are
extreme, for example in hot and cold deserts, photosynthetic organisms can be
driven into and under substrates where light is still sufficient for
photosynthesis. These communities exhibit no detectable surface spectral
signature to indicate life. The same is true of the assemblages of
photosynthetic organisms at more than a few metres depth in water bodies. These
communities are widespread and dominate local photosynthetic productivity. We
review known cryptic photosynthetic communities and their productivity. We link
geomicrobiology with observational astronomy by calculating the disk-averaged
spectra of cryptic habitats and identifying detectable features on an exoplanet
dominated by such a biota. The hypothetical cryptic photosynthesis worlds
discussed here are Earth-analogs that show detectable atmospheric biomarkers
like our own planet, but do not exhibit a discernable biological surface
feature in the disc-averaged spectrum.
