Asterothyrium atromarginatum

Asterothyrium atromarginatum is a species of foliicolous (leaf-dwelling) lichen in the family Gomphillaceae . Found in Veracruz, Mexico, it was formally described as a new species in 2002 by the lichenologists María de los Ángeles Herrera-Campos and Robert Lücking. The lichen forms small patches that are rounded to irregular in outline, 0.3 to 1 mm in diameter, smooth to silvery white in colour, and bordered by a distinct black line. The dark line appears to be a prothallus but is actually created by the intensely pigmented outer cells of the thallus cortex. The photobiont partner of Asterothyrium atromarginatum is a species of Trebouxia (a genus of green algae); their cells are more or less spherical and measure 6–8 μm in diameter. Asterothyrium microsporum is similar in morphology to A. atromarginatum, but its thallus lacks the black bordering line....read more on Wikipedia.