Top: Tidal flat close to Dangast (Jadebusen, Germany).
Bottom: In a field experiment a small area (beneath the
hand of the scientist) was covered for a few hours. This
area is darker brown than the surrounding sediment.
Have you ever been at a coast with tidal flats? Perhaps you have noticed the brown color of the the sediment surface during low tide. The color is caused by a group of photosynthetic microorganisms, the diatoms.