Certain characteristics make algae and fungi extremely useful to each other. Algae form simple carbohydrates that are excreted and subsequently absorbed and transformed by fungi cells. Algae also produce several vitamins that are required by fungi. In return, fungi absorb water vapor from the air and provide shade for the algae, which are very sensitive to light. Working together in such ways enables lichens to colonize extremely inhospitable areas, including bare rock, solidified lava flows, and regions at temperature extremes.