The influence of temperature and light penetration on the abundance of the hypolithic algae in the Negev desert of Israel.

Hypolithic algae were found under flint pebbles in the northern part of the Negev desert in Israel. The algae appeared in the contact area between the stone and the loess soil in which it is partially buried. Two types of flint were found in the research area: dark transparent ones and light opaque ones. A significant difference was found in the distribution of algae in these two types: 46.8% of the dark flints bear algae up to a thickness of 40 mm in the main range of 5 to 15 mm, whereas the respective figure for the light flints was 20.9% up to 30 mm only and in the thickness range of mainly 5 to 10 mm. No significant difference could be shown concerning the temperature underneath the two types of flint stones. The dark flint had a higher extinction coefficient than the light flint and this means that a greater amount of light penetrates to a deeper thickness in the dark flint than in the light. It may be assumed that the differences in light penetration explain the wider distribution under the dark flint than the light flint.