Color of Walls in Jerusalem
One of the prominent features of Jerusalem, seen by the visitor, is the use of stones in building. It is sufficient to see Figs. 6.1.1 through 6.1.5 to observe that after man's activity, building the houses, nature arrived with its special paint brushes and added colors. These colors repeat themeselves in a specific way. In this chapter, which is longer than normal, I wish to discuss the pattern of this phenomenon, both in Israel and abroad. We'll start with basic explanations, without going in depth into the roles played by the morphology, physiology and ecology of the organisms.
I arrived at the present chapter with great help from the annonymus reviewer of an article I once submitted to a scientific journal. I wrote there that the gray color of stones collected on the slopes of Givat Ram, that turns into black upon wetting, is caused by cyanobacterial activity. I contrasted it with the gray color of other stones that turn white-green upon wetting, due to the activity of endolithic lichens. Most of these lichens exist in the upper 0.1 mm of the rock. I wrote that I checked the colors using a magnifying glass and a dissecting microscope. The reviewer erased part of the sentence and added a comment that micro-organisms cannot be seen using a magnifying glass. Since then I try to tell every group of students about that reviewer, and emphasize that one should observe and understand the site of the sample intended for observation through a microscope or scanning electron microscope. The significance of each color should be understood before further investigations are undertaken.
I have written dozens of articles in this spirit, on understanding the essence of the habitats and their occupants in rainy and arid parts of Israel and a little bit in Rome and Turkey as well. I hope to bring many of these published articles in a simplified form, using new, contemporary photographs when necessary. A list of references to articles already published may assist those searching for further reading matter.
Organisms growing on limestone and dolomite (lithobionts)
Natural rocky habitats will be discussed here, as well as man-made habitats, i.e. houses and walls, The moment a stone becomes exposed to natural forces it becomes a part of a natural ecosystem and obeys the rules of that specific environment. Exposed rocks support drought resistant organisms which do not multiply by seeds. Many of these are poikilohydric – the quantity of water in their body is in equilibrium with the quantity of water in their adjacent environment. They may enter drought dormancy when there is not enough water in their vicinity. They resume activity when the rock they live on become wetted. Hard rocks absorb a very small quantity of water. Hard limestone or dolomite may absorb 1-2% of its weight in water, and make it available to micro-organisms. Soft rock, such as that used for building today, may absorb up to 15% of its weight. The types of organisms as related to their position in relation to the rock, are presented in Fig. 6.1.6. There are organisms growing above the rock (epiliths), inside the rock (endoliths), and below detached stones (hypoliths).The identity of materials released by the organisms into the substratum they live in, has not been fully studied in all cases.
Therefore, when we try to understand their impact on the rock, let us take a simple equation that is based on the release of CO2 into the water around the organism. As dealt with earlier, physiological activity of the poikilohydric organisms is resumed when they are wetted and we have to take into consideration what happens around them upon wetting and drying out. The basic equation we are using here displays the equilibrium among the components:
Ca(HCO3)2 <=> CO22O + CaCO3
Calcium carbonate <=> CO2 + water + Calcium bicarbonate
The addition of CO2 to the water increases its acidity by forming H2CO3, thus increasing the solubility of the limestone. On the other hand, when green organisms activate photosynthesis, they bind to the CO2, drawing the equilibrium to the right side of the equation, and may cause deposition of Ca COUsub>3 near the lithobionts, as we'll see in a few cases. Evaporating water may also pull the equilibrium to the right side, forming calcium carbonate sedimentation.
Many of the lithobionts form a crust or a film on rocks, sometimes less than 1 mm thick. Their position is epilithic (Figs. 6.1.7, 6.1.8). The orange-colored organism in Fig. 6.1.7 is an epilithic lichen, followed by a schematic cross-section in a similar lichen in Fig. 6.1.8. The three layers presented in the schematic illustration are easily seen in Fig. 6.1.9 as well. When organisms occur below the rock surface, even 0.1 mm deep, they are "endolithic" (Fig. 6.1.10, 6.1.11). There are instances when the microorganisms living in a deep layer, seem hidden (Fig. 6.1.12), and are known as crypto-endolithic. Organisms in such a position have also been found in Nubian sandstone (e.g. Timna, Sinai, and Jordan) and in the desert sandstone of Antarctica.
In sites where the penetration of microorganisms into the rock take place through fissures, lifting a stone which has become loosely connected to the rock will reveal the organisms hidden in the crevices (chasmo-endolith; Figs. 6.1.13-6.1.15). Later in this chapter we shall complain about the ecosystem of chasmoendolithic cyanobacteria that destroyed several candles in the relief of our Temple's Menorah (candlestick) carved in marble by Roman artists in Titus' Arch of Triumph in the Forum Romanum, Rome, Italy. We shall do our best to bring examples that may be seen near our feet, as we walk along our country's paths. We can provide a time dimension to the processes of the establishment of micro-organisms on rocks and stones, since the construction date of many buildings is known.