Formation of the Jigsaw-Puzzle-Like Pattern

פורסם: January 1st, 2009 | עודכן: 17/01/15

The diaspores that start the growth of thalli of these endolithic lichens (I do not know their nature but they evidently exist) arrive at the site and become established there (Fig. 6.8.1). A hard rock outcrop on Mt. Gilboa (Fig. 6.8.1) displays circular thalli, where the lichens expand at a similar rate in all directions. There are preferences of growth in a few lines that are the tips of joints. In many cases there is different rock material at the joint and possibly a better moisture regime for diaspore establishment. As the lichens expand laterally the thalli touch each other and their lateral growth ceases at the contact line (Fig. 6.8.2).

We (Danin & Garty 1983) wanted to understand the situation at this contact zone, so we dissolved the rock above the algal layer with a strong acid and photographed it with SEM. The central area of the thallus (not the periphery) is seen in Fig. 6.8.3; "1" points out the fungal hyphae and "2" are the algal cells. One can see in these photos that each of the two cell types dissolves the rock according to its own morphology. Thus, the researcher of micromorphology can discern the former presence of endolithic lichens long after their disappearance. At the periphery of the thalli their components meet ("3" in Fig. 6.8.3), and it is clear that in these places there are only fungal hyphae, that are locally much denser than in any other place.

Following rainfall during both day and night hours, the fungi release CO2~ into the water. The area of the thallus with algae releases CO~2~ into the water only during night wetting. During the day time, the algae use the CO~2~ near them for assimilation. When dissolved in the water, the CO~2 becomes a weak acid that dissolves the calcium carbonate more efficiently than plain water. We may therefore estimate that the time during which a weak acid is present near the meeting zone of thalli is double that of the regular surface of the lichen. When observing Fig. 6.7.6 one can see shallow channels among areas with plenty of small round depressions. The depressions are places where the fruiting bodies of the endolithic lichens have been, and the channels are the meeting zones of thalli.

Fig. 6.8.1: Limestone rock on Mt. Gilboa in the first stages of endolithic lichens establishment. Most thalli are round and many of them occur along joints (planes of contact between stones).

Fig. 6.8.2: A stone at an advanced stage of being populated with endolithic lichens. In each place where two thalli met, their lateral expansion ceased and the circles that typified the previous stage are gone unless the thallus is isolated. At the margins of the stone a "jigsaw puzzle-like" pattern is seen.

Fig. 6.8.3: A tangential section made on a stone, at a meeting zone of two lichen thalli, by dissolving its surface layer with a strong acid. The typical microscopic structure influences the appearance of a stone populated with such endolithic lichens: 1. Fungi hyphae and algal cells. 2. A bunch of algal cells. 3. Fungi hyphae.

Fig. 6.8.4: A tangential section made on a stone populated with endolithic lichens by scratching the surface with a knife. 1. Gray surface, 2. The green algal layer, 3. White lines of the meeting zones among thalli (with only fungi) seen below the algal layer.

A rough tangential section, that anyone can make in a "rolling stone" or in a rock populated with endolithic lichens (Fig. 6.8.4), will reveal the basic morphology of these kind of endolithic lichens. Wetting the rock facilitates the removal of its upper layers by means of a penknife or a simple knife. The green layer seen in "2", Fig. 6.8.4, is the algal layer inside the rock. Since the weathering induced by the fungi penetrates to a layer deep below the meeting zone of the thalli, their white lines are clear in "3", Fig. 6.8.4.
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There are areas in Israel where endolithic lichen activity encourages strong snail and slug activity; these prefer eating the organic matter of the fungi together with the rock at the meeting zone of the thalli. The result is clearly seen on Mt. Gilboa on rocks and stones, where the depth and width of the channels among the thalli may be 2 mm and more. Most meeting lines in "1", Fig. 6.8.5 were grazed to a considerable depth and width. Area "2" in that figure displays only lichen and fungi activity and the channels at the meeting zones are narrow and shallow. In area "3", Fig. 6.8.5, there are a few lines that were grazed by snails in an area that was shaped by lichens. Detached "rolling" stones on Mt. Gilboa, display in the active side (the left stone in Fig. 6.8.6), grazing of the gray stone by snails in the channels, producing white lines. The right stone (Fig. 6.8.6) was facing the soil for a long time and was not grazed.

Fig. 6.8.5: Rock surface of Mt. Gilboa, populated with endolithic lichens after many years of snail activity at the meeting zones of thalli. 1. Most of the meeting zones among thalli were grazed by snails. 2. Most of the meeting zones among thalli were not grazed. 3. A meeting zone among thalli grazed by snails.

Fig. 6.8.6: Stones from Mt. Gilboa, with weathering patterns induced by endolithic lichens. The left stone was facing the atmosphere and the lichens in it are active. Snails were grazing on this surface a few days ago and caused white lines on the channels made by previous generations of such snails. The surface of the right stone was facing the soil, became brownish, and the lichens living there died slowly, leaving their weathering pattern on the rock surface.

Lichens and the climate of the past

I traveled to various parts of the country to try dividing the study area into large subdivisions with homogeneous microorganism landscapes that differ from those of the neighboring districts. In the Samarian Desert, east of Ma'ale Efrayim, the prevailing microorganisms are cyanobacteria and cyanophilous lichens on most of the hard limestone rocks and stones. They often "paint" the limestone such an intense black that when it rains one would think that the area is covered with basalt rocks. At a depth of 1 mm the rock may be bright white.

When observing the rock surface, I was surprised to see remnants of a jigsaw puzzle-like pattern, as if endolithic lichens were there (Fig. 6.8.7). I scratched the rock surface but there was no green layer (unlike "2" in Fig. 6.8.4 and the discussion there). The conclusion is that the weathering pattern of a climate in the distant past is represented here. The lichens changed the chemical nature of the rock surface and today the contact lines among thalli of endolithic lichens are not populated by contemporary microorganisms.

Today, the lichens causing the jigsaw puzzle-like pattern develop in areas with a mean annual rainfall of 400-1000 mm. One can say now, that in the past, the area between En Poem on Nahal Prath (Ain Fawwar on Wadi Qilt) and Ma'ale Efrayim had a period with a mean annual rainfall of 400-1000 mm. Today that area receives a mean annual rainfall of 200-300 mm. Other rocks in that area look like Fig. 6.8.8. Here the dots of cyanobacteria and cyanophilous lichens as well as remnants of the jigsaw puzzle-like pattern are seen on most of the surface. However, remnants of pits are also seen here. The latter are formed in areas where the mean annual rainfall at present is 100 mm.

Hence, Fig. 6.8.8 may be regarded as a "meteorological station" that first measured a desert climate with a mean annual rainfall of 100 mm and later a mesic climate with a mean annual rainfall of more than 400 mm. At the present time the mean annual rainfall is 200 mm. Following the Samarian Desert discoveries, it was exciting to discover that the rocks near my laboratory in Jerusalem display the jigsaw puzzle-like pattern in pits of the remote past. Dating the drought- and high humidity eras demands further investigation.

Fig. 6.8.7: Surface of a rock from the Samarian Desert, east of Ma’ale Efrayim. During the prevailing climate cyanobacteria and cyanophilous lichens make the entire area blackish. In the past, under a moister climate, endolithic lichens formed a weathering pattern here that looks like a jigsaw puzzle. The present-day microorganisms do not inhabit the lines of contact between endolithic lichen thalli and the ancient pattern indicates a moister climate here in the past.

Fig. 6.8.8: Surface of a rock from that of Fig. 6.8.7. The dark-colored organisms and the jigsaw puzzle pattern are seen here as well. In addition there are remnants of pits (indicating the impact of desert climate), and above them a "jigsaw puzzle" pattern (indicating a more mesic climate). On the surface the prevailing cyanobacteria and cyanophilous lichens have adapted to this moderate climate.