Stromatolites are built around cyanobacteria. They are tiny cells without nuclei (prokaryotes), all identical.
|Cyanobacteria live in the water, and can manufacture their own food
through "photosynthesis." Although the oldest known fossils, more than 3.5
billion years old, are cyanobacteria, they are still around in large numbers; in fact,
they one of the largest and most important groups of bacteria. Individual
cyanobacteria are very small and usually just single cells, either round, ovoid, or
stringlike in shape. Some types grow in colonies that can be large. These colonies are
built of many layers, and are called stromatolites (if more or less dome-shaped) or
oncolites (if round). The characteristic layered structure of fossilized
stromatolites advertises their presence, helping scientists locate them and identify their
age through radioactive dating of the surrounding rocks.
To the upper left is a drawing of such a layered colony, then of the layered surface, and to the lower left of a fossil cynaobacterium such as would live near the surface.
As with all life, water is central to the life cycle of cyanobacteria. Because they are small in size and so simple that they cannot work cooperatively, each living cell needs to have access to a continuous supply. At the same time, they make food by photosynthesis, using chlorophyll. That is, as with complex, advanced plants, they absorb carbon dioxide from the atmosphere, and use the energy from the sun to build it into complex energy-containing sugars, while releasing oxygen. To carry out this process, each cell needs to be bathed in bright sunlight. To satisfy these two critical requirements - access to water and sunlight - the cyanobacteria grow at the shore of the ocean or the edges of ponds or pools of water. Single cells would be easily dislodged from this location and would float to regions where they could not survive, so the bacteria live in sheet-like films (thin enough so each cell has access both to water and sunlight) that are held together by a slime that they secrete. The stringlike ones can glide toward light at a speed of a few millimeters per hour, and become tangled in their competition to get positioned to receive sunlight. Thus, they develop into a felt-like structure that makes a particularly robust living mat. Another complication for stromatolites on the early Earth is that the gases in the atmosphere did not absorb ultraviolet photons from the sun well. Although stromatolites are somewhat UV-resistant, it is likely that they tended to grow just under the surface of the water to gain some additional protection. For colonies exposed to direct sunlight, a top level of bacteria killed by the UV light may have served to protect lower-lying layers.
The cyanobacteria form a veneer over a complex, layered colony. Underneath them is another layer of photosynthetic bacteria that absorb sunlight at wavelengths where the cyanobacteria are transparent. These bacteria are poisoned by oxygen, so they are termed "anaerobic"; the mat of cyanobacteria acts as a protective shield. Additional underlying layers, which can be millimeters or centimeters deep, contain other forms of anaerobic bacteria. Because they do not receive sunlight and do not conduct photosynthesis, these bacteria feed on dead photosynthetic bacteria that have been left behind by the gliding of the live ones toward the sun.
This structure by itself would be a stable colony. However, from time to time some external event - say a rainstorm - causes the top layer of the mat to be buried in a layer of mud. The cyanobacteria then stop secreting slime and, freed from the mat, glide upward toward the faint light filtering through until they are once again in the sun. The underlying anaerobic bacteria follow along, since their survival depends on the colony structure. In this way, a new layer grows on top of the silt, and after many repetitions the process yields a multi-layered structure that can grow to a foot or more in height. As the old, underlying layers dry out and are compressed, the silt in them solidifies into rock.
|To the far left is a fossil colony, and to the left a living one (from Schopf), showing how this pattern has remained stable for billions of years! Detailed comparisons of individual fossils of ancient bacteria also show them to be virtually identical to those found in living colonies. Much of what we describe about ancient stromatolites is based on our observations of the behavior of living ones, but it appears that this line of evidence should be quite reliable. Stromatolites appear to be the ultimate "living fossils", life forms that have survived for 3.5 billion years with virtually no modifications of their form or mode of surviving.|
|Stromatolites ruled the earth for billions of years. One line of evidence is the huge deposits of fossils they have left. Indeed, because of their striking appearance, they have been used as ornamental stonework. To the far left is a children's playground slide in China, made of 1.5-billion-year old stromatolite fossils. To the left is the Chinese parliamentary building; the columns are stromatolite fossils of the same age; note the bus in front of the building for scale (both pictures from Schopf).|
|Even more dramatically, the photosynthesis carried out by the stromatolites was on such a large scale that they consumed most of the carbon dioxide in the atmosphere and released enough oxygen to make permanent changes. The release of enough greenhouse gases by humans to change the climate through global warming makes us only the second species to have a major effect on the climate of the earth, and at least so far our influence is dwarfed by that of these bacteria!|
How did they do it? The simple cells of cyanobacteria can reproduce quickly, in only about 30 minutes. When conditions change, the members of a colony that are best able to cope are the ones that tend to survive, and since the reproduction time is so short, their offspring can be manifold and continue the growth of the colony*. In this process of "natural selection," the colony soon consists only of individual bacteria adapted to the new conditions. It is a different type of survival than we usually associate with more complex forms of life. For example, if there is a harsh winter, we are interested in whether enough individuals of lived through the winter and can establish a breeding stock to produce a next generation that can continue the species. With cynaobacteria in stromatolite, there may be as many as 10,000 generations in a single long, harsh winter. Thus, the survival depends on the most cold-tolerant members of the colony producing offspring, and the nature of the members of the colony will be subtly modified at the end of the ordeal. If a hot summer follows, then the process of natural selection will favor those that survived the harsh winter but still have some heat tolerance.
Modern stromatolites have used this process to search out ecological niches that are too harsh for other species. Thus, the snails and other animals that might feed on them cannot tolerate extremely salty water, so to avoid predators the colonies grow in bays where evaporation results in extremely saline pools of seawater.
Cyanobacteria have survived in other ways. One of the most interesting is through symbiosis, where a cell merges with another cell in a way that helps both survive. The most dramatic example is that the chloroplast with which plants make food for themselves is actually a cyanobacterium living within the plant's cell. The photosynthesis is centered in the chloroplast, while the other parts of the cell provide a protective environment for the chloroplast and integrate it into the plant.
*Mathematically, the number of generations needed for an advantage in survival of "s" to sweep through a colony of "N" inhabitants is (0.435/s) times log(2N). Without worrying about the details, this formula shows that a modification that improves the chances of survival by 1 part in 1000 will sweep through a colony of a billion stromatolites in about two years! (see "Endless Forms Most Beautiful" by Sean Carroll)