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| WHY BACTERIA? | Last Updated • December 2, 2014 | ||||||||||||||
Why do we need to know about bacteria, that are living organisms, in geology, a science that studies Earth? Earth is a planet that has been subject to change since its very formation, about 4.6 billion years ago. We have discussed in class how change always occurred within the limits set by James Hutton’s Principle of Uniformitarianism. We tend to think in terms of chemical and physical change, but Earth has been affected by something else, something that, so far, has not been recognized on other celestial bodies: life. Life on Earth has influenced the way Earth changed: from the introduction of oxygen gas in the atmosphere to the formation of carbonate rocks, from the variation in the rates of weathering and erosion to the sequestration of carbon dioxide from the atmosphere, and much more. Life itself changed over time, and this kind of change is particularly interesting, because it is non reversible: that is, life changes, or evolves, without possibilities of going back towards species that have been extinct. That is Charles Darwin’s Theory of Evolution by Natural Selection. An important consequence of evolution is that in time, Earth has been populated by progressively different kinds of organisms. Or, put in another form, different time intervals of Earth history have been characterized by different species. We can then use these organisms (their remains, or fossils) to define intervals of time, starting from the four major Eons we discussed, Hadean, Archean, Proterozoic, and Phanerozoic. As you will learn from William Schopf’s Cradle of Life, the first forms of life on Earth were prokaryotic chemosynthetic bacteria. These, which are still around today, consist of primitive, microscopic creatures that rely on the energy provided by different types of chemical reactions for their vital function (most organisms today, like us humans, rely instead on solar energy provided by plants and algae through the process of photosynthesis).
| WHAT ARE BACTERIA?
| Last Updated • December 2, 2014 | All life forms on Earth can be divided into three domains, known as Archea, Bacteria, and Eukarya. The first two domains include “bacteria” (lowercase) and they are known as prokaryotes (as opposed to eukaryotes). Prokaryotes are characterized by the lack of internal structures within their cells, while all more advanced organisms have cells with a nucleus and different types of organelles for cell functions. These more advanced organisms are the eukaryotes. All prokaryotes (or bacteria) are unicellular and most of them are extremely small in size, between 500 nm (nanometers) and 2µm (micrometers) (1000 nm = 1µm, and 1000 µm = 1 mm). As a consequence, they can only be studied under a microscope, and that is why they are also called microbes, or microorganisms. Since they are so small, they literally do not have the physical space within their cells for the organized bodies called organelles that we find in eukaryotes like plants and animals. The simplicity of the bacterial cell though favors basic life processes that allow these organisms to thrive in “extreme” environments that would be precluded to us eukaryotes. These extreme conditions prevailed on Earth at the very beginning of its existence. It is through the study of these bacteria and of their signature left in the geologic record that we can hope to get more information about the early stages of the evolution of life, and possibly also on the origin of life on Earth.
| LIFE IN EXTREME ENVIRONMENTS
| Last Updated • December 2, 2014 | As a consequence of what we said, microbes are the living organisms that establish what are the broadest limits for life in the environment, surviving in conditions that we would otherwise define as impossible. These conditions can be related to:
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© Alessandro Grippo, 1994-2014 | |||||||||