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M05_REEC5174_09_IE_27_40 - Notes to Instructors Chapter 27...

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Notes to Instructors Chapter 27 Bacteria and Archaea What is the focus of these activities? Activity 27.1 How diverse are the Archaea? Relatively recent studies indicate that prokaryotes are incredibly diverse, so diverse that they have been split into two domains: the Archaea and the Eubacteria. This activity helps students see that it is likely that early life- forms were more similar to the Archaea. In other words, life on the early Earth was very different from what we see today. Activity 27.2 How has small size affected prokaryotic diversity? This activity is designed to help students understand how small size has limited morphological diversity but promoted biochemical (metabolic) diversity among the prokaryotes. What misconceptions or difficulties can these activities reveal? Activity 27.2 We often state that the prokaryotes are more diverse metabolically and the eukaryotes are more diverse morphologically. Then we go on to indicate that a wide variety of metabolic types exist in the prokaryotes. As instructors, we understand that any single species of prokaryote has a specific type of metabolism. When we talk about the variety of metabolic types, we mean that different species of prokaryotes can display very different types of metabolism. Some students don’t have this same understanding, however. They have the impression that a single species can display many different types of metabolism. Answers Activity 27.1 How diverse are the Archaea? 1.The Archaea are divided into two major groups: the Euryarchaeota and the Crenarchaeota. a. What characteristics are used to place organisms into each of these groups? Archaean group Characteristics Euryarchaeota This group includes the methanogens, many extreme halophiles, and some extreme thermophiles. The methanogens are strictly anaerobic. The extreme halophiles are aerobic and require high environmental salt concentrations (>20%). Many nonextremophile species of Euryarcheota have also been discovered, for example, in soil samples and lake sediments. Most of these species were discovered by “genetic prospecting”—use of Euryarcheota DNA primer sequences and PCR on soil samples. As a result, we know that these species exist, but at present do not know much about their biochemistry or ecology. Crenarchaeota Most of the thermophilic species of Archaea fall into this category. Optimal temperatures for these species are between 608 and 808C. As with the Euryarcheota, Reece_IG Ch 27_40-1
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many nonextremophile species of Crenarcheota have also been discovered. As with the newly discovered Euryarcheota, at present, we have infor mation about the DNA sequences of these species, but little information about their biochemistry or ecology. b. Two additional groups of the Archaea have been proposed.
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