1. Describe the metabolic changes in oxidative metabolism
that must have accompanied the evolution and success
of the cyanobacteria?
Answers
Answer:
Cyanobacteria
Cyanobacteria (also called blue-green bacteria, blue-green algae, cyanophyceae, or cyanophytes) are a large and widespread group of photoautotrophic microorganisms, which originated, evolved, and diversified early in Earth's history. The earliest forms attributed to this group were found in sedimentary rocks formed 3.5 billion years ago, and it is commonly accepted that cyanobacteria played a crucial role in the Precambrian phase by contributing oxygen to the atmosphere (175).
At present, cyanobacteria are found in a wide range of habitats including aquatic (saltwater and freshwater), terrestrial, and extreme environments (like frigid lakes of the Antarctic or hot springs). Although called blue-green, cyanobacteria may display a variety of colors due to different combinations of the photosynthetic pigments chlorophyll a, carotenoids, and phycobiliproteins. All cyanobacteria combine the ability to perform an oxygenic photosynthesis (resembling that of chloroplasts) with typical prokaryotic features. The possession of chlorophyll a and the use of oxygenic photosynthesis distinguishes cyanobacteria from other photosynthetic bacteria, such as purple and green bacteria. Nevertheless, some cyanobacterial strains can perform anoxygenic photosynthesis by using hydrogen sulfide (H2S) as the electron donor. Many cyanobacteria can fix atmospheric dinitrogen (N2) (a capacity not possessed by any eukaryote) into ammonia (NH3), a form in which the nitrogen is further available for biological reactions. Although quite uniform in nutritional and metabolic respects, cyanobacteria are a morphologically diverse group with unicellular, filamentous, and colonial forms. Among certain filamentous cyanobacteria, there is some degree of cellular differentiation. Within the filament, vegetative cells may develop into structurally modified and functionally specialized cells: the akinetes (resting cells) or the heterocysts (cells specialized in nitrogen fixation). For more detailed information on cyanobacteria, see reference 218.
Unicellular and filamentous cyanobacteria can form symbioses with a wide diversity of hosts. In symbiosis, some cyanobionts perform both photosynthesis and nitrogen fixation while others exhibit only one of these properties (3, 22, 133, 161, 184). It is believed that ancestors of cyanobacteria evolved to become plastids after a long period of endosymbiosis. In biochemical and structural detail, cyanobacteria are especially similar to the chloroplasts of red algae (47, 104).
The taxonomy of cyanobacteria is still a controversial subject, with two prevailing approaches, the botanical approach (8-10, 102, 103) and the bacterial approach (167, 168, 190). Despite the differences, both botanists and bacteriologists divide the cyanobacteria into four or five major subgroups. In fact, the five sections recognized by Rippka et al. (167) coincide broadly with orders of other classifications: Chroococcales, Pleurocapsales, Oscillatoriales, Nostocales, and Stigonematales (for reviews on this subject, see references 218 and 219). At present, different molecular methods are being used and developed to infer phylogenetic relationships. However, the data available for cyanobacteria are still scarce (207, 219). Analysis of 16S rRNA sequences has given the most detailed hypothesis of the evolution of cyanobacteria (218, 219). A polyphasic taxonomy of cyanobacteria, integrating both phenotypic and genotypic characters is currently being assembled. A milestone in the study of cyanobacteria was the publication of the entire genome (3,573,470 bp) of the non-nitrogen-fixing unicellular cyanobacterium Synechocystis strain PCC 6803 (95, 144). A number of other cyanobacterial genome projects are being completed (see below).
Cyanobacteria may possess several enzymes directly involved in hydrogen metabolism: nitrogenase(s) catalyzing the production of hydrogen (H2) concomitantly with the reduction of nitrogen to ammonia, an uptake hydrogenase catalyzing the consumption of hydrogen produced by the nitrogenase, and a bidirectional hydrogenase which has the capacity to both take up and produce hydrogen ..
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