N2 Fixation





Yes (many)










Bact.-a, and c, d, or e


Bact.-a, b

Bact.-a, b


Figure 10.20 Anabaena, a filamentous cyanobacteria; with heterocyst.

structures called chlorosomes, in which much of their bacteriochlorophyll is concentrated. They are mostly autotrophic, but some photoheterotrophy has also been observed. Chlorobium is the best known genus.

10.5.6 Proteobacteria

The Proteobacteria is a vast kingdom, including many of the gram-negative species and many of the metabolic activities known among the bacteria. Based on 16S rRNA, it is broken into five classes, but these are more often referred to as the a, p, g, 8, and e subdivisions. Interestingly, the 16S rRNA of mitochondria (in eukaryotes) places them in this group also (Figure 10.19), suggesting that this organelle evolved from endosymbiotic early Proteobacteria. Table 10.5 lists some of the important and interesting genera. Because the kingdom is so large, and because organisms with similar activities may belong to different classes, we discuss them based mainly on phenotypic characteristics (i.e., based more on observable traits rather than on genetic similarity).

Phototrophs: Purple Sulfur and Nonsulfur Bacteria The purple sulfur bacteria, such as Chromatium, and the purple nonsulfur bacteria, such as Rhodospirillum, are both anoxy-genic phototrophic Proteobacteria (Table 10.4). Both groups show considerable morphological variability. They include species that are rods, spheres, and spirals, and may be flagellated or not. The purple nonsulfur group also includes some budding and appendage forming bacteria. All of the known purple sulfur bacteria, which deposit sulfur granules internally (unlike the external deposits of green sulfur bacteria), are g = Proteobacteria. Purple nonsulfur bacteria are found in both the a and p groups.

How do the different phototrophs all survive—or where are their places (i.e., niches; see Chapter 14)? The Cyanobacteria occupy the upper, oxygenated regions of a lake, or other oxic environments, perhaps competing with algae and plants. The purple sulfur bacteria are normally found in the anoxic depths of lakes, where hydrogen sulfide has been released from the underlying sediments. The green sulfur bacteria, which typically can tolerate higher concentrations of hydrogen sulfide, and which also can survive with lower levels of light, would be found in even deeper zones. Both might also occupy

TABLE 10.5 Kingdom Proteobacteria: Many of the Gram-Negative Bacteriaa

Class 1. Alphaproteobacteria

Acetobacter Agrobacterium Azospirillum Beijerinckia Brucella Caulobacter Hyphomicrobium Magnetospirillum Methylocystis Nitrobacter Paracoccus Rhodospirillum Rickettsia Rhizobium Sphingomonas Xanthobacter Class 2. Betaproteobacteria Achromobacter Alcaligenes Bordetella Burkholderia Gallionella Leptothrix Methylovorus Neisseria Nitrosomonas Nitrosospira Ralstonia Sphaerotilus Spirillum Thiobacillus Zoogloea Class 3. Gammaproteobacteria Acinetobacter Aeromonas
















Xanthomonas Family Enterobacteriaceae Citrobacter Enterobacter Escherichia Klebsiella Leminorella Proteus Salmonella Serratia Shigella Yersinia Class 4. Deltaproteobacteria Bdellovibrio Desulfovibrio Myxococcus Nitrospina Polyangium Class 5. Epsilonproteobacteria Campylobacter Helicobacter

"All five classes (subdivisions a to e) are listed, but not orders or families (except Enterobacteriaceae). Only some representative, important, and interesting genera are included.

other anaerobic environments where light and reduced sulfur are present, such as some hot springs. Purple nonsulfur bacteria have an advantage in lighted anaerobic environments in which organic matter is present, since they are able to grow well heterotrophi-cally. Green nonsulfur bacteria also grow heterotrophically and in low light, particularly in thick microbial mats found in hot springs and shallow marine systems. Because of differences in the chlorophylls and other pigments each contain, the groups also have light absorption profiles with optima at different wavelengths (Figure 10.21).


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