DESCRIPTION OF THE KINGDOM OXYPHOTOBACTERIA [ex GIBBONS AND MURRAY 1978 (MURRAY 1988)] (AND ITS SINGLE PHYLUM CYANOBACTERIA (STANIER 1974)

EUBACTERIA> OXYPHOTOBACTERIA> CYANOBACTERIA

Oxyphotobacteria (ak-se-fo-to-bak-TE-re-a) is a combination of three Greek roots that mean oxygen (oxygono -οξυγόνο), light (photos -φωτός), and little stick (bakterion -βακτήριον). The reference is to a photosynthetic bacterium that yields oxygen as a product.

INTRODUCTION TO THE OXYPHOTOBACTERIA AND ITS SINGLE PHYLUM CYANOBACTERIA Cyanobacteria are primary producers and therefore also called bluegreen algae. As bacteria go, they tend to be quite large, and morphological details are easily observed with a light microscope. Typically, they are aquatic and occur in both marine and freshwater environments, where they can be dominant. Generally considered nuisance taxa in freshwater, they can bloom when in environments enriched in phosphate, ultimately depleting the water of oxygen when the bloom dies and decays. Also, some are implicated in the formation of toxic blooms which can kill fish and lead to human health issues. Cyanobacteria are abundant in the microbial mats of salt marshes and significant members of the marine picoplankton (text with tooltip) Picoplankton are ultra small phytoplankton (<0.2-2µm). . Because many species can shroud themselves in mucilage, which holds water, some taxa grow on the surface of the soil. Among bacteria, the cyanobacteria are among the most complex. Some taxa exhibit morphological diversity, beyond differences in growth form. They may occur as single cells, colonies of cells, filaments and colonies of filaments. Cyanobacterial filaments are made of a linear array of cells (the trichome) that typically is surrounded by a complex set of mucilaginous layers called a sheath. Together, the trichome and the sheath make the filament. Most filamentous taxa do not branch. However, some taxa do have true branching (text with tooltip) True branching occurs in filamentous taxa which generally divide in a plane perpendicular to the axis of the filament (thus forming the filament). However, they sometimes divide in a plane parallel to the axis of the filament and form a branch. in which a cell within a filament divides in more than one plane and forms a branch. Others like Tolypothrix grow within the same sheath and emerge in what appears to be a branch, but actually the trichomes just grow past each other and make a branch-like structure if one breaks through the common sheath (Figure 1). Beyond the vegetative cells, cyanobacteria have resting or over wintering cells called akinetes, which are enlarged and very opaque. Most can withstand desiccation and extremes in temperature. In general, cyanobacteria can fix nitrogen (text with tooltip) Nitrogen fixation is the ability to use energy to take nitrogen gas and reduce it to ammonium, nitrite, or nitrate. The initial step requires a nitrogenase enzyme and the energy of about 38 ATP molecules to fix one nitrogen molecule. in microaerophilic conditions. However, some taxa have specialized cells (heterocysts) that retain the ATP-generating part of the photosynthetic system, but do not generate oxygen (Figure 2). Thus, they are able to use light energy to power the fixation of atmospheric nitrogen to a form that is biologically active. Not surprisingly, many species have entered into symbiotic relationships with plants that otherwise would be nitrogen starved. The cyanobacteria are organisms that use water as the electron donor in photosynthesis thereby releasing oxygen as a waste product. They all use chlorophyll A (text with tooltip) Chlorophyll A is a primary photosynthetic pigment of all photosynthetic eukaryotes (in the chloroplasts) and Cyanobacteria. It is membrane-bound on thyllakoids and absorbs mainly in the blue and red ranges of visible light. Its structure is that of a tetrapyrrole with a magnesium in the center. That is bound to a long aliphatic alcohol (phytol). Chlorophylls a, b, c, d, and e, together with the bacteriochlorophylls have similar structures and vary only in the structures of their side chains. and some use chlorophyll B (text with tooltip) Chlorophyll B is a secondary photosynthetic pigment in the Prochlorophytes, Euglenoids, Chlorophytes, and the Viridiplantae. See Chlorophyll A. in their photosynthetic machinery. They are called bluegreen algae because they have other pigments like phycobillins, carotenes, and xanthophylls that also serve to collect light energy. Except for the phycobillins, the pigments and photosystems are almost identical to eukaryotic chloroplasts. Indeed, the similarity is not superficial. The current view is that all chloroplasts were derived from the cyanobacteria in one or a few endosymbiotic events (Keeling 2004). As in chloroplasts, cyanobacterial cells have an internal membrane system (thylakoids, Figure 3) (Margulis 1990). Furthermore, chloroplasts have circular bacterial chromosomes, but they have only about 5% of the total DNA that is found in a typical cyanobacterial genome. However, thousands of cyanobacterial genes have been found in the nuclear genome of the flowering plant, Arabidopsis (Martin et al. 2002), suggesting that significant horizontal movement of the cyanobacterial genome to the nuclear genome occurred as a consequence of the endosymbiotic event. Fossil evidence suggests that the group is very old and likely responsible for the early formation of an oxidizing atmosphere. Using geochemical, paleontological, and molecular evidence, Tomitani et al. (2006) estimated that the cyanobacteria diverged from the rest of the bacteria between 2450 and 2100 mya. Fossils on the order of billions of years old have been found in petrified structures called stromatolites (Figures 4 and 5) and the preserved cyanobacteria strongly resemble their living descendants (Figure 6). Note that the basal position of the cyanobacteria in Figure 7A suggests that chlorophyll A likely was the photosynthetic pigment from which all other chlorophylls and bacteriochlorophylls emerged. Figure 7B illustrates the topology of the cyanobacteria according to Hoffman et al. (2005). By 2.2 billion years ago, oxygen levels in the oceans had begun to rise enough to cause iron to oxidize and precipitate out. This period of the “Rustball Earth” lasted millions of years, and the precipitation of iron formed the great iron deposits on earth. With free molecular oxygen in the oceans, the chemistry of the oceans began to change and then export molecular oxygen to the atmosphere as well. Not only did unbridled photosynthesis cause a shift to an oxidizing atmosphere, but available atmospheric carbon dioxide began to decrease. Just prior to the Cambrian, the cyanobacteria had become so abundant and so successful that they drew down carbon dioxide levels very low and the earth began to freeze. It went through several cataclysms, with the oceans nearly freezing pole to pole. It was during this period that multicellular life, the earliest animals, appeared, and likely began to consume the abundance of cyanobacteria.

FIGURE 1. Micrograph of Tolypothrix, showing the point of false branching. The two flaments are held by a common sheath. Image by Peter Siver. http://silicaseccidisk/conncoll.edu/	FIGURE 2. Micrograph of Anabaena. Note filament with many vegetative cells. The smaller clear cell is the heterocyst and the larger dark cell is the akinete. Image from NOAA, in the Public Domain	FIGURE 3. TEM micrograph of Anacystis. Note the small filaments within the cell membrane. Image from NOAA, in the Public Domain

FIGURE 4. Living stromatolites from Shark’s Bay, Australia. Image from NOAA, in the Public Domain	FIGURE 5. Fossil stromatolites from Pre-Cambrian strata in Glacier National Park. Image from the National Park Service, in the Public Domain	FIGURE 6. Fossil cyanobacterium from the Gunflint Chert (~2 billion years ago). Image from Systematics Biodiversity Image Archive

FIGURE 7A. The Oxyphotobacteria within the eubacterial tree. Note that the Oxyphotobacteria are basal among the photosynthetic bacteria.

	MAJOR CLADES OF THE CYANOBACTERIA 1. Chlorophyll A Clade 2. Plasmalemma -photosynthetic membrane 3. Thylakoid photosynthetic membrane 4. Thylakoids parallel to plasmalemma and peripheral 5. Thylakoids throughout the cell 6. No cellular differentiation 7. Cellular differentiation into heterocysts and sometimes akinetes.
FIGURE 7B. Major Clades of the Cyanobacteria. This cladogram was derived from the system of Hoffman et al. (2005). The major synapomorphies are based on the arrangements of thylakoids and the diversity of cell forms.

FURTHER READING: DISCOVERY OF THE DOMAINS OF LIFE INTRODUCTION TO THE DOMAIN EUKARYA DESCRIPTION OF THE DOMAIN ARCHAEA

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By Jack R. Holt. Last revised: 02/11/2013