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Chloroflexus aggregans is a photosynthetic bacterium known for its metabolic diversity and its physical grouping properties. Chloroflexus aggregans was originally isolated from hot springs in Japan in 1995. A group of Japanese scientists led by S. Hanada published their findings as "Chloroflexus aggregans sp. nov., a filamentous phototrophic bacterium which forms dense cell aggregates by active gliding movement" in the Journal of Bacteriology. Some of the characteristics of this species were generally similar to those of other Chloroflexus species, including their ability to grow anaerobically as photoheterotrophs or aerobically as chemoheterotrophs, and their ability to "glide" through the water. This species was named for its unique ability among the Chloroflexi phyla to "aggregate" forming small clusters.
The phylum of Chloroflexi represents a very large lineage within the Bacterial domain. Members of Chloroflexi are metabolically diverse and widely distributed throughout all of nature. Members of the Family Chloroflexaceae are unicellular rod-shaped bacteria that form filamentous strands in nature. These strands are formed by a process known as filamentation, where the individual cells group together end to end and elongate but do not divide. They hold this structure to increase their surface-area to volume ratio allowing them to intake nutrients from the environment and export waste through the cell membrane (Garrity 2001).
Hanada and his researchers were able to document two separate strains of C. aggregans growing in two separate freshwater hot springs in Japan. The first strain MD-66T isolated from bacterial mats found in an outflow of the Okukinu Meotobuchi hot spring in Tochigi Prefecture, Japan was thriving in 57C water at a neutral pH of 7.0. The second strain strain YI-9 was isolated from the Yufuin hot spring in Oh-ita Prefecture, Japan, thriving in 67C water and an alkaline pH of 8.2. Both strains upon further laboratory testing were found to flourish under the same conditions, with optimal growth at 55°C. Chemical concentrations in the water needed for growth were determined to be 18 mM of sodium chloride, 3 mM if sodium bicarbonate and trace amounts of sulfide. Both strains also required thiamine and folic acid as growth factors.
Chloroflexus aggregans forms mat-like aggregates when in a liquid medium that have the appearance of green-colored balls (Hanada et al., 2002). It is this aggregation property that originally tipped off the Japanese scientists that there was a unique species living in their springs, leading to its eventual binomial name. Cellular aggregates were found to reform rapidly whenever a growing culture was dispersed to form a uniform suspension. C. aggregans was grown with inserted illumination genes, allowing researchers to easily distinguish the aggregation taking place. Aggregation was observed within 20 to 30 minutes each time that the cells were dispersed by shaking.
Several filamentous cyanobacteria such as Anabaena cylindrical and Oscillatoria terebriformis also exhibit this ability of rapid cell aggregation, forming clumps of cells in liquid medium. Walsby concluded that their ability to aggregate so quickly was a result of their rapid gliding motility (Walsby 1982). They studied the speed at which a single bacterium filament could move dependent on their length. The gliding rate of C. aggregans was found to be 100 times greater than that of its known relative, Chloroflexus aurantiacus. Their gliding ability occurs when the cell is in contact with a solid or semi-solid surface. Unlike most motile bacteria, Chloroflexus does not create any flagella-like propulsive appendages. Their movement is very smooth and can be described as resembling the way a snail moves. Though this type of movement has been observed before among multicellular filamentous bacteria, such as cyanobacteria, the mechanism of gliding has not been clarified (Carr 1982). Currently, there are studies using electron microscopy to find structures that these so called "gliders" may have in common
While the physical source of movement is unknown, it has been proposed that chemotaxis may be involved in controlling the gliding movements of Chloroflexus aggregans. Chemotaxis is a process in which bacteria can adjust their direction of movement by sensing certain chemical signals in their habitat. This sensory ability is important in helping the bacteria find nutrients. They can move toward higher concentrations of glucose in the water, or away from deadly toxins. It has been proposed that Chloroflexus aggregans' gliding motility is affected by the concentration of cAMP in the environment. Cyclic adenosine monophosphate (cAMP) is a second messenger that controls a variety of biological processes. cAMP is originally derived from ATP and is used for intracellular signal transduction.(Carr 1982). According to Satoshi Hanada and his team of researchers, the aggregation of C. aggregans cell clusters was accelerated when 3-isobutyl-1-methylxanthine was introduced, inhibiting cyclic 3â€²,5â€²-AMP phosphodiesterase from breaking down the cell's cyclic AMP. How this directly affects their motility is still unknown, but research found that this increased the speed at which the cells moved, forming the aggregate mats quicker than without the introduction of the chemical.
Once aggregated in these high populations, these strands of bacteria form a microbial mat. A mat is a type of biofilm that is large enough to see with the naked eye and solid enough to survive moderate physical stresses such as daily heat fluctuations and wave currents. Chloroflexus aggregans however is not the only organisms living within this mat. A mat is a multi-layered sheet of several different micro-organisms, mainly Bacteria and Achaea. The studied bacterial mat consisted of a dark green layer of filamentous and unicellular cyanobacteria covered with bright yellow streamers that were composed of filamentous green bacteria (Hanada 2002). These mats only grow to be up to a few centimeters thick, yet they are known to create a wide range of chemical environments within the biofilm.
This variety means that the mats are generally composed of layers of micro-organisms that tolerate and even feed upon the prevailing chemicals at their site within the mat. Closely-related species are usually found in these mats, allowing them to grow in the same chemically controlled conditions. Chloroflexus usually forms these dense bacterial mats with or without thermophilic cyanobacteria layered throughout the mat. When present, the cyanobacteria produce substrates allowing the Chloroflexus to grow in a photoheterotrophic manner. Without the presence of the cyanobacteria, Chlorflexus will switch to autotrophically using sulfide and carbon dioxide to sustain its needs.
Microbial mats are the earliest form of life on Earth for which there is good fossil evidence from 3,500 million years ago. They also play a major role in the maintenance of the planet's ecosystem. Early microbial mat communities depended on hydrothermal vents for energy and chemical nutrients. It was not until the advent of photosynthesis that microbes began to move from the hydrothermal vents via utilizing a more widely-available energy source, the sun. However this transition did not happen overnight. There was a time in the oceans that microbes were developing photosynthetic pathways yet still relied on direct chemical intervention to produce their main energy components. Although it has been established that Chloroflexus aggregans is an anoxygenic phototroph, it seems to be a crossbreed phototroph. Its photosynthetic mechanism shows features typical of both purple bacteria and green sulfur bacteria. As in the green sulfur bacteria, Chloroflexus contains bacteriochlorophyll C and chlorosomes.
However, the structure of the photosynthetic reaction center is very similar to that of the purple bacteria due to the arrangement of bacteriochlorophyll A located in the cytoplasmic membrane. These discrepancies may indicate that Chloroflexus may be a remnant of a very early form of phototroph that perhaps first evolved a photosynthetic reaction center and then received chlorosome-specific genes by lateral gene transfer.
Researchers believe that further study of Chloroflexus can illuminate some of the unknowns that took place during this developmental period due to its facultative anaerobic abilities. The entire Chloroflexus genus is known for being anoxygenic phototrophs. This means that the bacteria produce their own energy via photosynthesis while the primary electron donor is something other than water. Unlike oxygenic phototrophs such as cyanobacteria, algae, and higher plants, there is no oxygen produced. They utilize type II photosynthetic reaction centers containing bacteriochlorophyll A. Purple bacteria were the first bacteria found to use this process. Instead of producing oxygen as their byproduct, they produce sulfur. Studies showed that under increasing amounts of oxygen, the bacteria actually moved away from the source, seeking non-oxygenated waters. Chloroflexus can be shown in stark contrast in that it can thrive both with and without the presence of oxygen in its surroundings.
Chloroflexus also utilizes light-harvesting chlorosomes. A Chlorosome is a complex photosynthetic antenna structure found in green sulfur bacteria (GSB) and green filamentous anoxygenic phototrophs (FAP) such as Chloroflexaceae. Chlorosomes are ellipsoidal bodies, shaped like a smooth, rounded river stone. Compared to other antenna complexes, chlorosomes are much larger and lack a protein matrix surrounding their photosynthetic pigments. Each chlorosome contains up to 250,000 chlorophyll molecules, mainly bacteriochlorophylls A and C. These pigments are closely related to chlorophylls, which are the main pigments found in plants, algae, and cyanobacteria. Consistent with Chloroflexus aggregans, organisms that contain bacteriochlorophyll use photosynthesis as their primary energy production process, but do not produce oxygen as a byproduct. Bacteriochlorophylls are specialized photosynthetic pigments found within various phototrophic bacteria ranging from purple bacteria to green sulfur and nonsulfur bacteria. They use wavelengths of light not absorbed by plants or Cyanobacteria, offering different colored organisms as a result. Chloroflexus aggregans' chlorosomes absorb wavelengths of 745-755 nm in length, giving them a green appearance.
The process of converting solar energy into biochemical energy in all organisms that contain chlorophyll or bacteriochlorophyll is very similar. First, the pigment molecules in the antenna regions (the chlorosomes) capture light energy, and store it temporarily in the bacteriochlorophylls as an excited electronic state. Next, the excess energy is forwarded to the reaction center of the photosystem. This arrival of the electrons at a specific bacteriochorophyll within the chlorosome triggers a photochemical reaction separating a positive and negative charge across the membrane. Finally this charge separation starts a series of electron transfers coupled to the movement of protons across the membrane, creating an electrochemical proton gradient. This gradient, or imbalance of charges from one side to the other across the membrane, can be used to power reactions such as the synthesis of ATP.
While oxygenic phototrophs (organisms using light and producing oxygen) use water as their electron donor for their respective phototropic reactions, Chloroflexus uses reduced sulfur compounds such as hydrogen sulfide, thiosulfate, or elemental sulfur. Different species of Chloroflexus can also use hydrogen as a source of electrons. The complete detailed electron transport chain for all of the various Chloroflexus species is not yet known. Physiological and biochemical studies suggest that Chloroflexus aggregans is distinct from Chloroflexus aurantiacus strains, and for this reason was given a different species designation.
Beyond these initial physiological studies, the properties of this organism are largely unknown. Hopefully processing the entire genome should contribute significantly to our knowledge of the origins and evolution of photosynthesis and will help to understand anoxygenic phototrophic metabolism in significantly greater detail. Carbon dioxide fixation in these organisms occurs by the 3-hydroxypropionate pathway (Klatt et al., 2007). This pathway for CO2 assimilation has only been found in bacterial species. The 3-hydroxypropionate cycle uses acetyl-CoA and propionyl-CoA carboxylases to fix CO2, ultimately forming malyl-CoA, which is then split back into two copies of acetyl-CoA to replenish the cycle. The second copy is converted into glyoxylate, for use as cell carbon. This pathway was originally discovered in Chloroflexus species, and only recently has it also been found in several autotrophic Achaea.
Anoxygenic photosynthetic bacteria are a widespread and metabolically-diverse group of organisms that make a significant contribution to energy input into the biosphere from sunlight. This specific species of bacteria, Chloroflexus aggregans was determined to be its own isolated species from the Chloroflexus genus. While the novel bacteria exhibited many characteristics that were similar to that of its already known relative, Chloroflexus auranticus, its ability to aggregate into cell masses is unique. While the precise machinery of their movement is still unknown, it has been proposed that chemotaxis involving cAMP effects their ability to do so. In addition to their native contribution, Cloroflexus aggregans are being used in leading laboratory research studying the mechanism of photosynthetic energy transduction by natural systems. This species may be found to be the "missing link" between the underwater hydrothermal vent-living bacteria and the oxygen producing surface bacteria that created the world that allowed higher forms of life to evolve. It is the hope that by understanding how these organisms first came to be, we can better understand the world around us.