Photosynthetic bacteria have been around for longer than the Earth’s atmosphere could sustain human life. It was only recently though that scientists began to unravel the mystery of how these micro-organisms execute the mechanisms of photosynthesis.
While scientists still have not been able to put all the pieces of the photosynthetic bacteria puzzle in the right places, they are actively studying them and are gaining valuable knowledge about the way they photosynthesize and how they have evolved. In fact, they believe that these micro-organisms may have had a huge impact on why the world evolved the way it did, and may show potential for life in places deemed uninhabitable, including extreme climates like Antarctica and even other planets.
What are photosynthetic bacteria?
Much like the name suggests, these micro-organisms are special types of bacteria that contain light absorbing pigments and reaction centers which make them capable of converting light energy into chemical energy.
Cyanobacteria contain chlorophyll while other forms of bacteria contain bacteriochlorophyll. Although bacteriochlorophyll resembles chlorophyll, it absorbs light of a longer wavelength than chlorophyll. Bacteriochlorophyll a is the most common form of bacteriochlorophyll but other forms include b, c, d, e, f and g.
Bacteria that contain bacteriochlorophyll do not use water as an electron donor and therefore do not produce oxygen. This is known as anoxygenic photosynthesis. Cyanobacteria perform photosynthesis using water as an electron donor in a similar manner to plants. This results in the production of oxygen and is known as oxygenic photosynthesis.
Classification of Photosynthetic Bacteria
Oxygenic photosynthetic bacteria perform photosynthesis in a similar manner to plants. They contain light-harvesting pigments, absorb carbon dioxide, and release oxygen. Cyanobacteria or Cyanophyta are the only form of oxygenic photosynthetic bacteria known to date. There are, however, several species of Cyanobacteria. They are often blue-green in color and are thought to have contributed to the biodiversity on Earth by helping to convert the Earth’s early oxygen-deficient atmosphere to an oxygen-rich environment. This transformation meant that most anaerobic organisms that thrived in the absence of oxygen eventually became extinct and new organisms that were dependent on oxygen began to emerge.
Cyanobacteria are mostly found in water but can survive on land, in rocks, and even in animal shells (or fur), and in coral. They are also known to be endosymbiont, which means they can live within the cells or body of another organism in a mutually beneficial way. Cyanobacteria also tend to live in extreme weather conditions, such as Antarctica, and are interesting to scientists because they may indicate a chance for life on other planets such as Mars.
Anoxygenic photosynthetic bacteria consume carbon dioxide but do not release oxygen. These include Green and Purple bacteria as well as Filamentous Anoxygenic Phototrophs (FAPs), Phototrophic Acidobacteria, and Phototrophic Heliobacteria. Let’s look at the differences between these types of bacteria a little more closely.
Purple bacteria can be divided into two main types – the Chromatiaceae, which produce sulfur particles inside their cells, and the Ectothiorhodospiraceae, which produce sulphur particles outside their cells. They cannot photosynthesize in places that have an abundance of oxygen, so they are typically found in either stagnant water or hot sulfuric springs.
Instead of using water to photosynthesize, like plants and cyanobacteria, purple sulfur bacteria use hydrogen sulfide as their reducing agent, which is why they give off sulfur rather than oxygen.
Purple bacteria are probably the most widely studied photosynthetic bacteria, being used for all sorts of scientific endeavors including theories on possible microbiological life on other planets.
Purple non-sulfur bacteria do not release sulfur because instead of using hydrogen sulfide as its reducing agent, they use hydrogen. While these bacteria can tolerate small amounts of sulfur, they tolerate much less than purple or green sulfur bacteria, and too much hydrogen sulfide is toxic to them.
Green sulfur bacteria generally do not move (non-motile), and can come in multiple shapes such as spheres, rods, and spirals. These bacteria have been found deep in the ocean near a black smoker in Mexico, where they survived off the light of a thermal vent. They have also been found underwater near Indonesia. These bacteria can survive in extreme conditions, like the other types of photosynthetic bacteria, suggesting an evolutionary potential for life in places otherwise thought uninhabitable.
Phototrophic Acidobacteria are found in a lot of soils and are fairly diverse. Some are acidophilic meaning they thrive under very acidic conditions. However, not much is known about this grouping of bacteria, because they are fairly new, the first being found in 1991.
Phototrophic Heliobacteria are also found in soils, especially water-saturated fields, like rice paddies. They use a particular type of bacteriochlorophyll, labelled g, which differentiates them from other types of photosynthetic bacteria. They are photoheterotroph, which means that they cannot use carbon dioxide as their primary source of carbon.
Green and red filamentous anoxygenic phototrophs (FAPs) were previously called green non-sulfur bacteria, until it was discovered that they could also use sulfur components to work through their processes. This type of bacteria uses filaments to move around. The color depends on the type of bacteriochlorophyll the particular organism uses. What is also unique about this form of bacteria is that it can either be photoautotrophic, meaning they create their own energy through the sun’s energy; chemoorganotropic, which requires a source of carbon; or photoheterotrophic, which, as explained above, means they don’t use carbon dioxide for their carbon source.
The slide presentation below provides additional information about photosynthetic bacteria.
Useful Applications for Photosynthetic Bacteria
Photosynthetic bacteria are currently being used in various applications which include water purification, bio-fertilizers, animal feed and bioremediation of chemicals among many others. They are used in the treatment of polluted water since they can grow and utilize toxic substances such as H2S or H2S203.
Researchers at Harvard’s Wyss Institute have engineered photosynthetic bacteria to produce simple sugars and lactic acid. In the video below, Dr. Jeffrey Way explains the science behind the innovation and the potential benefits of this technology.