Aerobic anoxygenic phototrophic bacteria

Aerobic anoxygenic phototrophic bacteria (AAPBs) are alphaproteobacteria and gammaproteobacteria that are obligate aerobes that capture energy from light by anoxygenic photosynthesis. Anoxygenic photosynthesis is the phototrophic process where light energy is captured and stored as ATP. The production of oxygen is non-existent and, therefore, water is not used as an electron donor. They are widely distributed marine plankton that may constitute over 10% of the open ocean microbial community. They can be particularly abundant in oligotrophic conditions where they were found to be 24% of the community. Aerobic anoxygenic phototrophic bacteria are photoheterotrophic (phototroph)microbes that exist in a variety of aquatic environments. Photoheterotrophs (Gk: photo = light, hetero = (an)other, troph = nourishment), are heterotrophic organisms that use light to produce energy, but are unable to utilize carbon dioxide as their primary carbon source. Most are obligately aerobic, meaning they require oxygen to grow. One remarkable aspect of these novel bacteria is that they, unlike other similar bacteria, are unable to utilize BChl (bacteriochlorophyll) for anaerobic growth. The only photosynthetic pigment that exists in AAPB is BChl a. Anaerobic phototrophic bacteria, on the contrary, can contain numerous species of photosynthetic pigments like bacteriochlorophyll a, b, c, d, e, f, etc. There is still a large void in the areas regarding the abundance and genetic diversity of the AAPB, as well as the environmental variables that regulate these properties.
Cellular structure.
Research suggests that all currently known AAPB contain Gram-negative cell walls. The majority, have shapes that resemble cylinders, as well as flagella and cilia. AAPB cell dimensions are normally, 1.2 μm long, 0.7 μm in diameter, and a cell volume of 0.5 μm3. Their dry weight is 0.05 pg and wet weight is 0.5 pg. 3 types of cell division are known to exist within AAPB, 2 daughter-cell division, 4 daughter-cell division, and the non-typical 3 daughter-cell division, commonly referred to as Y-cell division. AAPB are usually pink or orange in color when isolated from water. Current data suggests that marine bacteria have generation times of several days, whereas new evidence exists that shows AAPB to have a much shorter generation time. All species of AAPB produce large amounts of carotenoid pigments. The color of each species is due to the presence of carotenoids, giving peaks in the blue and green absorption spectra.
Taxonomy.
Aerobic anoxygenic phototrophic bacteria are classified in two marine ("Erythrobacter" and "Roseobacter") and six freshwater ("Acidiphilium", "Erythromicrobium", "Blastomonas" (synonym: "Erythromonas"), "Porphyrobacter", "Roseococcus", and "Sandaracinobacter") genera, which phylogenetically belong to the -1, -3, and -4 subclasses of the class Proteobacteria. Phylogenetically, they are not classified into single group. Species so far described are distributed rather widely within the α-subclass of Proteobacteria in which most of the purple nonsulfur bacteria as well as many non-photosynthetic bacteria are included. Apparently, these aerobic BChl-containing bacteria represent an evolutionary transient phase from anaerobic phototrophs to aerobic non-phototrophs. However, some characteristic features distinct from anaerobic phototrophs suggest that most of them are in an evolutionary stable state. Phototrophy is a noticeable and significant marker that should always play a primary role in bacterial classification.
Carbon cycling.
AAPBs play a key role in carbon cycling but to what extent is still questionable. The key to determining their role in marine ecosystems is the AAPB in total bacteria (AAPB%). Knowing that the BCHl a linked phototrophic function is an added feature to their standard heterotrophic diet of dissolved organic carbon, it is thought that AAPBs would fare better when a lack organic carbon electron donors existed in respiration. They also exhibit a selective advantage in oligotrophic environments.
Distribution.
They are widely distributed in coastal and oceanic environments. One study revealed that the surface water of the Indian Ocean ranked the highest of the oceans in AAPB% at 3.79. The Atlantic Ocean surface waters followed with 1.57 AAPB%. Last, the Pacific Ocean followed closely at 1.08 AAPB%. There was a positive correlation with oceans that held higher values of AAPB% and those with higher levels of chlorophyll a. More specific, the coastal/shelf waters of these oceans had greater amounts of AAPBs, some as high as 13.51% AAPB%. Phytoplankton also affect AAPB%, but little research has been performed in this area. They can also be abundant in various oligotrophic conditions, including the most oligotrophic regime of the world ocean. They are globally distributed in the euphotic zone and represent a hitherto unrecognized component of the marine microbial community that appears to be critical to the cycling of both organic and inorganic carbon in the ocean.