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.
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, 1.2μ long, 0.7μ 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.
AAPBs are currently classified taxonomically in 6 freshwater genera, Roseococcus, Erythromicrobium, Porphyrobacter, Acidiphilium, Erythromonas, and Sandaracinobacter. The remaining 2 marine freshwater genera are listed as, Erythrobacter and Roseobacter. The realization that the synthesized BChl a by obligately aerobic heterotrophs led to further research to determine their phylogenetic and taxonomic positions, in order to hopefully reveal their evolutionary origin. Studies revealed that the closest relatives were the purple nonsulfur phototrophic bacteria. Erythrobacter, Porphyrobacter, and Erythromicrobium are very similar genera. Phototrophy is a noticeable and significant marker that should always play a primary role in bacterial classification.
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.
One study revealed that, among the 3 oceans, the surface water of the Indian Ocean ranked the highest 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.