Aerobic anoxygenic phototrophic bacteria

Research suggests that all currently known AAPB contain Gram-negative cell walls. The majority have shapes that resemble cylinders, as well as flagella and pili. AAP bacteria are motile due to one polar or subpolar flagellum. Species of these bacteria such as Roseobacter denitrificans and Sandaracinobacter sibiricus have up to three subpolar flagella. 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. Light harvesting complexes with unusual absorption maxima have been discovered due to the isolation and characterization of new aerobic phototrophic species. A new strain of aerobic anoxygenic phototrophic bacteria, JF-1, was recently isolated from deep-ocean hydrothermal vent waters. These bacteria were found to be pleomorphic which shapes varying from coccoid and ovoid rods, to bean-shaped. The coccoid bacteria were from 0.4 to 0.5 μm in size. The ovoid rods were 0.4 to 0.5 μm in diameter, and 1.0 to 1.2 μm in length. Thread-like formations of up to five bacterial cells were also observed in these vents. ==Taxonomy==

Aerobic anoxygenic phototrophic bacteria are divided into two groups; there are two marine genera (Erythrobacter and Roseobacter), and five freshwater genera (Acidiphilium, Blastomonas (synonym: Erythromonas), Erythrobacter, Roseococcus, and Sandaracinobacter). ==Carbon cycling==

AAPB play a key role in carbon cycling but to what extent is still being investigated. The key to determining their role in marine ecosystems was originally thought to be the AAPB in total bacteria (AAPB%); however, this no longer seems to be the case because AAPB have been found to be much larger than other aquatic bacteria and so their actual effect on production is now thought to be much larger than their abundance would suggest. Since AAPB themselves lack a way to fix carbon, they instead rely on organic matter substrates as a carbon source. ==Distribution==

AAPB are widely distributed in coastal and oceanic environments. AAPB may constitute over 10% of the open ocean microbial community, being particularly abundant in oligotrophic conditions where they were found to make up around 24% of the microbes present. 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. == Isolation, enrichment, and maintenance ==

Aerobic phototrophic bacteria are unable to be isolated on selective medium. Instead, these bacteria are indicated by the colony color due to presence of carotenoids. Aerobic phototrophic bacteria can be isolated using media that are rich in organic compounds using direct inoculation of water samples or sand samples. Inoculated plates are prepared in conditions similar to the natural environments of the collected sample to increase survivability of the bacteria. The presence of Bchl a is what separates these bacteria from other heterotrophic bacteria. Aerobic phototrophic species can remain viable for at least 2 months when stored at 4°C in liquid or on agar surfaces. They can also be preserved long-term by storing in liquid nitrogen or at temperatures -70°C and below. == Limiting growth factors ==

Compared to most other bacteria in their natural environment, AAPB have a relatively large predation pressure, resulting in high growth rates which balance out the high level of grazing they experience. The removal of predators and this grazing pressure results in a large increase in AAP relative to the other bacteria in the environment. Additionally, phosphorus has been identified as a common limiting factor in AAPB growth, which has been shown to sometimes be a stronger limiting factor to AABP growth than predation. Light availability has also been shown to be a factor that stimulates AAPB population growth. Increased light exposure has also been shown to extenuate the growth rate increase caused by removal of predators and the amendment of phosphorus levels. ==References==