Candidatus Brocadia anammoxidans
A Microbial Biorealm page on the genus Candidatus Brocadia anammoxidans
Classification
Higher order taxa
Domain: Bacteria
Phylum: Planctomycetes
Class: Planctomycetacia
Order: Planctomycetales
Family: unclassified Planctomycetales
Genus: Candidatus Brocadia
Species
Candidatus Brocadia anammoxidans
NCBI: Taxonomy |
Description and significance
Candidatus Brocadia anammoxidans (Ca. B. anammoxidans) are aquatic autotrophs best known for their unique ability to anarobically oxidize ammonia to dinitrogen gas, a reaction that has been patented and is otherwise known as the “anammox” reaction (5). The bacteria were first discovered in a wastewater treatment plant in the Netherlands where it was observed that ammonia concentrations dropped while dinitrogen gas concentrations rose in airtight effluent reactors (5). Ca. B. anammoxidans were subsequently held responsible for this process and were deemed to be the first bacteria to demonstrate anaerobic ammonia oxidation abilities (5). The bacteria were isolated from enrichment cultures by density centrifugation and have received ample attention from ecologists who suspect the bacteria’s participation in consuming substantial amounts of nitrogen in the ocean, and from researchers who see the bacteria’s metabolism as a potential in revolutionizing wastewater treatment (3, 8). These microbes are no bigger than one micron in diameter and grow optimally in a pH range of 6.4-8.3 and in a temperature range of 20-43 oC (6, 9). The bacteria is named as such, as Candidatus indicates an uncultureable yet well-characterized organism, Brocadia refers to the Gist-Brocades, the place of its discovery, and anammoxidans describes the process of anaerobic ammonium oxidation (3, 10).
Genome structure
The genome of Ca. B. anammoxidans has yet to be sequenced. The unculturable nature of the bacteria (and of its closest relatives) has made it difficult to ascertain the its genomic particulars. Researchers have found through PCR, however, the complete sequences of the 16S ribosomal RNA gene, tRNA-Ile gene, and tRNA-Ala genes, and a partial sequence of the 23S RNA gene. The known sequences of these genes total 4032 contiguous base-pairs. The size of the genome, the shape and number of chromosomes, and the presence or absence of plasmids are not yet known (7).
Cell structure and metabolism
Ca. B. anammoxidans is a spherical bacterium that lacks peptidoglycan, a common compound found in most microbial cell walls, and displays small cavities known as ‘crateriform structures’ on its surface. The organism also exhibits a compartmentalized cytoplasm -- a rare find in bacteria. The anammoxosome, one of the cellular compartments, comprises 30-60% of the cell volume and is arguably the most integral structure Ca. B. anammoxidans possesses, as it plays the chief role in the bacteria’s unique metabolic process (3).
Nitrogen tracer studies have shown that Ca. B. anammoxidans obtain their energy by anaerobically combining ammonia and nitrite to produce dinitrogen gas (3, 6):
NH4+ + NO2- → N2 + 2H2O
The reaction, which takes place inside the ammoxosome, yields and requires two toxic intermediates, hydroxylamine (NH2OH) and hydrazine (N2H4, otherwise known as rocket-fuel). These toxic intermediates serve as electron generators for the initial step of the “ammox reaction”, the reduction of nitrite (2, 3). The safe containment of the noxious intermediates would be impossible were it not for the unparalleled molecular structure of the anammoxosome membrane. Lipids composed of five linearly cis-linked cyclobutane rings make the membrane unusually dense, limiting the diffusion of hydroxylamine and hydrazine. The diffusion limiting aspect of the membrane not only protects the rest of the cell from the toxic intermediates, but also prevents a substantial decrease in the bacteria’s metabolism. From a bioenergetics perspective, if one molecule of hydrazine diffuses through the anammoxosome, a 50% decrease in the catabolic activity will ensue. The bacteria have low growth rates to begin with; in an optimal environment, they double once every eleven days at best (9).
Ecology
Ca. B. anammoxidans plays a substantial role perpetuating the nitrogen cycle in the ocean. Nitrogen tracer studies and calculations have demonstrated that Ca. B. anammoxidans consume 20-40% of inorganic nitrogen that drops into the suboxic zones of the ocean. The bacterium use nitrite as the electron acceptor to anaerobically oxidize ammonia to dinitrogen gas, which promotes the growth and productivity of aquatic organisms by limiting the amount of inorganic nitrogen found in the ocean. Ca. B. anammoxidans cannot carry this reaction out however, if it is in an oxic ocean area, as the anammox bacteria are extrememy sensitive to concentrations of oxygen (as low as 2 μM) (4).
At the oxic/anoxic ocean interface, Ca. B. anammoxidans can cooperate with members of the genus Nitrosomonas, which are aerobic ammonium oxidizing bacteria, in carrying out the anammox reaction. The Nitrosomonas bacteria oxidize ammonia to nitrite and keep the oxygen concentration low, while Ca. B. anammoxidans takes the nitrite and combines it with ammonia to produce dinitrogen gas anaerobically (3, 4).
Pathology
Ca. B. anammoxidans is not known to cause disease.
Application to Biotechnology
The performance of the anammox reaction by Ca. B. anammoxidans has revolutionized wastewater treatment. Before the discovery of Ca. B. anammoxidans, wastewater treatment was carried out by aerobic bacteria that were obligated to perform nitrification in conjunction with denitrification to free the wastewater of ammonia (5). Furthermore, this process necessitated a supply of menthanol (5). On the other hand, the anaerobic removal of nitrogen from wastewater by Ca. B. anammoxidans leads to a 90% reduction in operational costs, as the anammox process bypasses the denitrification step of the nitrogen cycle completely and does not require expensive methanol as fuel (3).
Current Research
1) A recent phylogenic study in the University of Queensland was conducted to investigate the similarities and differences of the primary and secondary genetic sequences of Candidatus Brocadia anammoxidans and Candidatus Kuenenia stuttgartiensis. The two planctomycetes’ ribonuclease P RNA gene sequences and secondary structures were compared and found to be identical in helix number. Upon a phylogenetic analysis based on helices, the anammox organisms were repeatedly found to be closely clustered to Gemmata obscuriglobus, a sister planctomycete. It was found that P13, a unique bacterial helix insert was found in only these three planctomycetes. This find is significant as the helix insert is exclusively shared between three organisms, indicating a shared common ancestor for ribonuclease P RNA molecules among these three species (1).
2) Significant improvements on the quality of wastewater treatment with Ca. B. anammoxidans have been made through new anammox reactor developments. The newly developed anammox reactors yield a nitrogen removal rate of 25 kg-N m-3 per day, a number that triples the current nitrogen removal rate of 8.7 kg-N m-3 per day. A faster Ca. B. anammoxidans doubling rate was seen as well. It took 3.6-5.4 days for the bacteria to double in the new reactor, which is less than half the doubling time of Ca. B. anammoxidans mentioned in current reports. The increased nitrogen removal rate is attributed to the higher density of anammox bacteria (70% of total bacteria) used in the new reactors. The bacteria are retained and kept active inside the reactor with new non-woven fabric sheets, which are used as biofilm carriers, and by a high total nitrogen loading rate, which encourages internal substrate transport. Both bacteria containment devices encourage anammox bacteria division. These improvements made in nitrogen removal promise a more effective means of anaerobic wastewater treatment (12).
3) A study in Hokkaido University, Japan, is innovating current quantification methods of Ca. B. anammoxidans by quantifying the bacteria using a real-time polymerase chain reaction (PCR). The quantification of the bacteria has traditionally been carried out using fluorescence in situ hybridization (FISH) but the method has proved to be impractical. Anammox bacteria have low counts of rRNA molecules per cell and tend to form heavy clusters. These obstacles make FISH difficult to carry out. The use of real-time PCR for quantification is shown in the study to be a more convenient and advantageous method for anammox bacteria enumeration. The real-time method is better adapted to quantify dense microbial clusters and is sensitive enough to quantify the slow-growing Ca. B. anammoxidans in an uncultured environment, as it is based on continual fluorescent monitoring. Real-time PCR was used to quantify the 16S rRNA gene of anammox bacteria subsequent to the development of specific PCR primers for an enrichment culture of anammox bacteria from a rotating disk reactor biofilm (11).
The autotrophic and obligate anaerobic nature of Ca. B. anammoxidans makes culturing the bacteria very difficult, but the quantification of the bacteria’s 16S rRNA gene using real-time PCR sheds light on the intricate details of the bacteria’s physiology and kinetics, which is an important step in isolating Ca. B. anammoxidans in pure culture (11).
References
Edited by Daniel Calaguas, student of Rachel Larsen