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Introduction
"Candidatus Scalindua" is a bacterial genus, member of the order Planctomycetes. These bacteria lack peptidoglycan in its cell wall and have a compartmentalized cytoplasm. . Candidatus Scalindua can be further divided into three species: Scalindua brodae, Scalindua wagneri, and Scalindua sorokinii. They are ammonium oxidising bacteria found in marine environments. The genus Scalindua are the most abundant annamox bacteria in marine environments, so they are extremely vital in the Earth's nitrogen cycle.

Metabolism
Scalindua are anaerobic anammox (ammonium oxidizing) bacteria. The ammonium-oxidizing reaction composes a significant part of the global nitrogen cycle, by some estimates it is the cause of up to 50% of total nitrogen turnover in marine environments. It consists of the oxidization of ammonium using nitrite as an electron acceptor (both are fixed nitrogen) and subsequent generation of nitrogen gas:

“NH4+ + NO2- = N2 + 2H2O (ΔG° = -357 kj mol-1)”

This reaction uses nitrite (NO2-) as a terminal electron acceptor to produce nitric oxide (NO), which is then combined with ammonium (NH4+) to produce the intermediate hydrazine (N2H4) and water (H2O). Hydrazine, a very reactive molecule also used for rocket fuel, is then oxidized into nitrogen gas (N2). The half reactions may be represented as:

“NO2- + 2H+ + e- = NO + H2O (E° = +0.38V)

NO + NH4+ + 2H+ + 3e- = N2H4 + H2O (E° = +0.06V) N2H4 = N2 + 4H+ + 4e- (E° = -0.75V)”

This metabolic pathway occurs anaerobically, something that was once considered impossible as ammonium was thought to be inert in the absence of oxygen. In fact, the presence of oxygen over 2 uM inhibits the anammox pathway, which is why Scalindua respire anaerobically.

These reactions occur in a large membrane-bound cellular organelle called the anammoxosome, which contains an electron transport chain and an ATPase that pumps protons back into the cytoplasm from the anammoxosome lumen. It functions much like a mitochondrion in eukaryotic cells. The anammoxosome membrane is invaginated (folded in upon itself) to increase its surface area. The existence of membrane-bound cellular organelles is very unusual in prokaryotes, and appears to be limited to the planctomycetes phylum.

Scalindua and other anammox bacteria fix carbon using carbon dioxide as a carbon source. Metagenomic analysis has revealed the presence of genes responsible for the “reductive acetyl-CoA pathway (also known as the Wood-Ljungdahl pathway) which allows for the creation of the precursor molecule acetyl CoA from carbon dioxide.

Discovery and Distribution
Ammonium and methane are known to be relatively difficult to activate with reactions catalyzed by enzymes that make use of high-potential oxygen radicals, which are unavailable to anaerobic life leading to the assumption that both compounds where effectively inactive in low oxygen environments. Throughout the 1970s and 80s, results from several independent studies exploring relationships between methane and sulfate concentrations in marine sediments found indication that anaerobic methane oxidation was in fact a widespread occurrence. It was not until 1999 that the existence of aerobic ammonium oxidation was first discovered in a wastewater treatment plant in The Netherlands and given the name “anammox,” which would later prove to be a key player as part of the marine nitrogen cycle. Some known anammox bacteria include Candidatus Scalindua, Kuenenia, Brocadia, Jettenia and Anammoxoglobus. Of these bacteria, only Candidatus Scalindua spp. can be found in marine ecosystems.

During the past, many microorganisms such as anammox bacteria may have escaped discovery due to their relatively low growth rate requiring very efficient biomass retention absent from classical methods of cultivation. With the use of biofilms to improve the culturability of organisms that naturally occur in biofilms in combination with the use of biomass retention to study slowly growing microorganisms under substrate limitation, a technique using sequencing batch reactors (SBR) was developed for the long-term enrichment, cultivation, and quantitative analysis of a very slowly growing microbial community. Phylogenetic analysis of the first anamox bacteria discovered concluded that the organisms branched deeply into the Planctomycetes phylum, which was previously considered to be of limited environmental importance. Nitrogen loses that could only be explained by the process of anammox continued to be discovered in freshwater waste treatment facilities around the world including North America, Asia, and multiple regions throughout Europe. Scalindua’s role in the marine nitrogen cycle has been found to be of importance regarding the reduction of nitrate to atmospheric nitrogen in the anoxic regions of the ocean. Since primary productivity in the ocean is often limited by nitrogen availability, the removal of usable nitrogen in sediments through anammox by Scalindua may significantly affect biogeochemical cycles in anoxic waters. In certain regions, such as the Golfo Dulce in Costa Rica, up to %35 of atmospheric nitrogen production in the water column can be attributed to Scalindua spp. In other regions such as the Black Sea, the worlds largest anoxic basin characterized by a large gradient in ammonium concentrations (high levels in deep water tapering off to only trace amounts in the suboxic zone), the apparent ammonium sink in the suboxic zone was identified to be the result of anaerobic oxidation by Scalindua spp.

Morphology
Organisms within the genus “Candidatus Scalindua” are classified as gram-negative chemolithoautotrophic bacteria. This means that their carbon and energy largely come from inorganic sources. Furthermore, Scalindua are obligate anaerobes, so they are unable live in oxygen-rich environments.

As with all other organisms within the order Planctomycetes, the cell wall of these organisms does not contain peptidoglycan. Scalindua are spherical in shape, with a diameter of roughly one micrometer, and contain compartmentalized cytoplasms. Furthermore, organisms within Candidatus Scalindua have two inner membranes instead of one inner, and one outer surrounding the cell wall. Cells within Scalindua wagenri are oriented into compact clusters, whereas Scalindua brodae’s cells are found to be more loosely packed. All Scalindua contain unique organelles called Anammoxosomes, which are membrane bound within the cytoplasm. Anammoxosomes are where the anaerobic ammonium oxidation process occurs. The membrane that surrounds anammoxosomes in anammox bacteria contain unique lipids called “ladderane” lipids, which contain a series of cyclobutane ring structures. However, all other membranes within anammox bacteria are similar to organisms within the order Planctomycetes.

Evolutionary History
According to Strous et. al., anammox-capability is the result of a singular evolutionary event. All anammox bacteria are descendents of the same ancient planctomycetes species that first evolved the anammox reaction. The Scalindua genus is the most widespread of all the anammox bacteria genera described so far.

Currently, all anammox bacteria are thought to be members of the order Brocadiales.

Ecological Role
Candidatus Scalindua sp. were the only Candidatus species found in the Black Sea, the Benguela OMZ off the coast of Namibia, and the estuary in the Randers Fjord, Denmark. Globally, Candidatus Scalindua has been discovered in all of the marine environments that have been studied; other marine bacteria are not this ubiquitous.

The ideal environmental conditions, with regards to temperature, pH and salinity for “Candidatus Scalindua sp.” are: 10 to 30°C, 6.0 to 8.5 pH and 0.8% to 4.0% salinity with no activity when salinity was 0%, respectively.

Marine sediments located in deep-sea methane seeps contain anammox bacteria associated with the Candidatus Scalindua sp; these bacteria likely have a substantial role in the nitrogen cycle found in the sediments.

Two types of anammox bacteria, Scalindua (59% abundance) and Kuenenia (41% abundance), have been found in the non-rhizosphere area of the saltmarsh grass Spartina alterniflora while only Scalindua was present within the rhizosphere and at 1.5 times greater than the amount of anammox bacterial in the non-rhizosphere sediments. Changing seasons do not affect the make-up of the anammox bacterial communities within the sediments in and around the rhizosphere; however there was always a greater abundance of the anammox bacteria within the rhizosphere, which peaks during July and October when temperature are warmest. During the warmer parts of the year both communities of the anammox bacteria within and outside of the rhizosphere are more active and produce more N2 with the bacteria in the rhizosphere producing almost twice as much N2.

Taxonomy
Candidatus Scalindua is closely related genetically to the other Anammox bacteria within the order Planctomycetes, such as Candidatus Brocadia and Candidatus Kuenenia. . Yet, Scalindua is quite different from other genuses of Anammox bacteria on the 16s rRNA level. For example, Candidatus Scalindua and Candidatus Brocadia only share 85% similar sequences. Candidatus Scalindua can be further divided into three species: Scalindua brodae, Scalindua wagneri, and Scalindua sorokinii. Scalindua is the most abundant genus of Anammox bacteria known to date, making it very important in the world’s aquatic environments.

Applications
Scalindua wagneri are often used in wastewater treatment plants as a buffer of sorts to reduce the adverse effects of nitrification and denitrification on the local environment

.The use of annamox bacteria in wastewater treatment plants has a drastically reduced cost compared to previous dinitrification methods. Furthermore, it is a much more environmentally friendly method.