"Candidatus Scalindua" is a bacterial genus, and a proposed member of the order Planctomycetales. These bacteria lack peptidoglycan in their cell wall and have a compartmentalized cytoplasm. They are ammonium oxidizing bacteria found in marine environments.
Introduction "Candidatus Scalindua" is a bacterial genus, member of the order Planctomycetales. These bacteria lack peptidoglycan in their cell wall and have a compartmentalized cytoplasm. "Candidatus Scalindua" spp. 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 "Ca. Scalindua" are the most abundant anammox bacteria in marine environments, so they are vital to the Earth's nitrogen cycle.
Metabolism Members of the proposed genus 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 μM inhibits the anammox pathway, which is why members of the proposed genus 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 members of the phylum Planctomycetota. Anammox bacteria, including those belonging to Ca. Scalindua, 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 were 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 indications that anaerobic methane oxidation was in fact a widespread occurrence. It was not until 1999 that the existence of anaerobic 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 rates 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, combined 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 anammox bacteria discovered concluded that the organisms branched deeply in the phylum, Planctomycetota, 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. The role of bacteria belonging to Ca. Scalindua in the marine nitrogen cycle has been found to be of important in the reduction of nitrate to atmospheric nitrogen in 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 Ca. 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 Ca. Scalindua spp. In other regions such as the Black Sea, the world's 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 bacteria belonging to Ca. Scalindua spp.
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