Scalindua

"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)” In fact, the presence of oxygen over 2 μM inhibits the anammox pathway, which is why members of the proposed genus Scalindua respire anaerobically. The existence of membrane-bound cellular organelles is very unusual in prokaryotes, and appears to be limited to the members of the phylum Planctomycetota. == 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. 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. 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. 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. == 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, bacteria in the genus Ca. Scalindua are obligate anaerobes, so they are unable live in oxygen-rich environments. As with all other organisms within the order Planctomycetota, the cell wall does not contain peptidoglycan. The cells are spherical in shape, with a diameter of roughly one micrometer, and contain compartmentalized cytoplasms. Cells within Ca. Scalindua wagenri are oriented into compact clusters, whereas Ca. Scalindua brodae's cells are more loosely packed. Anammoxosomes are where anaerobic ammonium oxidation process occurs. The membrane that surrounds anammoxosomes in anammox bacteria contains 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 Planctomycetales. == Evolutionary history ==

According to Strous et al., anammox-capability is the result of a singular evolutionary event. All anammox bacteria are descendants of the same ancient Planctomycetota species that first evolved the anammox reaction. == Taxonomy ==

Members belonging to Candidatus Scalindua are close genetic relatives to other anammox bacteria within the order Planctomycetales, such as "Candidatus Brocadia" and "Candidatus Kuenenia". Cells belonging to Ca. Scalindua spp. are the most abundant members of Anammox bacteria known to date, making it very important in the world's aquatic environments. ==Phylogeny==

The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and National Center for Biotechnology Information (NCBI). Species incertae sedis: • "Ca. S. chemeplantae" Litti et al. 2024 • "Ca. S. erythraensis" Ali et al. 2020 • "Ca. S. flavimaris" corrig. Ahmed et al. 2017 • "Ca. S. marina" van de Vossenberg et al. 2007 • "Ca. S. pacifica" Dang et al. 2013 • "Ca. S. profunda" Van De Vossenberg et al. 2008 • "Ca. S. richardsii" Fuchsman et al. 2012 • "Ca. S. sinooilfield" Li et al. 2010 • "Ca. S. wagneri" Schmid et al. 2003 • "Ca. S. zhenghei" Hong et al. 2011 == Ecological role ==

Candidatus Scalindua sp. is the only taxonomic group of ammonium-oxidizing bacteria found in the Black Sea, the Benguela Oxygen minimum zone off the coast of Namibia, and the estuary of the Randers Fjord, Denmark. Globally, members of Candidatus Scalindua spp. have been discovered in all marine environments that have been studied; most other marine bacteria are not this wide spread. Marine sediments located in deep-sea methane seeps contain anammox bacteria associated with Candidatus Scalindua spp.; these bacteria likely have a substantial role in the nitrogen cycle in the sediments. Two types of anammox bacteria belonging to Ca. Scalindua (59% abundance) and Ca. Kuenenia (41% abundance), have been found in the non-rhizosphere area of the saltmarsh grass Spartina alterniflora while only Ca. Scalindua was present within the rhizosphere. Moreover, it was in 1.5 times greater abundance than for other anammox bacterial in the non-rhizosphere sediments. Changing seasons do not affect the make-up of anammox-capable bacterial communities within the sediments in and around the rhizosphere; however, there was always a greater abundance of anammox bacteria within the rhizosphere that peaked in abundance during July and October when temperatures are warmest. During the warmer parts of the year both communities of 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. == Applications ==

Bacteria belonging to "Ca. Scalindua wagneri" are often used in wastewater treatment plants to reduce the adverse effects of nitrification and denitrification on the local environment. The use of anammox bacteria in wastewater treatment plants has a drastically reduced cost compared to previous denitrification methods. Furthermore, it is a much more environmentally friendly method. ==Species==

• Scalindua brodae • Scalindua wagneri • List of bacterial orders • List of bacteria genera == References ==