The bacterial 'mitochondrium'.

Organelles are membrane-enclosed compartments that serve a dedicated physiological purpose. While eukaryotic organelles are common textbook knowledge, bacteria were long thought to lack such subcellular organization. However, Planctomycetes were proposed to comprise a compartmentalized cell plan, including membrane-enclosed organelles such as the paryphoplasm, the pirellulosome, a nucleus-like membrane system and the anammoxosome. The latter is the hallmark of anaerobic ammonium oxidizing (anammox) bacteria, which gain energy by coupling the oxidation of ammonium to the reduction of nitrite. Since calculations indicate that 50% of nitrogen gas in the earth atmosphere results from anammox activity, this process is key for the global nitrogen cycle. Despite strong evidence for a confined compartment housing this reaction, the concept of planctomycetal compartmentalization in general is currently under debate and the presence of organelles in these bacteria was questioned. However, Neumann et al. (2014) report the isolation of physiological functional anammoxosomes from 'Candidatus Kuenenia stuttgartiensis'. Subsequent proteomic and microscopic analysis revealed a confined organelle, paralleling eukaryotic mitochondria. This advance is of major importance for the understanding of bacterial compartmentalization in general and of the Planctomycetes in particular. Furthermore, the work of Neumann et al. leads to a better understanding of the anammox process.