Cell biology of anaerobic ammonium-oxidizing bacteria

Anammox bacteria perform anaerobic ammonium oxidation to dinitrogen gas and belong to the phylum Planctomycetes. Whereas most Prokaryotes consist of one compartment, the cytoplasm bounded by the cytoplasmic membrane and cell wall, the species within this phylum are compartmentalized by intracellular membranes. In the anammox case, compartmentalization results in the cytoplasm being divided into three compartments surrounded by individual bilayer membranes. The anammox cell plan thus consists of (from out- to inside): the paryphoplasm, the riboplasm (containing the ribosomes and the nucleoid) and the anammoxosome. The innermost compartment, the anammoxosome, has been speculated to be the locus of anammox catabolism. This hypothesis is based upon the immunogold localization of one of the key enzymes of the anammox reaction, hydrazine/hydroxylamine oxidoreductase, to the anammoxosome. In the hypothesis, the anammoxosome membrane is energized by the translocation of protons from the riboplasm to the anammoxosome. This creates a proton motive force that could be used to drive ATP synthesis by anammoxosome membrane-bound ATPases. The research presented in this thesis uses a combination of genome analysis, molecular tools and transmission electron microscopy including electron tomography to investigate whether the anammoxosome is a true bacterial organelle: a separate compartment with a specific function inside the cell. The results obtained support both the hypothesis that the anammoxosome compartment is indeed the place where the anammox catabolism takes place and that there is no connection between the anammoxosome membrane and the intracytoplasmic membrane. This suggests that anammox bacteria have evolved a true bacterial organelle with a similar function as the eukaryotic mitochondrion: energy metabolism