Anaerobic ammonium oxidation (anammox) is a recently discovered microbial pathway and a cost-effective way to remove ammonium from wastewater. Anammox bacteria have been described as obligate chemolithoautotrophs. However, many chemolithoautotrophs (i.e., nitrifiers) can use organic compounds as a supplementary carbon source. In this study, the effect of organic compounds on anammox bacteria was investigated. It was shown that alcohols inhibited anammox bacteria, while organic acids were converted by them. Methanol was the most potent inhibitor, leading to complete and irreversible loss of activity at concentrations as low as 0.5 mM. Of the organic acids acetate and propionate, propionate was consumed at a higher rate (0.8 nmol min ؊1 mg of protein ؊1 ) by Percoll-purified anammox cells. Glucose, formate, and alanine had no effect on the anammox process. It was shown that propionate was oxidized mainly to CO 2 , with nitrate and/or nitrite as the electron acceptor. The anammox bacteria carried out propionate oxidation simultaneously with anaerobic ammonium oxidation. In an anammox enrichment culture fed with propionate for 150 days, the relative amounts of anammox cells and denitrifiers did not change significantly over time, indicating that anammox bacteria could compete successfully with heterotrophic denitrifiers for propionate. In conclusion, this study shows that anammox bacteria have a more versatile metabolism than previously assumed.Anaerobic ammonium oxidation (anammox) is a recently discovered microbial pathway in the biological nitrogen cycle (9, 27) and a new cost-effective way to remove ammonia from wastewater (3, 6, 15, 16, 21, 22 34, 35). Anammox is carried out by the planctomycetes Candidatus "Brocadia anammoxidans" and Candidatus "Kuenenia stuttgartiensis" and several species of the genus Candidatus "Scalindua" (2,7,8,19,20). Nitrite is the electron acceptor for the anaerobic oxidation of ammonia to dinitrogen gas, and hydrazine is an important intermediate (32). Anammox bacteria have been described as strictly autotrophic, fixing CO 2 with nitrite as the electron donor, leading to the anaerobic production of nitrate (25, 33). The overall nitrogen balance showed a ratio of 1:1.32:0.26 for the conversion of ammonium and nitrite and the production of nitrate (26). The overall anammox reaction is presented in equation 1. Many other chemolithoautotrophs (i.e., nitrifiers) can grow mixotrophically; they can use organic compounds as a supplementory carbon source. This property is advantageous because mixotrophic growth can increase the growth rate and/or yield. In the case of anammox, this is especially advantageous, because both the growth rate (doubling time of 10 to 20 days) and yield (0.066 CO 2 fixed per mol of NH 4 ϩ ) are very low. Previously, it was found that some organic compounds inhibit anammox (32). In this study, the effects of organic compounds on the anammox bacteria and the potential for mixotrophic growth were investigated in detail. In experiments with anammox enrichment cultures an...
