Nitrifier denitrification is the pathway of nitrification in which ammonia (NH 3 ) is oxidized to nitrite (NO 2 − ) followed by the reduction of NO 2 − to nitric oxide (NO), nitrous oxide (N 2 O) and molecular nitrogen (N 2 ). The transformations are carried out by autotrophic nitrifiers. Thus, nitrifier denitrification differs from coupled nitrification–denitrification, where denitrifiers reduce NO 2 − or nitrate (NO 3 − ) that was produced by nitrifiers. Nitrifier denitrification contributes to the development of the greenhouse gas N 2 O and also causes losses of fertilizer nitrogen in agricultural soils. In this review article, present knowledge about nitrifier denitrification is summarized in order to give an exact definition, to spread awareness of its pathway and controlling factors and to identify areas of research needed to improve global N 2 O budgets. Due to experimental difficulties and a lack of awareness of nitrifier denitrification, not much is known about this mechanism of N 2 O production. The few measurements carried out so far attribute up to 30% of the total N 2 O production to nitrifier denitrification. Low oxygen conditions coupled with low organic carbon contents of soils favour this pathway as might low pH. As nitrifier denitrification can lead to substantial N 2 O emissions, there is a need to quantify this pathway in different soils under different conditions. New insights attained through quantification experiments should be used in the improvement of computer models to define sets of conditions that show where and when nitrifier denitrification is a significant source of N 2 O. This may subsequently render the development of guidelines for low-emission farming practices necessary.

Role of nitrifier denitrification in the production of nitrous oxide

Dissimilatory Sulfate- and Sulfur-Reducing Prokaryotes

Microbial mats: A joint venture

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Physiological and metabolic responses to hypoxia in invertebrates

A new chemolithotrophic bacterial metabolism was discovered in anaerobic marine enrichment cultures. Cultures in defined medium with elemental sulfur (S0) and amorphous ferric hydroxide (FeOOH) as sole substrates showed intense formation of sulfate. Furthermore, precipitation of ferrous sulfide and pyrite was observed. The transformations were accompanied by growth of slightly curved, rod-shaped bacteria. The quantification of the products revealed that S0 was microbially disproportionated to sulfate and sulfide, as follows: 4S0 + 4H2O → SO42- + 3H2S + 2H+. Subsequent chemical reactions between the formed sulfide and the added FeOOH led to the observed precipitation of iron sulfides. Sulfate and iron sulfides were also produced when FeOOH was replaced by FeCO3. Further enrichment with manganese oxide, MnO2, instead of FeOOH yielded stable cultures which formed sulfate during concomitant reduction of MnO2 to Mn2+. Growth of small rod-shaped bacteria was observed. When incubated without MnO2, the culture did not grow but produced small amounts of SO42- and H2S at a ratio of 1:3, indicating again a disproportionation of S0. The observed microbial disproportionation of S0 only proceeds significantly in the presence of sulfide-scavenging agents such as iron and manganese compounds. The population density of bacteria capable of S0 disproportionation in the presence of FeOOH or MnO2 was high, > 104 cm-3 in coastal sediments. The metabolism offers an explanation for recent observations of anaerobic sulfide oxidation to sulfate in anoxic sediments.

Bacterial disproportionation of elemental sulfur coupled to chemical reduction of iron or manganese.

This book contains 17 selections. Some of the titles are: Genetic regulation of nitrogen fixation; On the analysis of symbiotic genes of Rhizobium; Regulation of nitrogen assimilation by bacteria; Alternative and conventional nitrogenases; and The role of oxygen and hydrogen in nitrogen fixation.

S. J. Ferguson

Papers

The nitrogen and sulphur cycles