Production and consumption of nitrous oxide in nitrate-ammonifying Wolinella succinogenes cells

摘要

Global warming is moving more and more into the public consciousness. Besides the commonly mentioned carbon dioxide and methane, nitrous oxide (N 2 O) is a powerful greenhouse gas in addition to its contribution to depletion of stratospheric ozone. The increasing concern about N 2 O emission has focused interest on underlying microbial energy-converting processes and organisms harbouring N 2 O reductase (NosZ), such as denitrifiers and ammonifiers of nitrate and nitrite. Here, the epsilonproteobacterial model organism Wolinella succinogenes is investigated with regard to its capacity to produce and consume N 2 O during growth by anaerobic nitrate ammonification. This organism synthesizes an unconventional cytochrome c nitrous oxide reductase ( c NosZ), which is encoded by the first gene of an atypical nos gene cluster. However, W. succinogenes lacks a nitric oxide (NO)-producing nitrite reductase of the NirS- or NirK-type as well as an NO reductase of the Nor-type. Using a robotized incubation system, the wild-type strain and suitable mutants of W. succinogenes that either produced or lacked c NosZ were analysed as to their production of NO, N 2 O and N 2 in both nitrate-sufficient and nitrate-limited growth medium using formate as electron donor. It was found that cells growing in nitrate-sufficient medium produced small amounts of N 2 O, which derived from nitrite and, most likely, from the presence of NO. Furthermore, cells employing c NosZ were able to reduce N 2 O to N 2 . This reaction, which was fully inhibited by acetylene, was also observed after adding N 2 O to the culture headspace. The results indicate that W. succinogenes cells are competent in N 2 O and N 2 production despite being correctly grouped as respiratory nitrate ammonifiers. N 2 O production is assumed to result from NO detoxification and nitrosative stress defence, while N 2 O serves as a terminal electron acceptor in anaerobic respiration. The ecological implications of these findings are discussed.