Non-catalyzable denitrification intermediates induce nitrous oxide reduction in two purple nonsulfur bacteria

Abstract

AbstractDenitrification is a form of anaerobic respiration wherein nitrate is sequentially reduced via nitrite, nitric oxide, and nitrous oxide to dinitrogen gas. The pathway has mostly been characterized in bacteria that have the complete denitrification pathway. However, each individual step can serve as a form of anaerobic respiration and many bacteria only have partial denitrification pathways. The conditions under which these partial denitrification pathways are activated have received comparatively little attention. Here we report the activity of partial denitrification pathways in two purple nonsulfur bacteria, Rhodopseudomonas palustris CGA009 and Rhodobacter capsulatus SB1003. These bacteria can use one or more denitrification steps as an electron sink during phototrophic growth or to transform energy during anaerobic respiration in the dark. In each case, nitrous oxide reduction required supplementation with a non-catalyzable denitrification intermediate. Thus, bacteria that maintain partial denitrification pathways are still subject to regulation by denitrification intermediates that they cannot use.ImportanceDenitrification is a form of microbial respiration wherein nitrate is converted into dinitrogen gas, a major component of Earth’s atmosphere, via several nitrogen oxide intermediates. Unfortunately, denitrification of nitrate in agricultural fertilizers is often incomplete, resulting in the emission of the potent greenhouse gas, nitrous oxide. Some bacteria do not have all the steps for denitrification, but many can still reduce nitrous oxide into harmless dinitrogen gas. These partial denitrifying bacteria have received little attention. Here we characterized two partial denitrifying bacteria that are capable of nitrous oxide reduction. Surprisingly, nitrous oxide reduction was induced by denitrification intermediates that the bacteria could not respire, suggesting that regulation of nitrous oxide reduction was subject to the same cues as in complete denitrifiers. This work thus informs on how partial denitrifying bacteria can contribute to, and potentially combat, greenhouse gas emissions.