Abstract
The role of ammonia-oxidizing archaea (AOA) in nitrogen cycling in marine sediments remains poorly characterized. In this study, we enriched and characterized AOA from marine sediments. Group I.1a crenarchaea closely related to those identified in marine sediments and "Candidatus Nitrosopumilus maritimus" (99.1 and 94.9% 16S rRNA and amoA gene sequence identities to the latter, respectively) were substantially enriched by coculture with sulfur-oxidizing bacteria (SOB). The selective enrichment of AOA over ammonia-oxidizing bacteria (AOB) is likely due to the reduced oxygen levels caused by the rapid initial growth of SOB. After biweekly transfers for ca. 20 months, archaeal cells became the dominant prokaryotes (>80%), based on quantitative PCR and fluorescence in situ hybridization analysis. The increase of archaeal 16S rRNA gene copy numbers was coincident with the amount of ammonia oxidized, and expression of the archaeal amoA gene was observed during ammonia oxidation. Bacterial amoA genes were not detected in the enrichment culture. The affinities of these AOA to oxygen and ammonia were substantially higher than those of AOB. [(13)C]bicarbonate incorporation and the presence and activation of genes of the 3-hydroxypropionate/4-hydroxybutyrate cycle indicated autotrophy during ammonia oxidation. In the enrichment culture, ammonium was oxidized to nitrite by the AOA and subsequently to nitrate by Nitrospina-like bacteria. Our experiments suggest that AOA may be important nitrifiers in low-oxygen environments, such as oxygen-minimum zones and marine sediments.
Highlights
The role of ammonia-oxidizing archaea (AOA) in nitrogen cycling in marine sediments remains poorly characterized
Critical evidence for autotrophic archaeal ammonia oxidation was obtained by the characterization of the first cultivated mesophilic crenarchaeon, “Candidatus Nitrosopumilus maritimus SCM1,” from an aquarium [38], and a related archaeon from North Sea water [87] and subsequently by enrichment of thermophilic AOA [22, 31]
Factors leading to the selective enrichment of sedimentary AOA
Summary
The role of ammonia-oxidizing archaea (AOA) in nitrogen cycling in marine sediments remains poorly characterized. Using Archaea-specific PCR primers, novel archaeal 16S rRNA gene sequences were discovered in seawater [23, 27] Following these discoveries, an ever-increasing and unexpectedly high variety of archaeal 16S rRNA gene sequences has been reported from diverse “nonextreme” environments [67]. An ever-increasing and unexpectedly high variety of archaeal 16S rRNA gene sequences has been reported from diverse “nonextreme” environments [67] This indicates that archaea are, like bacteria, ubiquitous in the biosphere rather than exclusively inhabiting specific extreme niches. Molecular ecological studies indicated that these ammonia-oxidizing archaea (AOA) are often predominant over ammonia-oxidizing bacteria (AOB) in ocean waters [9, 53, 87], soils [17, 47], and marine sediments [61]. Archaeal amoA genes have been retrieved from marine and coastal sediments [8, 26, 61], and the potentially important role of AOA in nitrification has Downloaded from https://journals.asm.org/journal/aem on 27 January 2022 by 3.82.249.187
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