Abstract
Anaerobic ammonia oxidation (anammox) as an important nitrogen loss pathway has been reported in marine oxygen minimum zones (OMZs), but the community composition and spatial distribution of anammox bacteria in the eastern tropical North Pacific (ETNP) OMZ are poorly determined. In this study, anammox bacterial communities in the OMZ off Costa Rica (CRD-OMZ) were analyzed based on both hydrazine oxidoreductase (hzo) genes and their transcripts assigned to cluster 1 and 2. The anammox communities revealed by hzo genes and proteins in CRD-OMZ showed a low diversity. Gene quantification results showed that hzo gene abundances peaked in the upper OMZs, associated with the peaks of nitrite concentration. Nitrite and oxygen concentrations may therefore colimit the distribution of anammox bacteria in this area. Furthermore, transcriptional activity of anammox bacteria was confirmed by obtaining abundant hzo mRNA transcripts through qRT-PCR. A novel hzo cluster 2x clade was identified by the phylogenetic analysis and these novel sequences were abundant and widely distributed in this environment. Our study demonstrated that both cluster 1 and 2 anammox bacteria play an active role in the CRD-OMZ, and the cluster 1 abundance and transcriptional activity were higher than cluster 2 in both free-living and particle-attached fractions at both gene and transcriptional levels.
Highlights
For a long time, aerobic nitrification and anaerobic denitrification were thought to be the only major pathways for ammonium oxidation and nitrogen (N) loss in the marine N cycle
A subsurface peak of ammonium and nitrite concentrations in the euphotic zone was observed and nitrite concentration reached its maximum of 0.98-1.50 μM within the oxygen minimum zones (OMZs)
A low diversity of anammox communities has previously been detected at the 16S rRNA gene level in marine OMZ waters [48,49]
Summary
Aerobic nitrification and anaerobic denitrification were thought to be the only major pathways for ammonium oxidation and nitrogen (N) loss in the marine N cycle. The detection of widely distributed anammox bacteria in natural ecosystems has greatly changed our previous understanding of N sink in the N cycle [3]. More recent studies showed that anammox is the dominant process of N loss, with no denitrification activities detected, in the Benguela upwelling system [5], the Black Sea [6] and the Peruvian oxygen minimum zone [7]. The ubiquitous distribution of anammox bacteria in various oxygen-depleted ecosystems [3], as well as its potentially coupling relationships with denitrification [4], nitrification [6] and dissimilatory nitrate reduction to ammonium (DNRA) [9,10] , further emphasizes the critical role of anammox in the global N budget
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