Abstract
Anaerobic ammonium oxidizing (anammox) bacteria own a central position in the global N-cycle, as they have the ability to oxidize NH4+ to N2 under anoxic conditions using NO2−. They are responsible for up to 50% of all N2 released from marine ecosystems into the atmosphere and are thus indispensible for balancing the activity of N-fixing bacteria and completing the marine N-cycle. The contribution, diversity, and impact of anammox bacteria in freshwater ecosystems, however, is largely unknown, confounding assessments of their role in the global N-cycle. Here we report the activity and diversity of anammox bacteria in the world’s largest freshwater lake—Lake Superior. We found that anammox performed by previously undiscovered bacteria is an important contributor to sediment N2 production. We observed striking differences in the anammox bacterial populations found at different locations within Lake Superior and those described from other locations. Our data thus reveal that novel anammox bacteria underpin N-loss from Lake Superior, and if more broadly distributed across inland waters would play an important role in continental N-cycling and mitigation of fixed nitrogen transfer from land to the sea.
Highlights
As one of Earth’s great biogeochemical cycles, the N-cycle intimately couples to the carbon cycle, playing an important role in regulating global primary production[1]
Anammox bacteria have been detected in freshwater sediments[5,6] and appear diverse and widely distributed in soils[7,8,9,10]
Low total rates of N2 production were found at both stations (Table 1) and are consistent with the low overall biogeochemical activity of Lake Superior’s sediments, as indicated by oxygen uptake rates[13]
Summary
Holding 10% of the world’s surface freshwater, Lake Superior is an important natural resource It is characterized by low rates of primary production[12], and its sediments exhibit low rates of biogeochemical activity and very deep oxygen penetration (Fig. 1)[13]. Greater importance of anammox to N2 production at station SW compared to IR is in line with the general overall observation that anammox plays a proportionally greater role relative to denitrification in sediments with lower overall biogeochemical activity[18,19,20], as for example qualified by oxygen respiration rates. Anammox was observed at both stations, it was only observed at the shallowest depth at IR, and at lower rates than observed in the incubations with added 15N-NO3− We attribute this to nitrite limitation in the shallow sediment and a complete lack of nitrate and nitrite in the deeper sediment layer at IR. This possibility should be tested through further studies to expand the geographical range of our observations and to more completely map the diversity of anammox bacteria on the continents
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