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Dynamics of ammonia oxidizing Archaea in the South Atlantic Bight

261 Marine Science Bldg.

ABSTRACT:  Previous work on the Georgia, USA coast revealed consistent mid-summer peaks in the abundance of Thaumarchaeota accompanied by spikes in nitrite concentration.  We collected data on the distribution of Thaumarchaeota, ammonia-oxidizing Betaproteobacteria (AOB), nitrite-oxidizing Nitrospina, environmental variables and rates of ammonia oxidation during 6 cruises in the South Atlantic Bight from April to November 2014 to evaluate the areal extent and timing of the bloom.  The abundance of Thaumarchaeota marker genes (16S rRNA and amoA) increased at inshore and nearshore stations starting in July and peaked in August at >107 copies L-1, a period when free Cu2+ concentrations (sub-fM) were well below those believed to limit Thaumarchaeota growth.  The bloom did not extend onto the mid-shelf, where Thaumarchaeota genes ranged from 103 to 105 copies L-1.  Clone libraries from samples collected at mid-shelf stations generated using Archaea 16S rRNA primers were dominated by sequences from Marine Group II and III Archaea, whereas libraries from inshore and nearshore stations were dominated by Thaumarchaeota.  Thaumarchaeota were also abundant in oxygen-depleted waters at depth at the shelf-break.  This population was phylogenetically-distinct from the inshore/nearshore population.  Ammonia oxidation rates (AO) were highest at inshore stations and were at the limit of detection at mid-shelf stations.  AO correlated significantly with ammonium concentration (r2=0.23) and Thaumarchaeota abundance (r2=0.14).  Nitrite concentration correlated with AO (r2=0.74).  Our analysis of environmental data suggests that Thaumarchaeota distributions in the SAB are controlled primarily by photoinhibition and secondarily by water temperature.  Experiments with enrichment cultures verified the strong dependence of AO on temperature. Instantaneous rates of AO appear to controlled primarily by ammonium availability.

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