Seasonal Distribution and Activity of Ammonia-oxidizing Archaea in the South Atlantic Bight

Thaumarchaeota play an important role in the nitrogen cycle by oxidizing ammonia to nitrite. We quantified ammonia monooxygenase (amoA), and 16S rRNA genes from Bacteria and Archaea by qPCR in samples collected on cruises across the South Atlantic Bight (SAB) off Georgia, USA, from April to November, 2014. We measured Ammonia Oxidation (AO) rates at selected stations using ¹⁵NH₄. Thaumarchaeota 16S rRNA and archaeal amoA genes were abundant (>10⁶ copies L^-1) at the shelf-break over the depth range 75 – 200 m through the year, while the abundances across the continental shelf ranged from 10³ to 10⁵ copies L^-1. We detected a bloom of Thaumarchaeota (16S rRNA) and archaeal amoA at inshore stations beginning in July and peaking in August at >10⁷ copies L^-1. There was no bloom on the shelf, indicating that the bloom seen in inshore waters was not advected from offshore. Strong correlations between Thaumarchaeota 16S rRNA (r=0.67, p<0.0001)/archaeal amoA (r=0.69, p<0.0001) gene abundance and water temperature suggest that the onset of this bloom may be driven by water temperature. In comparison with archaeal amoA abundance, beta-Proteobacterial amoA genes were present at relatively low abundance (undetectable to 10⁴ copies L^-1). The abundance of 16S rRNA genes from Nitrospina, a nitrite oxidizing Bacteria, correlated with the abundance of Thaumarchaeota 16S rRNA and archaeal amoA genes, suggesting close coupling of ammonium oxidization and nitrite oxidation in the SAB. AO rates correlated with archaeal amoA gene abundance and varied seasonally, suggesting a major role of Thaumarchaeota in oceanic nitrification at this location.