Variable Influence of Light on the Activity of Ammonia-Oxidizing Thaumarchaeota in the North Pacific Ocean

Although many studies implicate Thaumarchaeota as the major ammonia oxidizers in the oceans, the abiotic factors that determine their distribution and activity are largely unexplored. A few studies with laboratory cultures and isolates have suggested differential sensitivity to sunlight, but a recent study of natural marine communities of AOA indicated no such sensitivity to sunlight. In this study, we examined the influence of light on ammonia oxidation rates for communities at the nitrite maximum across two transects in the North Pacific Ocean. Complementary examination of ammonia monooxygenase subunit A (amoA) genes and amoA transcript abundances and diversities allowed us to characterize the population. At four stations, representing different biogeochemical regimes, we measured the change in ammonia oxidation rate under different sunlight intensities by incubating ¹⁵NH₄⁺-spiked whole seawater collected from the nitrite maximum at five different sunlight intensities on a free floating in situ array. Molecular data suggested that activity originated primarily from AOA, not ammonia-oxidizing bacteria (AOB), at all locations: AOA cells were 48 - 76 times as numerous as AOB cells; AOA amoA transcripts ranged from ~9300 copies ml^-1 (oligotrophic gyre station) to ~56,000 copies ml^-1 (sub-arctic station); and AOB amoA transcripts were below detection at all stations. The fraction of ammonia oxidation inhibited by sunlight varied by location. Ammonia oxidation was usually, but not always, completely inhibited by surface PAR, 21 – 45% inhibited at 1% surface PAR, and there was no inhibition of ammonia oxidation at the depth of the nitrite max and deeper. Data from amplicon tag sequencing of AOA amoA genes suggest that closely related OTUs can have different responses to light. This study highlights differences in basic physiological responses in natural marine communities of Thaumarchaeota.