Oxidation of 15N from ammonium, urea and putrescine by bacterioplankton communities in coastal waters off the West Antarctic Peninsula (PAL-LTER)

James T Hollibaugh1, Brian N Popp2, Natalie Wallsgrove2, Aimee Oyinlade-Oyekan1, Julian Damashek1, Xiaozhen Jen Mou3 and Hugh W Ducklow4, (1)University of Georgia, Department of Marine Sciences, Athens, GA, United States, (2)University of Hawaiʻi at Mānoa, Department of Earth Sciences, Honolulu, United States, (3)Kent State University, Department of Biology, Kent, OH, United States, (4)Columbia University, Lamont Doherty Earth Observatory, New York, New York, United States
Abstract:
Previous studies (Massana et al. 1998, Murray et al 1998, Tolar et al. 2016) in continental shelf waters off the West Antarctic Peninsula (PAL LTER) have documented: 1) the seasonality of Thaumarchaeota populations in surface waters; 2) shown that they are likely responsible for most of the ammonia oxidation in these waters, and; 3) evaluated their contribution to carbon fixation during the polar night. These studies did not consider the contribution of nitrogen in urea to Thaumarchaeota activity, though it has been shown that some strains of Thaumarchaea can oxidize the N from urea. Other work off the Georgia coast (GCE-LTER) has suggested that 15N supplied as putrescine (1,4 diamino butane) is also oxidized, though the mechanism by which this occurs is not clear and cultured Nitrosopumilaceae will not grow on putrescine.

We compared potential rates of 15NO2+15NO3 (15NOx) production from ammonium, urea and putrescine in samples from 3 or 4 depths at 15 stations on ARSV LM Gould cruise 1801. Mean oxidation rates (nmol L-1 d-1) for N supplied as ammonium, urea or putrescine were: 0.06, 0.09 and 0.23 at 10 m; 1.8, 1.7 and 6.4 in Winter Water (50-100 m); 1.03, 1.5, and 5.2 in Circumpolar Deep Water (100-1000m); and 0.13, 0.03 and 0.57 at depths >1000m. We found a strong correlation between 15NOx production from ammonium and putrescine, but weaker correlations between 15NOx production from ammonium and urea. 15NOx production from the 3 substrates did not correlate with any of the qPCR estimates we made of the abundances of Thaumarchaeota, betaproteobacterial AOB or nitrite oxidizing bacteria 16S rRNA; or amoA or ureC genes. Total chemoautotrophy in these same water masses measured as dark incorporation of 14C-bicarbonate was (nmol L-1 d-1): 3.3 in the Winter Water, 2.3 in the Circumpolar Deep Water and 0.2 at depths >1000m, compared to rates of 66, 87 and 11 pmol L-1 d-1 expected from ammonium oxidation calculated as 0.037 moles C fixed per mole of N oxidized (Tolar et al. 2016). Carbon fixation in a subset of these samples was correlated with oxidation of N supplied as ammonium (r2=0.87), urea (r2=0.97) and putrescine (r2=0.88).

Literature Cited: Massana et al. (1998), Limnol Oceanogr 43: 607–617; Murray et al. (1998), Appl Environ Microbiol 64: 2585–2595; Tolar et al. (2016), ISME J 10: 2605–2619.

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