Award: OCE-1537995

Nitrogen is an essential nutrient for phytoplankton that often limits primary production in the ocean. The supply of bioavailable nitrogen, primarily in the forms of ammonium or nitrate, therefore plays a key role in global ocean productivity. The largest reservoir of bioavailable nitrogen in the ocean is nitrate in the deep sea, though ammonium cycles rapidly through surface water plankton communities, supporting more gross primary production. Microbial processes primarily control the amount and form in which nitrogen exists globally. One microbially-mediated process, known as nitrification, oxidizes nitrogen in the form of ammonium (N-3) to nitrate nitrogen (N+5). Conversion of that nitrate to nitrogen gas (N0) by microbes living in anoxic environments like sediments is a major loss term from the global reservoir of bioavailable nitrogen. This process may be particularly important in nearshore zones that are strongly influenced by terrestrial run-off that tends to be enriched in anthropogenic nitrogen. Oxidation of ammonium to nitrate is thus widely recognized as a key step in nitrogen biogeochemistry. However, recent results strongly suggest that polyamines, which are organic compounds containing a nitrogen atom linked directly to a carbon atom, may be converted directly to nitrate or nitrite by a process associated with the microorganisms that participate in nitrification. The mechanism of this process, and its importance to the global biogeochemical cycle of nitrogen, are unknown. The goals of this study have been to evaluate the biogeochemical significance of the direct oxidation of polyamine nitrogen to nitrate, compared to canonical nitrification based on ammonia regenerated as a consequence of heterotrophic degradation of the parent compound, and to elucidate the mechanism by which this oxidation occurs. We have examined oxidation of model polyamines, primarily putrescine (1,4 diamino butane), in experiments conducted with enrichment cultures and coastal bacterioplankton. Specifically, we aimed to determine: 1) the consequences of this novel process to ocean geochemistry; 2) the fate of the carbon present in polyamines; 3) what organisms are responsible for the oxidation of polyamine compounds; and 4) the chemical characteristics of the organic nitrogen compounds accessible to this form of oxidation. Intellectual Merit Data collected during this study demonstrated a highly significant correlation between oxidation of nitrogen supplied as polyamines versus ammonia (Figure 1), confirming that polyamine nitrogen is converted to nitrate or nitrite, likely by ammonia oxidizers. This correlation held regardless of the composition of the Thaumarchaeota community (sediment versus water column ecotypes), and location (South Atlantic Bight and the Southern Ocean off the Antarctic Peninsula). Furthermore, oxidation of putrescine nitrogen did not correlate as well with the abundance of Bacteria in the samples as it did with the abundance of Thaumarchaeota and with ammonia oxidation rate. Thaumarchaeota are an important phylum of archaea known to be responsible for oxidizing ammonia in the ocean. We found that Thaumarchaeota oxidized more nitrogen derived from the polyamine putrescine than nitrogen derived from the other organic N compounds we tested, including primary and secondary amines of various chain lengths, amides, quanidinium compounds and alpha amino acids. All of this evidence suggests that Thaumarchaeota are in some way responsible for the oxidation of nitrogen from these organic nitrogen compounds, yet the mechanism for this ? direct oxidation versus oxidation of ammonia regenerated by heterotrophs - was not clear. Broader Impacts The project resulted in training a postdoctoral researcher and provided opportunities for undergraduate students to gain hands-on experience with research on microbial geochemistry and coastal ecosystem processes. Project personnel also worked with outreach efforts led by the Georgia Coastal Ecosystems Long-Term Ecological Research program. We have presented our research results at numerous seminars and international conferences. We published our findings in six peer-reviewed papers and we have at least four additional manuscripts in preparation for submission for publication. Our data are publicly available through the Biological and Chemical Oceanography Data Management Office (https://www.bco-dmo.org/project/757587). Last Modified: 03/06/2019 Submitted by: Brian N Popp