Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Processing Description
  • Problem Description
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Water samples were collected during cruise AT50-10 in the eastern tropical and subtropical Pacific Ocean. Discrete water samples were obtained using a CTD rosette sampler equipped with 24 x 10 L Niskin bottles. Different depths were sampled ranging from 55 meters to 1500 meters.

For depths where O2 concentrations were above 20 µM, water was dispensed into 40 mL glass vials with teflon septa (TOC-certified, Fisher Scientific). For depths where in situ O2 concentrations were ≤20 µM, water was sampled directly from the Niskin bottle into 60 mL glass serum bottles using tygon tubing, allowing approximately three volumes of sample water to overflow the bottle prior to collection. Serum bottles were closed bubble-free with deoxygenated butyl rubber stoppers (De Brabandere et al. 2012) and sealed with aluminum crimps. A 20 mL helium (He) headspace was introduced to each serum bottle and oxygen was then added back to reach in situ concentrations by injecting air using gas-tight syringes (Hamilton) with volumes calculated using the solubility equations of Garcia and Gordon (1992).

For each depth, seven replicate bottles were filled, sealed and spiked with 50 µCi [14C]-bicarbonate (specific activity 56 mCi mmol−1/2.072 × 109 Bq mmol−1; PerkinElmer). To three bottles, 10 µM phenylacetylene dissolved in DMSO (0.01% final concentration) was added to inhibit ammonia oxidation activities. One bottle served as killed control to which formaldehyde (3% vol/vol) was added at the start of incubation. Bottles were incubated in the dark at in situ temperature and live incubations were terminated after 24-72 h by adding formaldehyde (3% vol/vol). After 30–60 min, samples were filtered onto 0.2 μm polycarbonate filters (GTTP, 25mm, Millipore) and rinsed with 10 mL of artificial seawater at a vacuum pump pressure of ~100 mbar. The filters were transferred to scintillation vials, 10 mL of scintillation cocktail (Ultima Gold; PerkinElmer) was added, samples were shaken for ca. 30 s and incubated in the dark for at least 24 h prior to counting the disintegrations per minute (DPM) in a scintillation counter (Perkin-Elmer Tri-Carb 2910 TR) for 15 min. Total radioactivity measurements were performed to verify added [14C]-bicarbonate concentrations and DIC fixation rates were calculated as described previously (Bayer et al. 2023).

The mean DPM of the samples were corrected for the DPM of the blank, converted into organic carbon fixed over time and corrected for the ambient seawater DIC concentration. 

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NSF abstract:
Though scarce and largely insoluble, trace metals are key components of sophisticated enzymes (protein molecules that speed up biochemical reactions) involved in biogeochemical cycles in the dark ocean (below 1000m). For example, metalloenzymes are involved in nearly every reaction in the nitrogen cycle. Yet, despite direct connections between trace metal and nitrogen cycles, the relationship between trace metal distributions and biological nitrogen cycling processes in the dark ocean have rarely been explored, likely due to the technical challenges associated with their study. Availability of the autonomous underwater vehicle (AUV) Clio, a sampling platform capable of collecting high-resolution vertical profile samples for biochemical and microbial measurements by large volume filtration of microbial particulate material, has overcome this challenge. Thus, this research project plans an interdisciplinary chemistry, biology, and engineering effort to test the hypothesis that certain chemical reactions, such as nitrite oxidation, could become limited by metal availability within the upper mesopelagic and that trace metal demands for nitrite-oxidizing bacteria may be increased under low oxygen conditions. Broader impacts of this study include the continued development and application of the Clio Biogeochemical AUV as a community resource by developing and testing its high-resolution and adaptive sampling capabilities. In addition, metaproteomic data will be deposited into the recently launched Ocean Protein Portal to allow oceanographers and the metals in biology community to examine the distribution of proteins and metalloenzymes in the ocean. Undergraduate students will be supported by this project at all three institutions, with an effort to recruit minority students. The proposed research will also be synergistic with the goals of early community-building efforts for a potential global scale microbial biogeochemistry program modeled after the success of the GEOTRACES program, provisionally called "Biogeoscapes: Ocean metabolism and nutrient cycles on a changing planet".

The proposed research project will test the following three hypotheses: (1) the microbial metalloenzyme distribution of the mesopelagic is spatially dynamic in response to environmental gradients in oxygen and trace metals, (2) nitrite oxidation in the Eastern Tropical Pacific Ocean can be limited by iron availability in the upper mesopelagic through an inability to complete biosynthesis of the microbial protein nitrite oxidoreductase, and (3) nitrite-oxidizing bacteria increase their metalloenzyme requirements at low oxygen, impacting the distribution of both dissolved and particulate metals within oxygen minimum zones. One of the challenges to characterizing the biogeochemistry of the mesopelagic ocean is an inability to effectively sample it. As a sampling platform, we will use the novel biogeochemical AUV Clio that enables high-resolution vertical profile samples for biochemical and microbial measurements by large volume filtration of microbial particulate material on a research expedition in the Eastern Tropical Pacific Ocean. Specific research activities will be orchestrated to test the hypotheses. Hypothesis 1 will be explored by comparison of hydrographic, microbial distributions, dissolved and particulate metal data, and metaproteomic results with profile samples collected by Clio. Hypothesis 2 will be tested by incubation experiments using 15NO2- oxidation rates on Clio-collected incubation samples. Hypothesis 3 will be tested by dividing targeted nitrite oxidoreductase protein copies by qPCR (quantitative polymerase chain reaction)-based nitrite oxidizing bacteria abundance (NOB) to determine if cellular copy number varies with oxygen distributions, and by metalloproteomic analyses of NOB cultures. The demonstration of trace metal limitation of remineralization processes, not just primary production, would transform our understanding of the role of metals in biogeochemical cycling and provide new ways with which to interpret sectional data of dissolved and particulate trace metal distributions in the ocean. The idea that oxygen may play a previously underappreciated role in controlling trace metals due not just to metals' physical chemistry, but also from changing biological demand, will improve our ability to predict trace metal distributions in the face of decreasing ocean oxygen content.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.