Response to Comment on >dilution limits dissolved organic carbon utilization in the deep ocean>

Our recent finding that dilution limits dissolved organic carbon (DOC) utilization in the deep ocean has been criticized based on the common misconception that lability equates to rapid and complete utilization. Even when considering the redefinition of recalcitrant DOC recently proposed by Jiao et al., the dilution hypothesis best explains our experimental observations.This is a contribution to the MALASPINA Expedition 2010 project, funded by the CONSOLIDER-Ingenio 2010 program of the Spanish Ministry of Economy and Competitiveness (ref. CSD2008-00077)Peer Reviewe

Exploring new frontiers in marine radioisotope tracing - adapting to new opportunities and challenges

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Cresswell, T., Metian, M., Fisher, N. S., Charmasson, S., Hansman, R. L., Bam, W., Bock, C., & Swarzenski, P. W. Exploring new frontiers in marine radioisotope tracing - adapting to new opportunities and challenges. Frontiers in Marine Science, 7, (2020): 406, doi:10.3389/fmars.2020.00406.Radioisotopes have been used in earth and environmental sciences for over 150 years and provide unique tools to study environmental processes in great detail from a cellular level through to an oceanic basin scale. These nuclear techniques have been employed to understand coastal and marine ecosystems via laboratory and field studies in terms of how aquatic organisms respond to environmental stressors, including temperature, pH, nutrients, metals, organic anthropogenic contaminants, and biological toxins. Global marine issues, such as ocean warming, deoxygenation, plastic pollution, ocean acidification, increased duration, and intensity of toxic harmful algal blooms (HABs), and coastal contamination are all impacting marine environments, thereby imposing various environmental and economic risks. Being able to reliably assess the condition of coastal and marine ecosystems, and how they may respond to future disturbances, can provide vital information for society in the sustainable management of their marine environments. This paper summarizes the historical use of radiotracers in these systems, describes how existing techniques of radioecological tracing can be developed for specific current environmental issues and provides information on emerging issues that would benefit from current and new radiotracer methods. Current challenges with using radioecological tracers and opportunities are highlighted, as well as opportunities to maximize the application of these methods to greatly increase the ability of environmental managers to conduct evidence-based management of coastal and marine ecosystems.The IAEA is grateful for the support provided to its Environment Laboratories by the Government of the Principality of Monaco. This contribution was made within the framework of the IAEA CRP on “Applied radioecological tracers to assess coastal and marine ecosystem health” (K41019)

Microbial Metabollism in the Deep Ocean

To address the major role microorganisms play in the biogeochemical cycling of carbon and nutrients in the marine environment, the work presented in this dissertation uses a combination of geochemical and molecular biological techniques to investigate carbon and nitrogen metabolism in the planktonic microbial community of the deep ocean. A method was developed to isolate microbial DNA from the marine water column suitable for radiocarbon analysis, which was then applied to determine the sources of carbon fueling microbial production in the mesopelagic. Fresh organic matter delivered from sinking particles was confirmed as an important carbon source for free-living microbes, but the extent of autotrophic carbon fixation was also significant and variable with depth, highlighting the requirement for particle-delivered reduced nitrogen by the total microbial community in the deep ocean. Both findings stressed the importance of constraining particle-derived carbon and nitrogen flux to the deep ocean. The importance of methane as a carbon source in unique environments above cold methane seeps was examined using both stable carbon isotope measurements of microbial DNA and quantitative PCR of the gene encoding for particulate methane monooxygenase. The carbon isotope measurements showed that methane-carbon played an insignificant role in fueling planktonic microbial production in the deep water column just above methane seeps. The PCR measurements substantiated this result by showing that the methane oxidizing community represented only a small percentage of the microbial community in this environment. And finally, bacterial heterotrophic nitrate assimilation was studied and determined to increase with depth and/or nitrate concentration, but appeared to only be a possible nitrogen acquisition method for a small (> 1%) fraction of the bacterial community. Sequencing of the heterotrophic nitrate assimilation gene isolated from the marine environment further demonstrated that these genes were depth stratified. In addition both isotopic analyses and molecular biological techniques such as quantitative PCR indicated distinct differences between free-living and particle-attached microbial communities. Overall, future research efforts should be focused on expanding the current data set, constraining particle flux, and gaining a better understanding of microbial physiology, including in situ growth rates, in the subsurface marine environment

Measuring the in situ carbon isotopic composition of distinct marine plankton populations sorted by flow cytometry

The carbon isotope ratio (δ^(13)C value) of marine particulates is a potentially useful tracer for elucidating pathways of carbon flow in the marine environment. Different species of phytoplankton vary in fractionation vs. CO_2 by up to 24‰ in laboratory cultures under varying nutrient and growth conditions, a signal that should propagate through the microbial food web. However, such contrasts have been difficult to confirm in field measurements due to analytical limitations. Here, we combine fluorescence-activated cell sorting (FACS) with a specialized micro-combustion interface and isotope-ratio mass spectrometry (SWiM-IRMS) to provide some of the first direct measurements of whole-cell δ^(13)C values for specific phytoplankton populations in the wild. For three samples collected off Scripps Pier in 2010–2011, Synechococcus averages δ^(13)C values of −25.7 ± 2.0‰, Prochlorococcus averages −23.0 ± 1.3, and diatoms average −20.8 ± 1.7‰. Diatoms were ∼3‰ enriched in ^(13)C when measured during a bloom (March 2011) as compared with mid-summer (July 2010). Sorted particles thought to represent living heterotrophic bacteria averaged −25.4 ± 2.5‰, whereas total filterable particles averaged −19.6 ± 1.0‰, indicating a strong similarity to diatom biomass. These variations demonstrate that in situ differences in δ^(13)C among different populations of particles can be exploited to follow carbon flow through successive trophic levels, and throughout organic matter remineralization, sinking, and preservation

Tracing iron-fueled microbial carbon production within the hydrothermal plume at the Loihi seamount

The Loihi hydrothermal plume provides an opportunity to investigate iron (Fe) oxidation and microbial processes in a system that is truly Fe dominated and distinct from mid-ocean ridge spreading centers. The lack of hydrogen sulfide within the Loihi hydrothermal fluids and the presence of an oxygen minimum zone at this submarine volcano’s summit, results in a prolonged presence of reduced Fe within the dispersing non-buoyant plume. In this study, we have investigated the potential for microbial carbon fixation within the Loihi plume. We sampled for both particulate and dissolved organic carbon in hydrothermal fluids, microbial mats growing around vents, and the dispersing plume, and carried out stable carbon isotope analysis on the particulate fraction. The δ^(13)C values of the microbial mats ranged from −23‰ to −28‰, and are distinct from those of deep-ocean particulate organic carbon (POC). The mats and hydrothermal fluids were also elevated in dissolved organic carbon (DOC) compared to background seawater. Within the hydrothermal plume, DOC and POC concentrations were elevated and the isotopic composition of POC within the plume suggests mixing between background seawater POC and a ^(13)C-depleted hydrothermal component. The combination of both DOC and POC increasing in the dispersing plume that cannot solely be the result of entrainment and DOC adsorption, provides strong evidence for in-situ microbial productivity by chemolithoautotrophs, including a likelihood for iron-oxidizing microorganisms

Radiocarbon in dissolved organic carbon by UV oxidation: an update of procedures and blank characterization at NOSAMS

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Xu, L., Roberts, M., Elder, K., Hansman, R., Gagnon, A., & Kurz, M. Radiocarbon in dissolved organic carbon by UV oxidation: an update of procedures and blank characterization at NOSAMS. Radiocarbon, 64(1), (2022): 195-199, https://doi.org/10.1017/rdc.2022.4.This note describes improvements of UV oxidation method that is used to measure carbon isotopes of dissolved organic carbon (DOC) at the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS). The procedural blank is reduced to 2.6 ± 0.6 μg C, with Fm of 0.42 ± 0.10 and δ13C of –28.43 ± 1.19‰. The throughput is improved from one sample per day to two samples per day.We gratefully acknowledge support from the U.S. National Science Foundation, via NSF-OCE-1755125

Comparability of radiocarbon measurements in dissolved inorganic carbon of seawater produced at ETH-Zurich

Radiocarbon observations (Δ14C) in dissolved inorganic carbon (DIC) of seawater provide useful information about ocean carbon cycling and ocean circulation. To deliver high-quality observations, the Laboratory of Ion Beam Physics (LIP) at ETH-Zurich developed a new simplified method allowing the rapid analysis of radiocarbon in DIC of small seawater samples, which is continually assessed by following internal quality controls. However, a comparison with externally produced 14C measurements to better establish an equivalency between methods was still missing. Here, we make the first intercomparison with the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) facility based on 14 duplicate seawater samples collected in 2020. We also compare with prior deep-water observations from the 1970s to 1990s. The results show a very good agreement in both comparisons. The mean Δ14C of 12 duplicate samples measured by LIP and NOSAMS were statistically identical within one sigma uncertainty while two other duplicate samples agreed within two sigma. Based on this small number of duplicate samples, LIP values appear to be slightly lower than the NOSAMS values, but more measurements will be needed for confirmation. We also comment on storage and preservation techniques used in this study, including the freezing of samples collected in foil bags.ISSN:0033-822

Experimental evaluation of the hypothesis that dilution limits DOC utilization in the deep ocean

The dilution hypothesis was tested by adding different concentrations of ambient DOC obtained by solid phase extraction to deep seawater samples. Microbial growth and consumption of DOC were assessed by flow cytometry, HTCO measurements of DOC and oxygen consumption measurements in 14 experiments using water collected from deep water masses of the Atlantic and Pacific Oceans.There are two kinds of experiments 14 (A-N) where prokaryotic growth was evaluated under increasing concentrations of ambient DOC and 2 additional experiments (O and P) where DOC composition and the utilization of different compounds was evaluated by means of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). A utilization index for each compound was derived from the FT-ICR-MS fingerprints, showing whether the relative signal for each compound remained stable (refractory or not used), decreased (was consumed) or increased (was produced). Detailed information on conditions and procedures can be found in the article. Enquiries can be sent to Jesús M. Arrieta at txetxu[at]mail.com.Access and reuse conditions: This database and its components are subject to a Creative Commons Attribution-Noncommercial-ShareAlike International licence 4.0.Experimental results on the hypothesis that deep-water DOC consists of many different, intrinsically labile compounds at concentrations too low to compensate for the metabolic costs associated to their utilization.This is a contribution to the MALASPINA Expedition 2010 project, funded by the CONSOLIDER-Ingenio 2010 program of the Spanish Ministry of Economy and Competitiveness (Ref. CSD2008-00077). J.M.A. was supported by a “Ramón y Cajal” research fellowship from the Spanish Ministry of Economy and Competitiveness. E.M. was supported by a fellowship from the JAE program of CSIC. G.J.H. and R.H. were supported by the Austrian Science Fund (FWF) projects: I486-B09 and P23234-B11 and by the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement No. 268595 (MEDEA project). We thank A. Dorsett for assistance with DOC analyses, participants in the Malaspina Expedition and the crews of the BIO Hespérides, and RV Pelagia and the personnel of the Marine Technology Unit of CSIC (UTM) for their invaluable support.Peer reviewe

Ocean chemistry: Dilution limits dissolved organic carbon utilization in the deep ocean

Oceanic dissolved organic carbon (DOC) is the second largest reservoir of organic carbon in the biosphere. About 72% of the global DOC inventory is stored in deep oceanic layers for years to centuries, supporting the current view that it consists of materials resistant to microbial degradation. An alternative hypothesis is that deep-water DOC consists of many different, intrinsically labile compounds at concentrations too low to compensate for the metabolic costs associated to their utilization. Here, we present experimental evidence showing that low concentrations rather than recalcitrance preclude consumption of a substantial fraction of DOC, leading to slow microbial growth in the deep ocean. These findings demonstrate an alternative mechanism for the long-term storage of labile DOC in the deep ocean, which has been hitherto largely ignored.This is a contribution to the Malaspina 2010 Expedition project, funded by the CONSOLIDER-Ingenio 2010 program of the from the Spanish Ministry of Economy and Competitiveness (Ref. CSD2008-00077). J.M.A. was supported by a “Ramón y Cajal” research fellowship from the Spanish Ministry of Economy and Competitiveness. E.M. was supported by a fellowship from the Junta para la Ampliación de Estudios program of CSIC. G.J.H. and R.L.H. were supported by the Austrian Science Fund (FWF) projects I486-B09 and P23234-B11 and by the European Research Council (ERC) under the European Community’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement 268595 (MEDEA project).Peer Reviewe