skip to main content


Title: Euphotic zone nitrification in the California Current Ecosystem
Abstract

Nitrification, the microbial conversion of ammonium to nitrite then to nitrate, occurs throughout the oceanic water column, yet the environmental factors influencing the production of nitrate in the euphotic zone (EZ) remain unclear. In this study, the natural abundances of N and O isotopes (δ15N and δ18O, respectively) in nitrate were used in an existing model framework to quantify nitrate contributed by EZ nitrification in the California Current Ecosystem (CCE) during two anomalously warm years. Model data estimated that between 6% and 36% of the EZ nitrate reservoirs were derived from the combined steps of nitrification within the EZ. The CCE data set found nitrification contributions to EZ nitrate to be positively correlated with nitrite concentrations () at the depth of the primary nitrite maximum (PNM). Building on this correlation, EZ nitrification in the southern California Current was estimated to contribute on average 20% ± 6% to EZ nitrate as inferred using the PNMof the long‐term California Cooperative Oceanic Fisheries Investigation (CalCOFI) survey record. A multiple linear regression analysis of the CalCOFI PNMtime series identified two conditions that led to positive deviations in. Enhanced PNM, and potentially enhanced EZ nitrification, may be linked to (1) reduced phytoplankton competition for ammonium () andas interpreted from particulate organic carbon:chlorophyll ratios, and/or (2) to increased supply of(and thenoxidation to) from the degradation of organic nitrogen as interpreted from particulate organic nitrogen concentrations.

 
more » « less
Award ID(s):
1637632 1756884
NSF-PAR ID:
10458716
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  
Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
Limnology and Oceanography
Volume:
65
Issue:
4
ISSN:
0024-3590
Page Range / eLocation ID:
p. 790-806
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    In hyperalkaline () fluids that have participated in low‐temperature (<150) serpentinization reactions, the dominant form of C is often methane (), but the origin of thisis uncertain. To assessorigin in serpentinite aquifers within the Samail Ophiolite, Oman, we determined fluid chemical compositions, analyzed taxonomic profiles of fluid‐hosted microbial communities, and measured isotopic compositions of hydrocarbon gases. We found that 16S rRNA gene sequences affiliated with methanogens were widespread in the aquifer. We measured clumped isotopologue (D and) relative abundances less than equilibrium, consistent with substantial microbialproduction. Furthermore, we observed an inverse relationship between dissolved inorganic C concentrations andacross fluids bearing microbiological evidence of methanogenic activity, suggesting that the apparent C isotope effect of microbial methanogenesis is modulated by C availability. An additional source ofis evidenced by the presence of‐bearing fluid inclusions in the Samail Ophiolite and our measurement of highvalues of ethane and propane, which are similar to those reported in studies of‐rich inclusions in rocks from the oceanic lithosphere. In addition, we observed 16S rRNA gene sequences affiliated with aerobic methanotrophs and, in lower abundance, anaerobic methanotrophs, indicating that microbial consumption ofin the ophiolite may further enrichin13C. We conclude that substantial microbialis produced under varying degrees of C limitation and mixes with abioticreleased from fluid inclusions. This study lends insight into the functioning of microbial ecosystems supported by water/rock reactions.

     
    more » « less
  2. Abstract

    Cadmium (Cd) is a trace metal whose distribution in the ocean bears a remarkable resemblance to the nutrient phosphate (PO43−). This resemblance has led to the use of Cd as a proxy for ocean nutrient cycling in paleoceanographic applications, but the processes governing the cycling of Cd in the modern ocean remain unclear. In this study, we use previously published Cd observations and an Artificial Neural Network to produce a dissolved Cd climatology that reproduces the observed subtle deviations between the Cd anddistributions. We use the Cd andclimatologies, along with an ocean circulation inverse model, to diagnose the biogeochemical sources and sinks of dissolved Cd and. Our calculations reveal that dissolved Cd, like, is removed in the surface ocean and has a source in the subsurface, consistent with the simultaneous incorporation of Cd andinto sinking organic particles. However, there are also contrasts between the cycling of dissolved Cd andIn particular, thesurface export ratio varies 8‐fold across latitudes, reaching highest values in the iron‐limited sub‐Antarctic Southern Ocean. This depletes Cd relative toin the low‐latitude thermocline while adding excess Cd to deep waters by the regeneration of Cd‐enriched particles. Also, Cd tends to regenerate slightly deeper thanin the subsurface ocean, and theregeneration ratio reaches a maximum at 700–1,500 m. These contrasts are responsible for a slight concavity in therelationship and should be considered when interpreting paleoceanographic Cd records.

     
    more » « less
  3. Abstract

    A detailed chemical kinetic model for oxidation of methylamine has been developed, based on theoretical work and a critical evaluation of data from the literature. The rate coefficients for the reactions of CHNH+ OCHNH/ CHNH + HO, CHNH+ HCH+ NH, CHNHCHNH, and CHNH + OCHNH + HOwere calculated from ab initio theory. The mechanism was validated against experimental results from batch reactors, flow reactors, shock tubes, and premixed flames. The model predicts satisfactorily explosion limits for CHNHand its oxidation in a flow reactor. However, oxidation in the presence of nitric oxide, which strongly promotes reaction at lower temperatures, is only described qualitatively. Furthermore, calculated flame speeds are higher than reported experimental values; the model does not capture the inhibiting effect of the NHgroup in CHNHcompared to CH. More work is desirable to confirm the products of the CHNH + NO reaction and to look into possible pathways to NHin methylamine oxidation.

     
    more » « less
  4. Abstract

    We present a statistical investigation of the effects of interplanetary magnetic field (IMF) on hemispheric asymmetry in auroral currents. Nearly 6 years of magnetic field measurements from Swarm A and C satellites are analyzed. Bootstrap resampling is used to remove the difference in the number of samples and IMF conditions between the local seasons and the hemispheres. Currents are stronger in Northern Hemisphere (NH) than Southern Hemisphere (SH) for IMF Bin NH (Bin SH) in most local seasons under both signs of IMF B. For Bin NH (Bin SH), the hemispheric difference in currents is small except in local winter when currents in NH are stronger than in SH. During Band Bin NH (Band Bin SH), the largest hemispheric asymmetry occurs in local winter and autumn, when the NH/SH ratio of field aligned current (FAC) is 1.180.09 in winter and 1.170.09 in autumn. During Band Bin NH (Band Bin SH), the largest asymmetry is observed in local autumn with NH/SH ratio of 1.160.07 for FAC. We also find an explicit Beffect on auroral currents in a given hemisphere: on average Bin NH and Bin SH causes larger currents than vice versa. The explicit Beffect on divergence‐free current during IMF Bis in very good agreement with the Beffect on the cross polar cap potential from the Super Dual Auroral Radar Network dynamic model except at SH equinox and NH summer.

     
    more » « less
  5. Abstract

    The potential for molecular hydrogen () generated via serpentinization to fuel subsurface microbial ecosystems independent from photosynthesis has prompted biogeochemical investigations of serpentinization‐influenced fluids. However, investigations typically sample via surface seeps or open‐borehole pumping, which can mix chemically distinct waters from different depths. Depth‐indiscriminate sampling methods could thus hinder understanding of the spatial controls on nutrient availability for microbial life. To resolve distinct groundwaters in a low‐temperature serpentinizing environment, we deployed packers (tools that seal against borehole walls during pumping) in two‐deep, peridotite‐hosted wells in the Samail Ophiolite, Oman. Isolation and pumping of discrete intervals as deep astobelow ground level revealed multiple aquifers that ranged in pH from 8 to 11. Chemical analyses and 16S rRNA gene sequencing of deep, highly reactedgroundwaters bearing up to,methane () andsulfate () revealed an ecosystem dominated by Bacteria affiliated with the class Thermodesulfovibrionia, a group of chemolithoheterotrophs supported byoxidation coupled toreduction. In shallower, oxidizedgroundwaters, aerobic and denitrifying heterotrophs were relatively more abundant. Highandof(up toand, respectively) indicated microbialoxidation, particularly inwaters with evidence of mixing withwaters. This study demonstrates the power of spatially resolving groundwaters to probe their distinct geochemical conditions and chemosynthetic communities. Such information will help improve predictions of where microbial activity in fractured rock ecosystems might occur, including beyond Earth.

     
    more » « less