An official website of the United States government
Abstract Nitrogen (N) bioavailability affects phytoplankton growth and primary production in the aquatic environment. N bioavailability is partly determined by biological N cycling processes that either transform N species or remove fixed N. Reliable estimates of their kinetic parameters can help understand the distribution of N cycling processes. However, available estimates of kinetic parameters are often derived from microbial isolates and may not be representative of the natural environment. Observations are particularly lacking in estuarine and coastal waters. We conducted isotope tracer addition incubations to evaluate substrate affinities of nitrification, denitrification and anammox in the Chesapeake Bay water column. The half‐saturation constant for ammonia oxidation ranged from 0.38 to 0.75 μM ammonium, substantially higher than observed in the open oceans. Half‐saturation constants for denitrification—0.92–1.86 μM nitrite or 1.15 μM nitrate—were within the lower end or less than those reported for other aquatic environments and for denitrifier isolates. Interestingly, water column denitrification potential was comparable to that of sedimentary denitrification, highlighting the contribution of the water column to N removal during anoxia. Mostly undetectable anammox rates prevented us from deriving the half‐saturation constants, suggesting a low affinity of anammox. Using these substrate kinetics, we were able to predict in situ N cycling rates and explain the vertical distribution of N nutrient concentrations. Our newly derived substrate kinetics parameters can be useful for improving model representation of N nutrient dynamics in estuarine and coastal waters, which is critical for assessing the ecosystem productivity and function.
more »
« less
In situ aerobic methane oxidation rates in a stratified lake
Abstract Microbial aerobic methane oxidation is an important sink for aquatic methane worldwide. Despite its importance to global methane fluxes, few aerobic methane oxidation rates have been obtained in freshwater or marine environments without imposing changes to the microbial community through use of ex situ methods. A novel in situ incubation method for continuous time‐series measurements was used in Jordan Lake, North Carolina, during 2020–2021, to determine reaction kinetics for aerobic methane oxidation rates across a wide range of naturally varying methane (55–1833 nM) and dissolved oxygen (DO; 28–366 μM) concentrations and temperatures (17–30°C). Methane oxidation began immediately at the start of each of 21 incubations and methane oxidation rates were 1storder with respect to methane. The data density allowed for accurate calculation of 1st‐order rate constants,k, that ranged from 0.018 to 0.462 h−1(R2 > 0.967). Addition of ammonium (20–45 μM) to natural concentrations ranging from 0.057 to 2.4 μM did not change aerobic methane oxidation rate kinetics, suggesting that the natural population of aerobic methane oxidizers in this eutrophic lake was not nitrogen limited. Values ofkinversely correlated most strongly with initial DO concentrations (R2 = 0.82) rather than temperature. Values forkincreased with Julian day throughout our sampling period, suggesting seasonal influences on methane oxidation via responses to geochemical changes or shifts in microbial community abundance and composition. These experiments demonstrate a high variability in the enzymatic capacity for 1st‐order methane oxidation rates in this eutrophic lake that is tightly and inversely coupled to oxygen concentrations. Measurements of in situ aerobic methane oxidation rate constants allow for the direct quantification and modeling of the microbial community's capacity for methane oxidation over a wide range of natural methane concentrations.
more »
« less
- Award ID(s):
- 1948720
- PAR ID:
- 10567541
- Publisher / Repository:
- Limnology and Oceanography
- Date Published:
- Journal Name:
- Limnology and Oceanography
- ISSN:
- 0024-3590
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract During aerobic oxidation of methane (CH4) in seawater, a process which mitigates atmospheric emissions, the12C‐isotopologue reacts with a slightly greater rate constant than the13C‐isotopologue, leaving the residual CH4isotopically fractionated. Prior studies have attempted to exploit this systematic isotopic fractionation from methane oxidation to quantify the extent that a CH4pool has been oxidized in seawater. However, cultivation‐based studies have suggested that isotopic fractionation fundamentally changes as a microbial population blooms in response to an influx of reactive substrates. Using a systematic mesocosm incubation study with recently collected seawater, here we investigate the fundamental isotopic kinetics of aerobic CH4oxidation during a microbial bloom. As detailed in a companion paper, seawater samples were collected from seep fields in Hudson Canyon, U.S. Atlantic Margin, and atop Woolsey Mound (also known as Sleeping Dragon) which is part of lease block MC118 in the northern Gulf of Mexico, and used in these investigations. The results from both Hudson Canyon and MC118 show that in these natural environments isotopic fraction for CH4oxidation follows a first‐order kinetic process. The results also show that the isotopic fractionation factor remains constant during this methanotrophic bloom once rapid CH4oxidation begins and that the magnitude of the fractionation factor appears correlated with the first‐order reaction rate constant. These findings greatly simplify the use of natural stable isotope changes in CH4to assess the extent that CH4is oxidized in seawater following seafloor release.more » « less
-
The Effects of Engineered Aeration on Atmospheric Methane Flux From a Chesapeake Bay Tidal TributaryEngineered aeration is one solution for increasing oxygen concentrations in highly eutrophic estuaries that undergo seasonal hypoxia. Although there are various designs for engineered aeration, all approaches involve either destratification of the water column or direct injection of oxygen or air through fine bubble diffusion. To date, the effect of either approach on estuarine methane dynamics remains unknown. Here we tested the hypotheses that 1) bubble aeration will strip the water of methane and enhance the air-water methane flux to the atmosphere and 2) the addition of oxygen to the water column will enhance aerobic methane oxidation in the water column and potentially offset the air-water methane flux. These hypotheses were tested in Rock Creek, Maryland, a shallow-water sub-estuary to the Chesapeake Bay, using controlled, ecosystem-scale deoxygenation experiments where the water column and sediments were sampled in mid-summer, when aerators were ON, and then 1, 3, 7, and 13 days after the aerators were turned OFF. Experiments were performed under two system designs, large bubble and fine bubble approaches, using the same observational approach that combined discrete water sampling, long term water samplers (OsmoSamplers) and sediment porewater profiles. Regardless of aeration status, methane concentrations reached as high as 1,500 nmol L−1in the water column during the experiments and remained near 1,000 nmol L−1through the summer and into the fall. Since these concentrations are above atmospheric equilibrium of 3 nmol L−1, these data establish the sub-estuary as a source of methane to the atmosphere, with a maximum atmospheric flux as high as 1,500 µmol m−2d−1, which is comparable to fluxes estimated for other estuaries. Air-water methane fluxes were higher when the aerators were ON, over short time frames, supporting the hypothesis that aeration enhanced the atmospheric methane flux. The fine-bubble approach showed lower air-water methane fluxes compared to the larger bubble, destratification system. We found that the primary source of the methane was the sediments, however,in situmethane production or an upstream methane source could not be ruled out. Overall, our measurements of methane concentrations were consistently high in all times and locations, supporting consistent methane flux to the atmosphere.more » « less
-
Abstract Microbial aerobic oxidation is known to be a significant sink of marine methane (CH4), contributing to the relatively minor atmospheric release of this greenhouse gas over vast stretches of the ocean. However, the chemical kinetics of aerobic CH4oxidation are not well established, making it difficult to predict and assess the extent that CH4is oxidized in seawater following seafloor release. Here we investigate the kinetics of aerobic CH4oxidation using mesocosm incubations of fresh seawater samples collected from seep fields in Hudson Canyon, U.S. Atlantic Margin and MC118, Gulf of Mexico to gain a fundamental chemical understanding of this CH4sink. The goals of this investigation were to determine the response or lag time following CH4release until more rapid oxidation begins, the reaction order, and the stoichiometry of reactants utilized (i.e., CH4, oxygen, nitrate, phosphate, trace metals) during CH4oxidation. The results for both Hudson Canyon and MC118 environments show that CH4oxidation rates sharply increased within less than one month following the CH4inoculation of seawater. However, the exact temporal characteristics of this more rapid CH4oxidation varied based on location, possibly dependent on the local circulation and biogeochemical conditions at the point of seawater collection. The data further suggest that methane oxidation behaves as a first‐order kinetic process and that the reaction rate constant remains constant once rapid CH4oxidation begins.more » « less
-
In adaptive importance sampling and other contexts, we haveK> 1 unbiased and uncorrelated estimates μ^kof a common quantity μ. The optimal unbiased linear combination weights them inversely to their variances, but those weights are unknown and hard to estimate. A simple deterministic square root rule based on a working model that Var(μ^k) ∝k−1/2gives an unbiased estimate of μ that is nearly optimal under a wide range of alternative variance patterns. We show that if Var(μ^k)∝k−yfor an unknown rate parametery∈[0,1], then the square root rule yields the optimal variance rate with a constant that is too large by at most 9/8 for any 0 ⩽y⩽ 1 and any numberKof estimates. Numerical work shows that rule is similarly robust to some other patterns with mildly decreasing variance askincreases.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1002/lno.12583
