Enzyme level N and O isotope effects of assimilatory and dissimilatory nitrate reduction

To provide mechanistic constraints to interpret nitrogen (N) and oxygen (O) isotope ratios of nitrate (NO3−), 15N/14N and 18O/16O, in the environment, we measured the enzymatic NO3− N and O isotope effects (15ε and 18ε) during its reduction by NO3− reductase enzymes, including (1) a prokaryotic respiratory NO3− reductase, Nar, from the heterotrophic denitrifier Paracoccus denitrificans, (2) eukaryotic assimilatory NO3− reductases, eukNR, from Pichia angusta and from Arabidopsis thaliana, and (3) a prokaryotic periplasmic NO3− reductase, Nap, from the photoheterotroph Rhodobacter sphaeroides. Enzymatic Nar and eukNR assays with artificial viologen electron donors yielded identical 18ε and 15ε of ∼28‰, regardless of [NO3−] or assay temperature, suggesting analogous kinetic mechanisms with viologen reductants. Nar assays fuelled with the physiological reductant hydroquinone (HQ) also yielded 18ε ≈ 15ε, but variable amplitudes from 21‰ to 33.0‰ in association with [NO3−], suggesting analogous substrate sensitivity in vivo. Nap assays fuelled by viologen revealed 18ε:15ε of 0.50, where 18ε ≈ 19‰ and 15ε ≈ 38‰, indicating a distinct catalytic mechanism than Nar and eukNR. Nap isotope effects measured in vivo showed a similar 18ε:15ε of 0.57, but reduced 18ε ≈ 11‰ and 15ε ≈ 19‰. Together, the results confirm identical enzymatic 18ε and 15ε during NO3− assimilation and denitrification, reinforcing the reliability of this benchmark to identify NO3− consumption in the environment. However, the amplitude of enzymatic isotope effects is apt to vary in vivo. The distinctive signature of Nap is of interest for deciphering catalytic mechanisms but may be negligible in most environments given its physiological role.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/136031/1/lno10393-sup-0001-suppinfo.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/136031/2/lno10393_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/136031/3/lno10393.pd

Convex Functions and Spacetime Geometry

Convexity and convex functions play an important role in theoretical physics. To initiate a study of the possible uses of convex functions in General Relativity, we discuss the consequences of a spacetime $(M,g_{\mu \nu})$ or an initial data set $(\Sigma, h_{ij}, K_{ij})$ admitting a suitably defined convex function. We show how the existence of a convex function on a spacetime places restrictions on the properties of the spacetime geometry.Comment: 26 pages, latex, 7 figures, improved version. some claims removed, references adde

Seasonality of nitrogen sources, cycling, and loading in a New England river discerned from nitrate isotope ratios

Coastal waters globally are increasingly impacted due to the anthropogenic loading of nitrogen (N) from the watershed. To assess dominant sources contributing to the eutrophication of the Little Narragansett Bay estuary in New England, we carried out an annual study of N loading from the Pawcatuck River. We conducted weekly monitoring of nutrients and nitrate (NO3-) isotope ratios (15N / 14N, 18O / 16O, and 17O / 16O) at the mouth of the river and from the larger of two wastewater treatment facilities (WWTFs) along the estuary, as well as seasonal along-river surveys. Our observations reveal a direct relationship between N loading and the magnitude of river discharge and a consequent seasonality to N loading into the estuary – rendering loading from the WWTFs and from an industrial site more important at lower river flows during warmer months, comprising ∼ 23 % and ∼ 18 % of N loading, respectively. Riverine nutrients derived predominantly from deeper groundwater and the industrial point source upriver in summer and from shallower groundwater and surface flow during colder months – wherein NO3- associated with deeper groundwater had higher 15N / 14N ratios than shallower groundwater. Corresponding NO3- 18O / 16O ratios were lower during the warm season, due to increased biological cycling in-river. Uncycled atmospheric NO3-, detected from its unique mass-independent NO3- 17O / 16O vs. 18O / 16O fractionation, accounted for &lt; 3 % of riverine NO3-, even at elevated discharge. Along-river, NO3- 15N / 14N ratios showed a correspondence to regional land use, increasing from agricultural and forested catchments to the more urbanized watershed downriver. The evolution of 18O / 16O isotope ratios along-river conformed to the notion of nutrient spiraling, reflecting the input of NO3- from the catchment and from in-river nitrification and its coincident removal by biological consumption. These findings stress the importance of considering seasonality of riverine N sources and loading to mitigate eutrophication in receiving estuaries. Our study further advances a conceptual framework that reconciles with the current theory of riverine nutrient cycling, from which to robustly interpret NO3- isotope ratios to constrain cycling and source partitioning in river systems.</p

On the Quasi-Linear Elliptic PDE −∇⋅(∇u/1−∣∇u∣2)=4π∑kakδsk-\nabla\cdot(\nabla{u}/\sqrt{1-|\nabla{u}|^2}) = 4\pi\sum_k a_k \delta_{s_k} in Physics and Geometry

It is shown that for each finite number of Dirac measures supported at points $s_n$ in three-dimensional Euclidean space, with given amplitudes $a_n$, there exists a unique real-valued Lipschitz function $u$, vanishing at infinity, which distributionally solves the quasi-linear elliptic partial differential equation of divergence form $-\nabla\cdot(\nabla{u}/\sqrt{1-|\nabla{u}|^2})=4\pi\sum_{n=1}^N a_n \delta_{s_n}$. Moreover, $u$ is real analytic away from the $s_n$. The result can be interpreted in at least two ways: (a) for any number of point charges of arbitrary magnitude and sign at prescribed locations $s_n$ in three-dimensional Euclidean space there exists a unique electrostatic field which satisfies the Maxwell-Born-Infeld field equations smoothly away from the point charges and vanishes as $|s|\to\infty$; (b) for any number of integral mean curvatures assigned to locations $s_n$ there exists a unique asymptotically flat, almost everywhere space-like maximal slice with point defects of Minkowski spacetime, having lightcone singularities over the $s_n$ but being smooth otherwise, and whose height function vanishes as $|s|\to\infty$. No struts between the point singularities ever occur.Comment: This is the preprint of the version published in 2012 in Commun. Math. Phys. PLUS an errata which has been accepted 08/13/2018 for publication in Commun. Math. Phy

Seasonal variations in the nitrogen isotopic composition of settling particles at station K2 in the western subarctic North Pacific

Intensive observations using hydrographical cruises and moored sediment trap deployments during 2010 and 2012 at station K2 in the North Pacific western subarctic gyre (WSG) revealed seasonal changes in δ15N of both suspended and settling particles. Suspended particles (SUS) were collected from depths between the surface and 200 m; settling particles by drifting traps (DST; 100-200 m) and moored traps (MST; 200 and 500 m). All particles showed higher δ15N values in winter and lower in summer, contrary to the expected by isotopic fractionation during phytoplankton nitrate consumption. We suggest that these observed isotopic patterns are due to ammonium consumption via light-controlled nitrification, which could induce variations in δ15N(SUS) of 0.4-3.1 ‰ in the euphotic zone (EZ). The δ15N(SUS) signature was reflected by δ15 N(DST) despite modifications during biogenic transformation from suspended particles in the EZ. δ15 N enrichment (average: 3.6 ‰) and the increase in C:N ratio (by 1.6) in settling particles suggests year-round contributions of metabolites from herbivorous zooplankton as well as TEPs produced by diatoms. Accordingly, seasonal δ15 N(DST) variations of 2.4-7.0 ‰ showed a significant correlation with primary productivity (PP) at K2. By applying the observed δ15 N(DST) vs. PP regression to δ15 N(MST) of 1.9-8.0 ‰, we constructed the first annual time-series of PP changes in the WSG. Moreover, the monthly export ratio at 500 m was calculated using both estimated PP and measured organic carbon fluxes. Results suggest a 1.6 to 1.8 times more efficient transport of photosynthetically-fixed carbon to the intermediate layers occurs in summer/autumn rather than winter/spring

Journal reflections shed light on challenges students face in an introductory field biology course

Abstract Field biology courses—which expose students to concepts through outdoor experimentation and observation—are a staple in undergraduate natural science education. Participation in field courses has been associated with increased academic success, retention, and strengthening of science identity. However, for some students, learning outdoors can pose barriers to comfort, enjoyment, and success and may contribute to alienation and attrition. To better understand the kinds of challenges encountered by students and how those experiences might vary by student demographics and prior experience, we analyzed journal reflections from undergraduates in an introductory field biology course using a mixed‐methods approach. We used open coding to characterize the challenges that students described and found that students identified challenges related to four categories: (1) scientific challenges, concerning concepts, field methods, and data; (2) logistic challenges, relating to factors such as the timing of the course and equipment; (3) conditions challenges, regarding difficulties with organisms, the environment, and inclement weather; and (4) additional challenges, encompassing a broad array of student difficulties. Using multilevel binomial modeling, we examined the effect of course factors (e.g., field lab topic and section) and student factors (e.g., demographics and prior outdoor experience) on the occurrence of each type of challenge within journal reflections. Field lab topic was the only effect likely to increase the presence of all four types of challenges for students. In addition, students with the most prior outdoors experience were less likely to report a logistic challenge compared with their peers in the course who had less outdoors experience. Based on our findings, we offer recommendations for how instructors can support students through challenges they face in introductory field biology

The Controls of Iron and Oxygen on Hydroxyl Radical (•OH) Production in Soils

Hydroxyl radical (&bull;OH) is produced in soils from oxidation of reduced iron (Fe(II)) by dissolved oxygen (O2) and can oxidize dissolved organic carbon (DOC) to carbon dioxide (CO2). Understanding the role of &bull;OH on CO2 production in soils requires knowing whether Fe(II) production or O2 supply to soils limits &bull;OH production. To test the relative importance of Fe(II) production versus O2 supply, we measured changes in Fe(II) and O2 and in situ &bull;OH production during simulated precipitation events and during common, waterlogged conditions in mesocosms from two landscape ages and the two dominant vegetation types of the Arctic. The balance of Fe(II) production and consumption controlled &bull;OH production during precipitation events that supplied O2 to the soils. During static, waterlogged conditions, &bull;OH production was controlled by O2 supply because Fe(II) production was higher than its consumption (oxidation) by O2. An average precipitation event (4 mm) resulted in 200 &micro;mol &bull;OH m&minus;2 per day produced compared to 60 &micro;mol &bull;OH m&minus;2 per day produced during waterlogged conditions. These findings suggest that the oxidation of DOC to CO2 by &bull;OH in arctic soils, a process potentially as important as microbial respiration of DOC in arctic surface waters, will depend on the patterns and amounts of rainfall that oxygenate the soil