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"Bloom, J."
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Photorespiration and nitrate assimilation: a major intersection between plant carbon and nitrogen
C₃carbon fixation has a bad reputation, primarily because it is associated with photorespiration, a biochemical pathway thought to waste a substantial amount of the carbohydrate produced in a plant. This review presents evidence collected over nearly a century that (1) Rubisco when associated with Mn²⁺generates additional reductant during photorespiration, (2) this reductant participates in the assimilation of nitrate into protein, and (3) this nitrate assimilation facilitates the use of a nitrogen source that other organisms tend to avoid. This phenomenon explains the continued dominance of C₃plants during the past 23 million years of low CO₂atmospheres as well as the decline in plant protein concentrations as atmospheric CO₂rises.
Journal Article
Nitrate assimilation is inhibited by elevated CO2 in field-grown wheat
Reductions in the protein and nitrogen content of plants grown under enhanced atmospheric CO
2
concentrations could adversely affect the quality of food grown in the future, but the mechanisms of change remain unclear. Now research investigating plant responses to enhanced levels of atmospheric CO
2
under field conditions finds that wheat nitrate assimilation was slower for elevated CO2 than for ambient CO
2
.
Total protein and nitrogen concentrations in plants generally decline under elevated CO
2
atmospheres
1
,
2
. Explanations for this decline include that plants under elevated CO
2
grow larger, diluting the protein within their tissues
3
,
4
; that carbohydrates accumulate within leaves, downregulating the amount of the most prevalent protein Rubisco
2
; that carbon enrichment of the rhizosphere leads to progressively greater limitations of the nitrogen available to plants
4
; and that elevated CO
2
directly inhibits plant nitrogen metabolism, especially the assimilation of nitrate into proteins in leaves of C
3
plants
5
. Recently, several meta-analyses have indicated that CO
2
inhibition of nitrate assimilation is the explanation most consistent with observations
6
,
7
,
8
. Here, we present the first direct field test of this explanation. We analysed wheat (
Triticum aestivum
L.) grown under elevated and ambient CO
2
concentrations in the free-air CO
2
enrichment experiment at Maricopa, Arizona. In leaf tissue, the ratio of nitrate to total nitrogen concentration and the stable isotope ratios of organic nitrogen and free nitrate showed that nitrate assimilation was slower under elevated than ambient CO
2
. These findings imply that food quality will suffer under the CO
2
levels anticipated during this century unless more sophisticated approaches to nitrogen fertilization are employed.
Journal Article
Measuring and mitigating PCR bias in microbiota datasets
PCR amplification plays an integral role in the measurement of mixed microbial communities via high-throughput DNA sequencing of the 16S ribosomal RNA (rRNA) gene. Yet PCR is also known to introduce multiple forms of bias in 16S rRNA studies. Here we present a paired modeling and experimental approach to characterize and mitigate PCR NPM-bias (PCR bias from non-primer-mismatch sources) in microbiota surveys. We use experimental data from mock bacterial communities to validate our approach and human gut microbiota samples to characterize PCR NPM-bias under real-world conditions. Our results suggest that PCR NPM-bias can skew estimates of microbial relative abundances by a factor of 4 or more, but that this bias can be mitigated using log-ratio linear models.
Journal Article
A Fermi-degenerate three-dimensional optical lattice clock
Strontium optical lattice clocks have the potential to simultaneously interrogate millions of atoms with a high spectroscopic quality factor of 4 × 1017. Previously, atomic interactions have forced a compromise between clock stability, which benefits from a large number of atoms, and accuracy, which suffers from density-dependent frequency shifts. Here we demonstrate a scalable solution that takes advantage of the high, correlated density of a degenerate Fermi gas in a three-dimensional (3D) optical lattice to guard against on-site interaction shifts. We show that contact interactions are resolved so that their contribution to clock shifts is orders of magnitude lower than in previous experiments. A synchronous clock comparison between two regions of the 3D lattice yields a measurement precision of 5 × 10−19 in 1 hour of averaging time.
Journal Article
Assembly and coherent control of a register of nuclear spin qubits
The generation of a register of highly coherent, but independent, qubits is a prerequisite to performing universal quantum computation. Here we introduce a qubit encoded in two nuclear spin states of a single
87
Sr atom and demonstrate coherence approaching the minute-scale within an assembled register of individually-controlled qubits. While other systems have shown impressive coherence times through some combination of shielding, careful trapping, global operations, and dynamical decoupling, we achieve comparable coherence times while individually driving multiple qubits in parallel. We highlight that even with simultaneous manipulation of multiple qubits within the register, we observe coherence in excess of 10
5
times the current length of the operations, with
T
2
echo
=
40
±
7
seconds. We anticipate that nuclear spin qubits will combine readily with the technical advances that have led to larger arrays of individually trapped neutral atoms and high-fidelity entangling operations, thus accelerating the realization of intermediate-scale quantum information processors.
In large qubit registers, long coherence times and individual qubit control are difficult to achieve at the same time. Here, the authors assemble a 2D register of qubits in an array of fermionic alkaline-earth atoms, where tailored pulses can be applied to subsets of individual qubits in parallel.
Journal Article