ward mixing is inconsistent with long-term
observations of a relatively stable chloro-
phyll concentration in this region. Histori-
cal marine nitrogen fixation measurements
do not support an increase in regional ni-
trogen fixation. Hence, Ren et al. conclude
that the lighter isotope ratio of coral-bound
organic nitrogen is due to an increase in the
deposition of light anthropogenic nitrogen
in this region.
The Dongsha Atoll data imply that anthropogenic inputs have increased the flux
of nitrogen to the surface of the South China
Sea. Although substantial, this estimate is
near the lower end of the range of previous
estimates from atmospheric measurements.
The timing of the anthropogenic fixed-nitro-
gen increase is later than anticipated from
the history of many Chinese anthropogenic
fixed-nitrogen sources, such as agriculture
and fertilizer production. The late rise does,
however, coincide with a major increase in
coal consumption and vehicle use (10), sug-
gesting that these fossil fuel sources may
account for the increased anthropogenic ni-
trogen flux into the South China Sea.
One consequence of the recognition of
substantial anthropogenic additions of iso-topically light nitrogen is that some light
upper-ocean nitrate, which has been attributed solely to natural biological nitrogen
fixation in the ocean (11), is actually anthropogenic. Better understanding and modeling of the integrated rates of biological
nitrogen fixation in the ocean will require a
clearer quantification of the anthropogenic
sources throughout the ocean (12). j
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converts N2 gas
to fxed nitrogen.
Fossil-fuel burning and
agriculture produce increasing
amounts of fxed nitrogen.
Deep-ocean nitrate is mixed
upward, circulates, and
deposited as proteins in coral.
Energy of lightning
breaks N2 to produce
Growing source Small natural sources Main natural source
in coral skeleton
A major energy carrier
of the cell also promotes
By Allyson M. Rice and Michael K. Rosen
Adenosine triphosphate (ATP) is the canonical energy carrier that powers cellular machines, drives metabolic reactions, and serves as a precursor to nucleic acids. On page 753 of this issue, Patel et al. (1) propose a previously unknown function for ATP in maintaining protein solubility and preventing
macromolecular aggregation. They show that
at physiologic concentrations (2 to 8 mM),
the physical properties of ATP alone enable
it to dissolve liquid-liquid phase–separated
droplets and amyloid fibers. These findings
suggest an additional way for cells to use ATP
to maintain proteostasis in the crowded cytoplasm and also fine-tune the material properties of nonmembrane-bound organelles and
the cell interior in general.
Small molecules called hydrotropes have
been used for decades in industrial and pharmaceutical settings to increase the solubility of hydrophobic species and polymers in
aqueous solution—sometimes by orders of
magnitude (2). Hydrotropes are amphiphiles,
often containing a hydrophobic ring and an
ionic moiety. Although individual hydrotrope
molecules can self-associate like surfactants,
hydrotropes cannot assemble into larger
organized structures, such as surfactant micelles, because they lack long hydrophobic
chains (2). Current theories suggest that the
most efficient hydrotropes use their hydrophobic regions to interact with the hydrophobic elements of a solute, decreasing surface
tension of a solute and its tendency to separate from solution into a distinct liquid phase
[to undergo liquid-liquid phase separation
ATP—amphiphilic in nature and containing a charged triphosphate and a large aromatic ring system—has characteristics of a
hydrotrope. Patel et al. examined the actions
of ATP toward a class of biomolecules with
limited aqueous solubility and the propensity
to undergo LLPS and assemble into amy-
Department of Biophysics and Howard Hughes Medical
Institute, UT Southwestern Medical Center, Dallas, TX 75390,
USA. Email: firstname.lastname@example.org
Nitrogen isotope data from Dongsha Atoll
in the South China Sea provide evidence for
rising levels of anthropogenic pollution.
19 MAY 2017 • VOL 356 ISSUE 6339 701
Sources of fixed nitrogen in surface waters
The isotope signatures of fixed-nitrogen sources, given in per mil (‰), can be used to estimate the flux
strengths. Upwelling of deep-ocean nitrate is the main natural source of fixed nitrogen. Coral skeleton data now
show the rising contribution of anthropogenic emissions.