By
Dawn Levy Stanford University News
Shake a snow
globe and clear skies turn to a blizzard in your hand. The Earth
is a similarly closed system, with chemical reactions that occur
in soil affecting chemical reactions that occur in skies and seas.
Chemistry is destiny.
Advanced technologies
and methods are helping scientists take a new look at one of the
Earth`s most abundant elements - phosphorus - to better understand
how it cycles through soil, sea and living organisms. Phosphorus
is an essential nutrient for all organisms including plants, which
through photosynthesis remove carbon dioxide - the greenhouse
gas most responsible for global warming - from the atmosphere
and bind it into organic matter. Phosphorus also plays a key role
in the cycles of such biologically essential elements as nitrogen,
oxygen and sulfur. Because of analytical limitations, little was
known about phosphorus cycling until recently.
``We`re at
the stage where we can make another jump in our understanding
of phosphate cycling because of the availability of new technology
and new methods that can be applied to natural systems,`` says
Adina Paytan, an assistant professor in Stanford`s Department
of Geological and Environmental Sciences. Paytan is leading a
special session Dec. 15 with Stanford postdoctoral scholar Barbara
Cade-Menun at this year`s San Francisco meeting of the American
Geophysical Union (AGU), an international scientific society with
more than 35,000 members dedicated to advancing the understanding
of Earth and its environment.
The new technologies
and methods allow scientists to characterize phosphorus compounds
in water and soil, estimate how fast phosphate cycles through
natural reservoirs, estimate long-term phosphorus burial in the
oceans through geologic time, pinpoint phosphorus sources in different
ecosystems, determine fluctuations in phosphate concentrations
at certain times and places, and study the effects of phosphorus
pollution in estuaries and lakes.
In recent
years, AGU meetings, traditionally devoted to geology and geophysics,
have increasingly included biology-related topics. This year`s
meeting, however, marks the first at which ``biogeosciences``
is an official heading. The new heading describes an interdisciplinary
field that treats the Earth system as a whole and attempts to
understand connections and interactions between the atmosphere
(air), hydrosphere (water), biosphere (life) and lithosphere (rock).
It also attempts to evaluate human impact on the system.
Phosphorus
is crucial to life as a building block of nucleic acids, proteins
and lipids. It is a component of adenosine triphosphate (ATP)
and nicotinamid adenosine dinucleotide phosphate (NADP), molecules
that transfer energy in living systems. In photosynthesis, plants
use ATP and NADP to harness the sun`s energy to convert atmospheric
carbon dioxide and water into sugar and oxygen. Through photosynthesis,
organisms as tiny as algae and as mighty as redwoods become part
of the global ``sink`` that absorbs excess carbon dioxide. In
ecosystems that are relatively unaffected by human activities,
plant growth may be limited by the availability of phosphorus
and/or nitrogen.
Most phosphorus
is found in rocks, where it is bound in minerals. In these minerals
it usually exists as phosphate - that`s one phosphorus atom bound
to four oxygen atoms. Weathering releases phosphorus into soil,
where it is utilized and recycled by plants and bacteria. The
phosphorus eventually travels through streams to oceans and into
marine sediments.
A ``feast
or famine`` element, phosphorus is unequally distributed throughout
the Earth, with too much in North American and European soils
and some coastal estuaries and not enough in soils of the tropics,
sub-Saharan Africa, South America and in the open ocean.
Though phosphorus
is necessary, too much of a good thing can be bad. Excess phosphate
derived from fertilizers, detergents and other human sources makes
its way into lakes and coastal waters by runoff, leaching or erosion,
causing massive algae blooms that can affect taste and clarity
of drinking water. Moreover, when algae die, thick pads sink to
the bottom and oxidize, reducing dissolved oxygen and creating
an environment inhospitable to fish.
Too little
phosphorus also is a problem. In old, weathered soils and in ocean
ecosystems, plants have enough nitrate (another nutrient whose
scarcity limits growth) but not enough phosphorus. Crop harvesting
can deplete phosphorus from agricultural lands, and old, weathered
soils hold onto phosphate tightly, reducing its availability to
plants. Most phosphate fertilizer is derived from rock phosphate
- a nonrenewable resource. ``Organic fertilizers could replace
this source, but we need to know the forms of phosphorus available
and how quickly they turn over,`` says Cade-Menun, who studies
phosphorus as it cycles through soils.
Cade-Menun
studies phosphorus cycling in temperate forests including warm,
dry woods of the Sierra Nevada and cool, wet rainforests of the
Pacific Northwest. In addition to traditional analytic techniques,
she uses nuclear magnetic resonance (NMR) spectroscopy to identify
the kinds and amounts of phosphorus compounds that are distributed
throughout forests. She also is collaborating with Paytan to adapt
techniques for soils to ocean sediment trap samples to track which
compounds degrade first, releasing their phosphorus back into
the water and making it available to organisms.
``Unlike nitrogen,
little is known about the role of specific organisms in soil phosphorus
cycling,`` says Cade-Menun. ``I think it would be exciting to
link the phosphorus forms revealed by NMR spectroscopy back to
the soil organisms producing them, and to link specific enzyme
production to the transformation of these phosphorus forms.``
Cade-Menun
also is interested in the effect of land management practices
in forestry, such as clear cutting and slash burning, on soil
nutrient dynamics: ``My Ph.D. work in the coastal forests of British
Columbia showed that during slash burning, fire, as a strong mineralizing
agent, converted much of the soil phosphorus from organic to inorganic
forms. In this high-rainfall environment, where natural forest
fires rarely occur, the phosphorus became occluded and unavailable
to replanted trees, leading to phosphorus deficiencies despite
high soil total phosphorus concentrations. But in drier ecosystems,
such as the Sierra Nevada where fire regularly occurs, it may
be an important means by which phosphorus is released back to
plant-available forms.``
While Cade-Menun
studies land systems, Paytan, a marine biogeochemist, investigates
the intimate interactions between the solid Earth and the oceans
through biogeochemical cycles that affect global environment and
climate.
Funded by
the National Science Foundation, her work focuses on phosphorus
cycles of both the present and the past. To study cycling in today`s
oceans, she boards research vessels during different seasons and
collects water samples from various ocean regions and depths.
She also obtains water samples from Professor Rob Dunbar (geological
and environmental sciences) and Assistant Professor Kevin Arrigo
(geophysics), faculty colleagues in Stanford`s new ocean sciences
program.
Studying the
phosphate cycles of the past may yield valuable clues about the
future. To learn how phosphorus cycled through oceans millions
of years ago, Paytan boards research vessels with Monterey Bay
Aquarium Research Institute scientists to collect samples of phosphate
preserved in marine sediments. She also obtains samples from the
core repository of the Ocean Drilling Program, an international
program partially funded by the National Science Foundation. Postdoctoral
scholar Kristina Faul will join Paytan`s lab in the spring to
further these studies.
``From the
chemistry of marine sediments, you can tell a lot about the chemistry
of seawater, about where substances precipitated, about the biology,
about circulation,`` Paytan says. ``All the information about
past oceans is basically retrieved from the sediments. If you`re
looking at shorter time scales, of course you have other archives,
such as coral, tree rings, ice cores and cave and lake deposits.
But for the longer time scales you have to go to marine sediments.``
Reading the
chemical signatures of the sediments, Paytan tries to correlate
phosphate burial in the ancient ocean with carbon dioxide levels
in the ancient atmosphere. She would like to know, for example,
if the weathering of the Himalayas, which potentially transferred
lots of phosphate from continents to oceans, led to an increase
in photosynthesis, and if that, in turn, helped reduce the levels
of carbon dioxide in the atmosphere. By understanding the consequences
of natural fluctuations in the ancient phosphorus cycle, Paytan
hopes to gain an understanding of how similar fluctuations today
might affect modern climate.
With Stanford
graduate student Karen McLaughlin and researchers Carol Kendall
and Steve Silva of the U.S. Geological Survey in Menlo Park, Calif.,
Paytan analyzes phosphates using advanced techniques, including
oxygen isotope analysis. Isotopes are atoms of a chemical element
with the same atomic number and nearly identical chemical behavior
but with different atomic masses. Oxygen has several naturally
occurring stable isotopes - 16O, 17O and 18O. Because of the differences
in mass, the individual isotopes of oxygen in phosphate tend to
participate at different rates in biochemical reactions, resulting
in isotopic fractionation (partitioning of the isotopes between
reactants and products).
The phosphorus-oxygen
bond is so strong that it is resistant to breakage over the range
of the Earth`s surface temperatures. But when a phosphorus-containing
compound finds itself in a biological environment, such as a cell,
enzymes easily mediate breakage of that bond, and the oxygen isotopes
in the phosphate exchange with the oxygen in the surrounding water.
This exchange is temperature dependent. That means scientists
can look at changes in oxygen isotope ratios and gain information
about phosphate cycling and the water temperature in which an
ancient sea-dwelling organism lived. In terms of their isotopic
ratios, organisms are not what they eat but rather what they drink,
as well as the temperature of their drink.
Eventually,
Paytan and Dr. Ken Caldeira at Lawrence Livermore National Laboratory
would like to use lab and field data to build a model that describes
phosphorus cycling in the ocean. And someday phosphorus scientists
studying marine cycling may collaborate with those studying terrestrial
cycling to create a ``big picture`` of planet-wide cycling of
phosphorus and even linked elements, such as carbon and nitrogen.
This AGU session
marks the first official meeting between marine and terrestrial
phosphorus scientists. But this small step may lead to giant leaps
in understanding that shape public policy in areas ranging from
forestry to global warming.
A better scientific
understanding of Earth`s complex chemical cycles couldn`t come
too soon. Consider that on Nov. 25, 2000, negotiations broke down
between officials trying to put the finishing touches on the Kyoto
Protocol, a 1997 treaty drafted by more than 170 countries to
cut greenhouse gases. A major reason: persistent disagreement
over the role of trees and managed farmland as ``sinks`` to absorb
carbon dioxide.
CONTACT: Dawn
Levy, News Service (650) 725-1944; e-mail: dawnlevy@stanford.edu
COMMENT: Adina
Paytan, Geological/Environmental Sciences (650) 724-4073; e-mail:
apaytan@pangea.stanford.edu
EDITORS: The
American Geophysical Union will hold its annual fall meeting Dec.
15 to 19 at the Moscone Convention Center, 747 Howard St., San
Francisco, CA 94103. Adina Paytan and Barbara Cade-Menun will
moderate AGU Session B51A, ``Phosphate Cycling in the Marine and
Terrestrial Environments,`` on Friday, Dec. 15, at 8:30 a.m. PST
in Hall D
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