Diatoms, the secret sequesterer

Diatoms, the secret sequesterer

Posted In: R&D Daily | Climate | Global Climate Change | Oceanography | Biology | Chemistry | Argonne National Laboratory (DOE)

Friday, April 23, 2010

Even though you can’t see them with the naked eye, certain tiny sea algae make a big difference to the world’s climate. By taking in carbon dioxide from the atmosphere, they convert it into solid plant matter and sequester it in the world's oceans.

But what makes these particular algae, called diatoms, of interest to scientists at Argonne and around the country is their ability to sequester a different organic compound: phosphorous. That's because phosphorous in the seas helps the algae grow faster, which allows them to remove more carbon from the atmosphere during their lifetimes.

A photomicrograph of an oceanic diatom, which can turn dissolved phosphorous into an inorganic mineral shell.

Though recent attention has focused more strongly on the relationship between atmospheric carbon and climate, researchers like Argonne X-ray physicist Ian McNulty also believe that the balance of dissolved phosphorous in the world’s oceans also plays a vital role in maintaining the planet’s fragile ecological equilibrium.

"If we can understand how phosphorus uptake and sequestrations takes place, we could uncover information that might give us clues as to how carbon uptake and sequestration take place in the ocean and affect the global carbon balance," said McNulty, who leads a collaborative effort to study how diatoms sequester various dissolved compounds. "This research is of huge interest to climatologists and bears directly on and the potential to combat global climate change."

McNulty and his colleagues have spent years studying diatoms, which absorb phosphorous from the surrounding water during photosynthesis. Unlike the carbon dioxide or several other elements that diatoms take in during their lifetimes, absorbed phosphorous does not re-enter the environment in its original state. Instead, the diatoms convert it into an inorganic mineral known as apatite. During the course of a diatom’s life, naturally occurring dissolved phosphorous is transformed into a mineral shell. When a diatom dies, this shell sinks to the ocean floor, sequestering the phosphorous from the ecosystem for millennia.

“Even though each individual diatom is exceptionally small, the scale at which they sequester phosphorous and carbon from the environment is vast,” McNulty said. “When you add it all up, the diatoms in the world’s ocean are taking up gigatons of phosphorous.”

“The phosphorous balance in the oceans is intimately connected with the carbon balance in the atmosphere – you can’t alter one without altering the other,” he added. “High phosphorous levels in the environment allow the algae to grow and reproduce, and as they expand they take in more carbon dioxide from the atmosphere.”

Ellery Ingall and Julia Diaz, both of Georgia Tech, rinse particle samples aboard a research vessel. The diatoms collected in these samples were then taken to Argonne’s Advanced Photon Source for analysis.

Phosphorus is one of the principal ingredients of fertilizer, and makes up a large portion of agricultural runoff that winds up in large bodies of water, said oceanographer Jay Brandes of the Skidaway Institute of Oceanography in Georgia, who collaborated with McNulty on the research. Researchers from Skidaway and the Georgia Institute of Technology helped to collect and analyze the diatom samples.

"Oceans are the repositories of everything that washes off the lands, and phosphorus is an important nutrient for all kinds of life, especially plant life," Brandes said. "Because these diatoms need it to survive, the levels of phosphorus will control the size of the algae population. As the diatoms use up the available phosphorus and turn it into polyphosphates, they will die off in large numbers, altering the phosphorus balance."

In order to study the molecular dynamics that underlie how diatoms capture and convert phosphorous, scientists need a high-energy synchrotron light source that can generate just the right type of light to illuminate phosphorous’s chemical structure. Fortunately, Argonne is home to the Advanced Photon Source (APS), which provides exactly the kind and intensity of X-rays that McNulty and his colleagues need. “In order to study the chemistry of phosphorous, you need a very specialized facility,” he said. “The APS is and will remain at least for a few years the brightest star on the horizon for this kind of research.”

Experiments performed on the APS use a physical phenomenon known as X-ray diffraction, in which the object under study – in this case, the phosphorous compounds contained with the diatom – scatter the oncoming X-ray beam. The pattern produced by the scattering allows scientists to determine the precise atomic configuration of the phosphorous in the sample. Argonne also is home to a world-class scanning X-ray microscope that provides another key that can unlock the chemical secrets of phosphorous compounds.

The APS allows researchers from around the world to observe and analyze structures that cannot be seen anywhere else, and an anticipated upgrade to the facility will give scientists an even more comprehensive view of diatoms at the molecular level. For instance, at the upgraded APS, Argonne researchers and users could study cryogenically preserved living algae to see the exact mechanism that allows them to form their apatite coatings.

After the more concentrated X-ray beams are built, physicists from Argonne and partner institutions could also examine the diatoms' ability to sequester other trace elements, such as iron and arsenic. Some of these elements are toxic, not only to the environment but also to people, and McNulty and his colleagues are eager to find new ways to prevent these chemicals from ending up in our bodies. “If you can image the concentrations of trace elements in cells, you can understand the root cause of many diseases or monitor the uptake of anti-cancer drugs. All of these advances depend on improving the sensitivity and resolution of the facility we have here,” McNulty said.