New Insights in Silicon Cycle Variability

in the Sargasso Sea

Diatoms are single-cell phytoplankton found everywhere in the ocean. Because of their role in capturing and storing energy, diatoms are a key biological component to understanding the ocean, its influence on the carbon cycle and global climate.

• Diatoms are consumed in large numbers by zooplankton and are an important part of the marine food chain.
  
• When conditions are right, the microscopic algae multiply into a huge biomass, causing algal blooms in the ocean.
  
• Diatoms also move carbon from the atmosphere to the seafloor, an export that could be key to the global climate.

“Biology is responsible for taking carbon dioxide out of the atmosphere and putting it into the deep ocean," OSU PhD student Jeffrey Krause explains. "The concentration of dissolved inorganic carbon in the deep ocean is 40–50 times greater than that in the atmosphere. Without the sequestration of carbon by biology, the atmosphere would be a lot warmer. We need to understand what circumstances promote this type of activity.”

Diatoms take the elements they need from the upper ocean water, including dissolved silicate, Si(OH)4, to build their shells and carbon dioxide, CO2, to form the organic matter of their bodies. When diatoms die, they fall through the water column, raining down on the seafloor below. As they fall, some of the silicon and carbon dissolves back into the water. The undissolved shells and carbon matter become buried on the bottom of the ocean.

Krause is now testing a new technique for measuring previously unknown rates in the diatom silica cycle in the Sargasso Sea. The role of diatoms in the open ocean—the majority of the surface of Earth—is not well understood.

On cruises in the Sargasso Sea in 2004 and 2005, Krause and advisor Dave Nelson looked for the effect of winter storms on production of diatoms. They rode through winter storms, sampling seawater (and suspended diatoms). Krause dismissed the discomfort of the cruise, “It was rough, but we got very interesting numbers. Diatom production was ten times greater than what is normally present—much more than previous studies, in quieter seas, had shown.”

During the winter-storm cruises, Krause and Nelson used upper water-column sediment traps to catch the dead diatoms and other detritus sinking toward the bottom. However, even though diatom production had increased tenfold, the biomass caught by the sediment traps increased only by three to four times. Where were the diatoms going? Being eaten by zooplankton? Being dissolved into the seawater? Being swept out of the sediment traps by turbulence?

To learn how much silica (and carbon) was being exported to the seafloor, Krause and Nelson devised a new, highly sensitive, radioisotope method for measuring silica dissolution in the open ocean and first tested it in the equatorial Pacific in September 2005. Krause is further testing and refining the method (March 14 to May 21, 2006) at the Bermuda Biological Research Station.

Their method involves putting the radioisotope, Silicon-32, into the dissolved samples of seawater and incubating for a day. Diatoms take up this radio-tracer and incorporate it into their shells. The samples are later measured with liquid scintillation counting, to see the rate at which diatoms are producing. It is much easier to measure small changes with this method, rather than using a mass spectrometer to measure stable isotopes, as is done in coastal waters.

If the method is successful, Krause and Nelson (and other biological oceanographers) will have an important tool for determining the extent to which silicate is being recycled in the surface ocean or exported to the deep ocean.

During his PhD work, Jeffrey Krause has spent five months at sea. Here, he samples seawater from aboard the R/V Explorer in the Sargasso Sea. (Photo by Dr. Peter Sedwick.)

Microscopic diatoms build shells of silica. Diatoms are an important part of the marine food chain and also transfer silica and carbon to the deep ocean. (Photo courtesy of National Oceanic and Atmospheric Administration.)

The Sargasso Sea off Bermuda has long been a place of mystery. This salty sea is named for the huge mats of seaweed, sargassum, which floats atop it. Jeffrey Krause has been cruising from the Bermuda Biological Station for Research, BBSR, for his research into the silica cycle of the open ocean. (Photo courtesy of BBSR.)