Every schoolchild knows that drifting plankton make excellent fodder for whales, but these tiny, rudderless microorganisms do more than sustain the entire marine food web. They also transport chemical compounds such as silica, an essential ocean nutrient that makes up the intricate glass skeletons of unicellular plankton such as diatoms and some radiolaria. Such organisms take up vast quantities of carbon dioxide from Earth’s atmosphere—affecting the climate—so scientists are keen to track where silica comes from and where it ends up. Now, a new study conducted along the California coast reveals that one group of these organisms, Phaeodaria, plays a much larger role in oceanic silica transport than previously thought.
Silica is a form of silicon, the seventh most abundant element in the universe. Many marine creatures require it to produce their skeletons, including phytoplankton, which float near the ocean’s surface and capture carbon dioxide through photosynthesis. Without a steady supply of silicon from rivers, deep-sea vents, and other sources, these organisms would not be able to produce the biogenic silica—also known as opal—that forms their skeletons. When diatoms and other plankton die, they sink to the bottom of the ocean, and their skeletons are buried in marine sediment.
In the new study, Biard et al. focused on silica transport by giant phaeodarians, unicellular zooplankton that consume phytoplankton and whose delicate spherical shapes resemble dandelion seed globes. During four ocean cruises off the coast of California, the team measured the abundance of several different families of Phaeodaria at depths down to 500 meters. They also calculated the silica content of 65 individual specimens, which ranged from 0.5 to 3.9 millimeters in size.
Using those measurements, the team extrapolated how much silica the organisms were transporting from the euphotic zone—a layer of ocean water that receives enough light for photosynthesis to occur—to the deep, dark, biologically sluggish mesopelagic zone, which typically begins around 200 meters below the surface. Of the estimated total silica transport between these two zones over 4 years, phaeodarians accounted for just 10%. At greater depths, however, this one group of organisms was responsible for 26% of the total downward silica transport, far more than previously thought. Additionally, within the world’s oligotrophic gyres—a nutrient-poor ecosystem of ocean currents that covers nearly 40% of the planet’s surface—they represented between 10% and 80% of silica transport, suggesting an underestimated role of these organisms in silica transport. (Global Biogeochemical Cycles, https://doi.org/10.1029/2018GB005877, 2018)
—Emily Underwood, Freelance Writer
Correction, 3 July 2018: The article has been updated to reflect the fact that phaeodarians are zooplankton.