Led by Dr. Stergios Zarkogiannis from the University of Oxford's Department of Earth Sciences, the research reveals that changes in ocean density - not just chemistry - significantly affect the ability of planktonic organisms to calcify and incorporate carbon into their shells. Foraminifera, a key group of shell-bearing microorganisms, play an essential role in sequestering carbon dioxide (CO2) through calcification. When these organisms die, their shells sink to the ocean floor, contributing to long-term carbon storage.
The study focused on Trilobatus trilobus, a widely distributed planktonic foraminifera species, and demonstrated its sensitivity to variations in ocean density and salinity. Unlike many marine organisms, T. trilobus relies on buoyancy forces - determined by ocean density - to maintain its position in the water column.
Dr. Zarkogiannis explained: "Our findings demonstrate how planktonic foraminifera adapt their shell architecture to changes in seawater density. This natural adjustment, potentially regulating atmospheric chemistry for millions of years, underscores the complex interplay between marine life and the global climate system."
The research showed that T. trilobus reduces its calcification as ocean density decreases, making its shells lighter to counteract sinking. This response increases ocean surface alkalinity, enhancing the ocean's ability to absorb atmospheric CO2. This effect is especially relevant in the context of climate change, as ice sheet melting introduces freshwater into oceans, reducing density and further amplifying this mechanism.
Dr. Zarkogiannis analyzed fossil shells of T. trilobus from sediment sites along the Mid-Atlantic Ridge, using advanced X-ray microcomputed tomography and trace element geochemistry. The findings revealed that the species forms thinner shells in equatorial regions where ocean density is lower, and thicker shells in denser subtropical waters.
"The study reframes the narrative around ocean calcification," said Dr. Zarkogiannis, "showing that physical ocean changes, such as density and salinity, play as much of a role as chemical factors do."
These findings suggest that reduced calcification in response to lower ocean density, anticipated in a warming climate, may lead to short-term increases in CO2 absorption by the oceans. This contrasts with the long-term carbon storage traditionally associated with planktonic calcification. However, questions remain about whether similar mechanisms apply to other planktonic organisms, such as coccolithophores, or to species that form silica-based or organic shells.
"Although planktonic organisms may passively float in the water column, they are far from passive participants in the carbon cycle," added Dr. Zarkogiannis. "By actively adjusting their calcification to control buoyancy and ensure survival, these organisms also regulate the ocean's ability to absorb CO2. This dual role underscores their profound importance in understanding and addressing climate challenges."
Future research aims to explore whether these calcification principles are universal among other planktonic groups and across diverse ocean regions.
Research Report:Calcification and ecological depth preferences of the planktonic foraminifer Trilobatus trilobus in the central Atlantic
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