Ocean density identified as a key driver of carbon capture by marine plankton

New findings, published in Royal Society Open Science, have revealed that changes in ocean density have a significant impact on the rate at which marine plankton incorporate carbon into their shells. This has profound implications for carbon cycling and the ocean’s ability to absorb atmospheric CO2 in response to climate change.

Up to now, researchers have focused on how ocean chemistry and acidification affect the biomineralization of marine plankton. This study, led by Dr Stergios Zarkogiannis from the Department of Earth Sciences, University of Oxford, breaks new ground by highlighting the critical role of physical ocean properties—specifically density—in influencing this process.

Foraminifera, abundant microscopic shell-bearing organisms, play a pivotal role in the carbon cycle, due to their ability to sequester carbon dioxide into their calcium carbonate shells (a process called calcification). These sink to the ocean floor when they die, contributing to long-term carbon storage. Yet, the factors driving calcification remain poorly understood.

Planktonic foraminifera

This new study focused on Trilobatus trilobus, an abundant planktonic foraminifera species. The findings reveal that this species is highly sensitive to changes in ocean density and salinity—not just chemistry—and fine-tunes its calcification process in response. A key reason for this is that T. trilobus – similar to other planktonic foraminifera – cannot actively move itself and relies on buoyancy forces (a function of ocean density) to keep its position in the water column.

According to the new results, as ocean density decreases (and buoyancy forces decrease with it), T. trilobus reduces calcification to decrease its weight and stop itself from sinking. This ultimately leaves surface waters more alkaline and increases their ability to absorb CO2.

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The results have important implications for climate change. When ice sheets melt, this introduces freshwater into oceans, causing ocean density to decrease. Reduced calcification in less dense waters, anticipated in a future ocean impacted by climate-driven ice sheet melting and freshening, could increase ocean alkalinity and enhance its capacity to absorb CO2. For short-term climatic cycles, increased absorption of CO2 by the oceans would have a greater influence than the reduced incorporation of carbon into planktonic foraminifera (which stores carbon over longer cycles).

Shell architecture

Dr Stergios Zarkogiannis said: “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.”

In the study, Dr Zarkogiannis analysed modern (late Holocene) T. trilobus fossil shells collected from deep-sea sediment sites along the Mid-Atlantic Ridge in the central Atlantic Ocean. Using advanced techniques such as X-ray microcomputed tomography (which rotates specimens to capture thousands of X-ray images), reconstructing them in three dimensions to reveal hidden anatomical details, and shell trace element geochemistry, the study connected calcification patterns to variations in salinity, density, and carbonate chemistry.

The results demonstrated that the species produces thinner, lighter shells in equatorial waters and thicker, heavier shells in the denser subtropical regions.

Physical ocean changes

According Dr Zarkogiannis, the study reframes the narrative around ocean calcification, showing that physical ocean changes, such as density and salinity, play as much of a role as chemical factors do. These findings provide a critical view of how marine ecosystems adapt to climate change.

Dr Zarkogiannis added: “Although planktonic organisms may passively float in the water column, they are far from passive participants in the carbon cycle. 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.”

While this study reveals critical insights into how T. trilobus adapts its calcification, more research is needed to determine whether buoyancy regulation influences calcification in other groups of organisms that contribute to ocean and atmosphere chemistry regulation, such as coccolithophores.

Additionally, it remains unclear whether this is a universal process affecting all planktonic organisms, including those which form shells using silica or organic materials. Future studies by Dr Zarkogiannis will investigate whether these principles apply across diverse groups and oceanic regions.