Ocean density drives carbon capture by marine plankton

Recent research has demonstrated that variations in ocean density significantly influence the rate at which marine plankton assimilate carbon into their shells. This finding has major implications for carbon cycling and the ocean’s capacity to absorb CO₂ from the atmosphere in response to climate change.

Previously, scientists have concentrated on the ways ocean chemistry and acidification impact the biomineralization of marine plankton.

However, this research, led by Dr. Stergios Zarkogiannis from the Department of Earth Sciences at the University of Oxford, brings attention to the crucial influence of physical properties of the ocean—specifically density—on this process.

Foraminifera, tiny organisms that produce shells, are integral to the carbon cycle as they effectively sequester carbon dioxide into their calcium carbonate structures through calcification. When these shells eventually sink to the ocean floor, they contribute significantly to long-term carbon storage. Nonetheless, the factors that drive calcification are still not well understood.

This recent research examined Trilobatus trilobus, a widely distributed species of planktonic foraminifera that plays a critical role in marine ecosystems. This research highlights the species’ remarkable sensitivity to variations in ocean density and salinity—not just its chemistry—and its ability to adapt its calcification process accordingly.

Unlike many organisms, T. trilobus lacks the ability to swim actively, depending instead on buoyancy forces related to ocean density to maintain its position within the water column. The study found that when ocean density decreases, T. trilobus lowers its calcification to reduce its weight and prevent sinking.

This adjustment not only helps the organism maintain its buoyancy but also leads to higher alkalinity in surface waters, ultimately enhancing its capacity to absorb CO₂. These findings have significant implications for our understanding of climate change.

As ice sheets continue to melt, the influx of freshwater reduces ocean density. This reduction in calcification within less dense waters could increase ocean alkalinity and, in turn, boost CO₂ absorption capabilities in a future ocean shaped by climate change. Importantly, this enhanced absorption of CO₂ in the short term is likely to outweigh the effects of reduced carbon storage within planktonic foraminifera, which contributes to longer-term carbon cycling.

“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,” Dr Stergios Zarkogiannis said.

In the research, Dr. Zarkogiannis examined contemporary (late Holocene) T. trilobus fossil shells gathered from deep-sea sediment locations along the Mid-Atlantic Ridge in the central Atlantic Ocean.

By employing sophisticated techniques like X-ray microcomputed tomography (which rotates samples to obtain thousands of X-ray images), he reconstructed them in three dimensions to uncover concealed anatomical features and analyzed shell trace element geochemistry, linking calcification patterns to changes in salinity, density, and carbonate chemistry.

The findings revealed that this species develops thinner, lighter shells in equatorial waters, while in the denser subtropical areas, it forms thicker, heavier shells. According to Dr. Zarkogiannis, the research shifts the perspective on ocean calcification, indicating that variations in physical ocean attributes like density and salinity are equally significant as chemical factors. These results offer an important insight into how marine ecosystems adjust 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.”

This study offers vital insights into the adaptive calcification of T. trilobus, highlighting an intriguing relationship between buoyancy regulation and calcification. However, it is crucial to expand this research to determine if similar mechanisms influence calcification in other key organisms, such as coccolithophores, that significantly impact ocean and atmospheric chemistry.

Moreover, the team must address the uncertainty surrounding whether this phenomenon is universally applicable to all planktonic organisms, including those that create shells from silica or organic components. The anticipated future studies by Dr. Zarkogiannis aim to clarify whether these critical principles are consistent across a variety of marine groups and oceanic regions, which could transform our understanding of ecological dynamics.

Journal reference:

1. Stergios D. Zarkogiannis. Calcification and ecological depth preferences of the planktonic foraminifer Trilobatus trilobus in the central Atlantic. Royal Society Open Science, 2024; DOI: 10.1098/rsos.240179