This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: A process known as the biological carbon pump (BCP) moves organic carbon from near the ocean's surface to the ocean floor. That carbon often stays there for hundreds or thousands of years, making the BCP a primary source of carbon sequestration in the oceans.
Scientists have thought that relatively large phytoplankton are more efficient at exporting carbon to the deep ocean than tiny cyanobacteria, but new research by Jingjing Zhang and colleagues published in AGU Advances now calls that belief into question. Using sediment traps placed at a depth of 1,000 meters (3,280 feet) in the South China Sea, paired with multiple years of observations of surface phytoplankton communities, the authors show that about two-thirds of the carbon sequestered in the ocean comes not from eukaryotic microalgae but from another major group: cyanobacteria. These tiny, often unicellular prokaryotic phytoplankton are so named because of the blue-green color seen when they grow in large colonies.
The main reason cyanobacteria play a larger role is that much of the carbon from microalgae is lost on its journey to the deep ocean, while carbon from cyanobacteria is not. These findings could change how models of ocean carbon sequestration are constructed and have implications for understanding how the BCP will shift in response to climate change, the authors say. Evidence of taxon-specific divergence in carbon sequestration comes from amino acid carbon isotope signatures found 1,000 meters (3,280 feet) deep, which allowed the researchers to tie the carbon they collected to phytoplankton groups at the surface.
The authors found that carbon production and phytoplankton structure at the surface vary considerably by season and corresponding nutrient availability. But surprisingly, the efficiency and origin of carbon collected deeper down do not change much by season. The linked processes of aggregation and ballasting, in which carbon from cyanobacteria is incorporated into larger clumps that also contain other minerals, help carbon from cyanobacteria sink more efficiently and reduce its loss during sinking by limiting microbial degradation.
These findings may explain why high-latitude systems dominated by large phytoplankton do not always exhibit higher organic carbon sequestration efficiency than low-latitude oligotrophic gyres rich in cyanobacteria, the authors note. Jingjing Zhang et al, Cyanobacteria Sustain Deep Ocean Carbon Sequestration Despite Eukaryotic Loss, AGU Advances (2026). DOI: 10.1029/2025av002148 BA art history, MA material culture.
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