Fourteen thousand years ago, during a pivotal transition period at the end of the last ice age, massive algal blooms in the Southern Ocean played an unexpectedly significant role in modulating Earth’s climate. Recent ground-breaking research led by the Alfred Wegener Institute has unveiled that these blooms, dominated by the genus Phaeocystis, absorbed substantial amounts of carbon dioxide from the atmosphere, thereby considerably slowing the rise of this potent greenhouse gas during a phase known as the Antarctic Cold Reversal (ACR). The findings, recently published in Nature Geoscience, provide new insight into complex biological-climate feedbacks that were previously undetectable by conventional paleoenvironmental methods.
The Antarctic Cold Reversal represents a unique climatic event characterized by a temporary cooling trend in the southern hemisphere roughly between 14,500 to 12,500 years ago, interrupting the general warming that marked the end of the last glacial period. This research elucidates how the seasonal expansion and retreat of sea ice around Antarctica created prime conditions for expansive blooms of Phaeocystis algae. In particular, the extensive sea ice cover during winter followed by nutrient-rich meltwater influx during spring favored the proliferation of these algae. This phenomenon contributed to a biological carbon drawdown mechanism which had profound implications for atmospheric CO₂ concentrations and global climate regulation.
Traditional paleoceanographic proxies have made it challenging to investigate Phaeocystis because this genus does not produce the classic microfossils that allow for their detection in sediment cores. To overcome this obstacle, the Alfred Wegener Institute team employed an innovative approach centered on sedimentary ancient DNA (sedaDNA) analyses. This technique allows for the detection of genetic material preserved in ocean floor sediments over thousands of years, thus granting visibility into past ecosystems that had previously remained invisible. By extracting and scrutinizing this genetic archive from sediment cores taken from nearly 2,000 meters depth in the Bransfield Strait, north of the Antarctic Peninsula, the researchers reconstructed changes in past biological communities alongside geochemical data.
One compelling piece of evidence supporting the role of Phaeocystis blooms as a driver for carbon sequestration involves the ratio of barium to iron in the sediment core. Elevated Ba/Fe ratios, which serve as proxies for organic carbon export and deposition, coincide with the period of intensified Phaeocystis blooms. This coupling implies that increased biological productivity linked to algal proliferation directly enhanced carbon fluxes from the surface to the deep ocean, thereby facilitating long-term carbon storage in marine sediments. In this way, biological responses to sea ice dynamics contributed to modulating atmospheric carbon dioxide during a critical phase of Earth’s climatic history.
The relationships illuminated by this study underscore the intricate interplay between sea ice cover, nutrient availability, and algal productivity. Extensive sea ice formation during winter expanded habitats capable of sustaining blooms following spring melt. Nutrient-rich meltwater spreading across larger surface areas enhanced growth conditions for Phaeocystis, thus intensifying photosynthetic carbon uptake. This suggests that seasonal sea ice cycles critically influenced biogeochemical feedback loops impacting not only Southern Ocean ecosystems but also Earth’s carbon budget on millennial timescales.
Beyond carbon sequestration, Phaeocystis blooms influenced Southern Ocean food webs and nutrient cycling by altering the composition and interactions within planktonic communities. Such shifts affect the vertical transport of organic material, which governs sequestration efficiency, and influence local ecosystem structure. The study highlights how these biological processes created a complex web of feedbacks linking marine productivity, nutrient availability, and climate regulation during the ACR period. These insights illuminate the ocean’s multifaceted role as a dynamic component of the Earth’s climate system, mediated through living organisms.
In the present day, these algal populations face unprecedented threats due to ongoing environmental changes. The steady decline in sea ice extent, exacerbated by anthropogenic warming, is altering the delicate balance of the Southern Ocean’s ecosystems. As habitats shrink and seasonal patterns shift, Phaeocystis blooms are becoming less frequent and less intense. This disruption threatens to undermine their role as carbon sinks and destabilize marine food webs, potentially causing cascading effects throughout the Antarctic ecosystem. Unlike other phytoplankton such as diatoms that may adapt favorably to less ice-covered waters, the decline of Phaeocystis could lead to diminished carbon export and fundamental restructuring of ecological networks.
A particularly critical aspect of Phaeocystis is its contribution to the production of dimethyl sulfide (DMS), a sulfur-containing gas with a significant climatic impact. DMS released into the atmosphere acts as a precursor to sulfate aerosols that promote cloud nucleation, thus increasing cloud reflectivity and cooling the Earth’s surface by reflecting sunlight back to space. A decline in the frequency and extent of Phaeocystis blooms could therefore reduce DMS emissions, weakening this natural climate regulation mechanism and potentially creating a positive feedback loop that exacerbates regional and global warming.
The novel methodological approach combining sedaDNA with traditional geochemical proxies represents a leap forward for paleoceanography and climate science. By integrating biological and geochemical archives, researchers can now reconstruct past climate and ecosystem dynamics with unprecedented precision and nuance. This interdisciplinary advance challenges earlier assumptions based solely on microfossils and chemical indicators, revealing previously hidden dimensions of the Earth’s ancient climate system. As such, it calls for a reevaluation of marine ecosystem contributions to carbon cycling in Earth system models.
Looking ahead, these results emphasize the urgent necessity of incorporating marine biological feedbacks into climate models and forecasts. The influence of microalgal blooms on carbon sequestration and climate regulation underscores the complexity and sensitivity of ocean ecosystems to environmental change. Improving the resolution and scope of paleoenvironmental reconstructions is crucial to predict how ongoing changes in sea ice and oceanic conditions will affect carbon dynamics and climate trajectories. Furthermore, focusing on key biological players like Phaeocystis will enhance our understanding of ecosystem resilience and vulnerability in a warming world.
This research also hints at the potential of continued sedaDNA analyses to uncover additional hidden players in Earth’s past climate changes, offering fresh perspectives on the biological underpinnings of natural carbon sinks. Through detailed genetic examination of sediment cores from diverse marine settings, scientists may uncover further biological-climate linkages that challenge existing paradigms and help refine climate mitigation strategies. Understanding these ancient oceanic processes is more vital than ever as humanity faces accelerated climate change driven by fossil fuel emissions.
In sum, the Alfred Wegener Institute’s study marks a crucial step in unraveling the complex biogeochemical interactions governing Earth’s climate system. It highlights the Antarctic Cold Reversal as a natural experiment in which large-scale Phaeocystis blooms modulated carbon dioxide levels and influenced global climate on millennial timescales. Today, as these critical algal communities face threats from ongoing environmental change, recognizing their historical importance reinforces the imperative to protect marine biodiversity and maintain oceanic carbon sinks in the face of accelerating anthropogenic impacts.
Subject of Research: Role of Phaeocystis algal blooms in carbon dioxide drawdown during the Antarctic Cold Reversal revealed through sedimentary ancient DNA analysis.
Article Title: Carbon drawdown by algal blooms during Antarctic Cold Reversal from sedimentary ancient DNA
News Publication Date: 25-Aug-2025
Web References: http://dx.doi.org/10.1038/s41561-025-01761-w
Image Credits: Gerhard Drebes
Keywords: Ancient DNA, Carbon sinks, Antarctica, Climate change
