Some Fourteen thousand years ago, algal blooms in the Southern Ocean helped to massively reduce the global carbon dioxide content of the atmosphere – as has now been revealed by new analyses of ancient DNA published by a team from the Alfred Wegener Institute in the journal Nature Geoscience. In the ocean around the Antarctic continent, these algal blooms had a significant impact on global carbon dynamics. The current and expected future decline in sea ice in this region now poses a serious threat to these algae, which could incur global consequences.
At the end of the last ice age, the warming in the southern hemisphere slowed temporarily in a phase known as the Antarctic Cold Reversal (ACR). A new study by the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) reveals that the special climatic conditions of this period – in particular involving a vast sea ice cover in winter, followed by strong seasonal melting in the springtime – favoured massive algal blooms of the genus Phaeocystis in the Southern Ocean. These blooms absorbed large amounts of carbon dioxide, markedly slowing the increase of this climate-damaging gas in the atmosphere. The AWI research team was able to prove this connection for the first time by examining so-called sedimentary ancient DNA (sedaDNA) – genetic material that has been preserved in the seabed for thousands of years. This is due to the fact that Phaeocystis does not leave behind classic microfossils and therefore remained invisible in previous climate archives. To date, it has not been possible to detect its presence by way of classic geochemical methods.
In conducting their study, the AWI team examined a sediment core from a depth of almost 2,000 metres in the Bransfield Strait north of the Antarctic Peninsula. The core contains sedaDNA from the last 14,000 years. The researchers extracted this from the sediment cores to study changes in biological communities over time. "Our study shows that these algal blooms contributed to a significant reduction in global atmospheric CO₂ levels during a climatically important transition phase characterized by high sea ice extent," explains Josefine Friederike Weiß from AWI, lead author of the study. This is because the sediment core exhibits a high ratio of barium (Ba) to iron (Fe) for this phase – a ratio considered as an indicator of organic carbon input and deposition, due to the fact that it is linked to biological productivity in surface water.
"The further the sea ice expands in winter, the larger the area in spring where nutrient-rich meltwater enters the surface sea – and therefore the zone where Phaeocystis algae find ideal growth conditions. As a result, greater sea ice extent leads directly to a larger region with high algal productivity." Such biological processes in the ocean are closely linked to global climate events – even if they remain invisible to the human eye. In addition, the large-scale Phaeocystis blooms impacted on food webs and nutrient distribution in the Southern Ocean, triggering a complex chain reaction: From changes in plankton composition and shifted biogeochemical cycles through to increased carbon transport into the depths, they influenced the ecological balance and the carbon cycle over long periods of time.
Today, Phaeocystis is particularly endangered in Antarctica, given that the long-term trend towards sea ice loss and, in particular, the recent dramatic decline in the Southern Ocean is altering its living conditions significantly. The loss of these important algal blooms could destabilise local ecosystems. Although other algae species such as diatoms could benefit from ice-free conditions, the structure of the food web would change fundamentally. What is more, Phaeocystis is particularly efficient in transporting carbon to the deep sea. Therefore, a decline in its blooms could mean that less carbon is stored in the ocean overall, which could exacerbate climate change in the long term.
Furthermore, Phaeocystis produces dimethyl sulphide (DMS), a gas that promotes cloud formation, thereby increasing the reflection of sunlight. Consequently, the loss of algal blooms could also incur a negative impact on cloud formation and therefore on climate regulation, which in turn would lead to an additional, amplifying impact on climate.
On the one hand, the study by the AWI scientists provides new insights into the role of the Southern Ocean and its microorganisms in the global climate events of the past, which could not have previously been detected using traditional methods in sediment archives. On the other hand, it shows for the first time that previous geological investigation methods, in combination with sedimentary ancient DNA, give rise to a more realistic reconstruction of past ecosystems and our understanding of earlier carbon dioxide fluctuations. This will pave the way for more differentiated assessments of future developments in the climate system. The analysis of these geological processes underscores the crucial role that biological processes play in climate regulation. This finding highlights the significance of giving greater consideration to marine ecosystems in current climate research and in future forecasts.
With regard to further research, this means that the combination of DNA analyses and geological methods should be further improved in order to obtain and outline an even more accurate picture of past climate changes. In addition, individual significant organisms, such as Phaeocystis, should be studied in greater detail to be able to better understand their influence on the carbon cycle and climate. This will not only result in better climate predictions, but also allow potential profound ecological changes in the ocean to be identified at an early juncture and their impacts assessed accordingly.
