Humans trigger genetic shift in ancient Baltic diatom populations
A new study of ancient DNA extracted from Baltic Sea sediments has revealed that human activity, particularly over the last century, has fundamentally altered the genetic makeup of diatom populations that had remained stable for thousands of years. The findings, based on a meticulous reconstruction of the sea’s ecological history, show an abrupt genetic shift that coincides with the dramatic increase in nutrient pollution from industrial agriculture and urbanization.
This research provides the first direct genetic evidence that the foundational layer of the Baltic food web is rapidly evolving in response to anthropogenic pressures. Diatoms, a group of single-celled algae responsible for a significant portion of the world’s oxygen production, are critical to the health of marine ecosystems. The study highlights how modern environmental changes are not only changing which species are present but are also driving unprecedented evolutionary change at a pace far exceeding natural historical cycles, raising new questions about the long-term resilience of the Baltic Sea.
A Stable Past, A Turbulent Present
By analyzing sediment cores drilled from the seabed, scientists created a high-resolution timeline of the Baltic’s microbial life. These cores act as natural archives, with deeper layers representing the more distant past. Using advanced molecular techniques on these layered samples, the research team was able to track the genetic identity and diversity of diatoms over millennia.
The analysis revealed a remarkable consistency in the diatom community’s genetic structure for nearly 7,000 years following the last Ice Age. While species composition naturally fluctuated with long-term climate cycles, the underlying genetic variants within key diatom groups showed little turnover. This long period of stability ended abruptly in the 20th century.
The study’s key findings include:
- Millennia of Stability: Genetic data showed that diatom populations were in a state of relative equilibrium for thousands of years, adapted to the naturally low-nutrient (oligotrophic) conditions of the pre-industrial Baltic Sea.
- An Abrupt Breakpoint: Beginning around the 1950s, a period often called the “Great Acceleration” of human impact, the genetic signatures within the sediment cores changed dramatically. Ancient, stable lineages were rapidly replaced by new, genetically distinct variants.
- Rise of New Competitors: The emergent diatom genotypes are those known to thrive in nutrient-rich waters. Species and strains that can rapidly consume nitrogen and phosphorus—the primary pollutants from fertilizers and wastewater—outcompeted the historical populations.
- Loss of Diversity: While new variants appeared, the overall genetic diversity within certain historical diatom groups declined, suggesting that specialized strains adapted to the pristine, older Baltic environment have been lost.
The Power of Ancient DNA
The breakthrough in this research lies in its methodology. Traditional paleoecological studies rely on identifying the silica shells, or frustules, of diatoms preserved in sediment. While this method is excellent for identifying species, it cannot reveal genetic changes within a species. This study, however, employed the analysis of sedimentary ancient DNA (sedaDNA).
The process involved several sophisticated steps. First, researchers carefully extracted long sediment cores from the anoxic, or oxygen-free, deep basins of the Baltic Sea, where organic material like DNA is better preserved. Each core was then precisely dated using radiometric techniques, such as Lead-210 for the most recent centuries and Carbon-14 for older layers, creating a reliable chronology.
From each time-stamped layer, the team extracted trace amounts of preserved DNA. Using a technique called DNA metabarcoding, they amplified and sequenced specific genetic markers unique to diatoms. This allowed them to identify not just the species present at a given time but also the specific genetic variants, providing a far more detailed picture of community structure and evolutionary change. The authors noted that this approach effectively “unlocked a molecular fossil record” that was previously inaccessible.
The Human Fingerprint: Eutrophication
The timing of the genetic shift points unequivocally to a human cause: eutrophication. This process, driven by the massive influx of nitrogen and phosphorus into waterways, has plagued the Baltic Sea for decades. The primary sources are runoff from agricultural fields laden with industrial fertilizers and the discharge of poorly treated sewage from surrounding urban centers.
These excess nutrients act as a powerful selective pressure. Diatom strains that were historically successful in a nutrient-poor environment were suddenly at a competitive disadvantage. In their place, new or previously rare genotypes capable of rapid growth in the high-nutrient soup flourished, leading to the massive seasonal algal blooms that now characterize the modern Baltic Sea. These blooms have cascading effects, most notably the creation of vast oxygen-depleted “dead zones” on the seafloor as the dead algae sink and decompose.
The study’s genetic data provides a direct link between this well-documented pollution history and the biological response at the very base of the food web. The researchers correlated the genetic turnover directly with historical records of fertilizer use and population growth in the Baltic catchment area, finding a near-perfect match between the spike in nutrient loading and the diatom community’s genetic upheaval.
Implications for a Sea in Crisis
The discovery that the Baltic’s microscopic engine is being genetically rewired carries significant implications. Diatoms are not just passive inhabitants; they influence the entire ecosystem’s chemistry and structure. They are the primary food source for zooplankton, which are in turn eaten by fish such as herring and cod—species of immense commercial and ecological importance.
A shift in the dominant diatom types can alter the nutritional quality of the food available to grazers, potentially affecting fish stocks. Furthermore, different diatom species play different roles in biogeochemical cycles, particularly the cycling of silica, which they use to build their intricate shells. Altering the diatom community could have unforeseen long-term consequences for nutrient cycling throughout the Baltic.
Looking ahead, the research team plans to apply sedaDNA techniques to investigate how other parts of the microbial food web, including bacteria and archaea, have responded to the same pressures. They also aim to use their historical data to build more accurate predictive models of how the Baltic ecosystem will respond to future climate change and nutrient management efforts. The study serves as a stark reminder that human impacts extend beyond the visible world, reaching deep into the genetic code of the planet’s most essential organisms.
