Unravelling the Past and Present : Marine Phytoplankton Population Dynamics in the face of the Anthropocene

Abstract

This research investigates the resilience and adaptability of the Baltic Sea's phytoplankton populations over millennia, with a particular focus on the diatom Skeletonema marinoi. It demonstrates how genomic monitoring can provide a comprehensive picture of ecosystem health in response to environmental changes. By means of a series of multi-layered studies, the study establishes a new framework for ecosystem assessment, combining marker-based genetics with advanced genetic techniques, including analyses of sedimentary ancient DNA (sedaDNA) and resurrected individuals. This approach permits the reconstruction of phytoplankton population dynamics, here S. marinoi, across both recent decades and millennia, thereby providing insight into shifts that have occurred due to natural, climatic and anthropogenic influences. In Chapter 1, a molecular index is developed to track the dominance relationship between a diatom species (S. marinoi) and a dinoflagellate species (Apocalathium malmogiense) over time. This index provides valuable historical data that extends beyond traditional monitoring programs, allowing us to detect responses to changes. In Chapter 2, we examine the genetic structure of S. marinoi populations through hybridization capture of sedaDNA over the last ~8,000 years. Our findings reveal historical resilience, with profound changes in genetic structure occurring over the last 1,000 to 2,000 years. This demonstrates the species’ capacity for resilience despite historical disturbances and underscores the potential of genomic data to reflect long-term ecosystem stability. In Chapter 3, the revival of diatom S. marinoi resting cells from sediments over 6,800 years old demonstrates that these populations retain functional traits even after millennia of dormancy, though phenotypic convergence can be observed, whereby similar phenotypes exist despite genetic differences. This allows for the real-time study of evolutionary adaptations. Finally, in Chapter 4, the genomes of these resurrected strains are compared to modern locally distributed Baltic Sea strains of S. marinoi, revealing patterns in genetic structure. Results suggest that while there are marginal spatial impacts, there are significant temporal ones. This indicates the existence of a single large Baltic Sea population with high genetic diversity. The findings reveal that genomic monitoring, which encompasses both genetic and physiological information, represents an instrument for evaluating ecosystem health. Importantly, this research suggests that maintaining genetic diversity in phytoplankton could be essential for resilience against rapid environmental shifts, a conclusion with considerable implications for marine conservation. The Baltic Sea serves as a case study in this research, offering a model for monitoring marginal seas and coastal regions worldwide, where biodiversity is under similar pressures from climate change, pollution, and eutrophication. In light of these findings, it is evident that genomic monitoring has the potential to profoundly impact marine conservation. It can facilitate more responsive and adaptive management strategies, thereby enhancing the capacity to protect ecosystems in the face of the Anthropocene.