Reconstruction of Marimo Population Dynamics Over 200 Years Using Molecular Markers and Fossil Plankton Remains

Abstract

Recent efforts have focused on reconstructing the historical abundance of unfossilized organisms using environmental DNA preserved in sediments (sedDNA). This information is crucial for understanding long-term changes in ecosystems. However, because sedDNA is prone to degradation, its quantification may not always provide accurate estimates of past abundances. To address this issue, we developed a novel method to correct for sedDNA degradation by incorporating plankton remains and applied it to estimate the historical abundance of marimo—large spherical colonies of the green alga Aegagropila brownii (formerly A. linnaei)—in Lake Akan, Japan, which is the only known habitat for large marimo. We first quantified marimo sedDNA in lake sediments dating back over 200 years. We then used our new method to estimate historical changes in their abundance from sedDNA. Analyses revealed that marimo were historically 10–100 times more abundant than they are today, but that their abundance declined in the early 20th century when influxes of muddy water and water level fluctuations occurred due to deforestation and the operation of a hydroelectric power plant. These findings align with historical eyewitness accounts, indicating that, when corrected for degradation using fossilized remains, sedDNA can be a powerful tool for reconstructing the past abundance of unfossilized organisms.