Complementarity of ancient DNA and microfossil records in a marine sedimentary archive from the Arctic

Diatom frustules and dinoflagellate cysts preserved in sedimentary archives are routinely utilized as indicators of past climate and environmental change. However, fragile or lightly silicified diatom frustules can be prone to fragmentation and dissolution, and indicator species are often rare or not preserved at all. For dinoflagellates, only a fraction of species are known to form fossilizable organic-walled cysts, thus the large majority remains unaccounted for in the microfossil record. Sedimentary ancient DNA ( sed aDNA) can potentially fill this gap due to its ability to trace biota even in the absence of physical remains, yet direct comparisons of microfossil records with marine sed aDNA are still limited. Here, we present a comparative analysis of marine sed aDNA metabarcoding with microfossil records of diatoms and dinoflagellates in a marine sediment core taken off North-West Greenland spanning the past ~8300 years. Our results show that the microfossil and sed aDNA records are complementary, both with regard to uncovering past diversity and describing temporal changes. Incongruencies between data types were due to database incompleteness, primer biases, marker resolution for sed aDNA, while the microfossil records were affected by low abundance or differential preservation potentials. For dinoflagellates, unknown cyst-theca relationships and differential preservation potentials were additionally limited comparability. Community efforts are needed to generate reference DNA sequences through culturing and to determine cyst-theca relationships via life-cycle studies. Our study highlights the importance of combining sed aDNA with classical approaches to gain a better understanding of long-term climate and marine ecosystem changes in the Arctic and for marine biodiversity assessments and monitoring. • Complementarity is found for both overall diversity and temporal occurrences. • 46% of diatoms recorded as microfossils are not in the genomic reference database. • sed aDNA revealed a broad diversity of non-cyst producing dinoflagellates. • Cyst-producing dinoflagellates were underrepresented in the sed aDNA record.