Not dead yet: Diatom resting spores can survive in nature for several millennia.

Microscopic marine planktons have played a major role in the Earths ecosystem and accounts for half the globes primary production. Predicting the fate of marine planktons is imperative to understanding how ocean life will change in future centuries and how organisms have adapted to changes in the past over long timescales (over thousands of years). Studies of revived resting spores of marine diatoms will serve as excellent proxies of environmental change in marine environments. Thus far, only about a century old marine diatom resting spores could be revived. This severely limited the use of diatom resting spores to investigate the effects of environmental perturbations over longer time scales. Here, we, for the first time report revival of thousands of years old resting spores from the deposits of ancient diatoms (Chaetoceros) in sub-seafloor sediments using a revival protocol that involved recreating the ancient environmental conditions (salinity, temperature and light). Our revived diatom resting spores samples range from recent (0-80 years) to [~]7200 years old. Importantly, we also extracted the DNA from the ancient resting spores of Chaetoceros species from the Baltic Sea. Our findings will enable us to compare DNA sequence data obtained from these natural archives of resurrected organisms and provide predictive models to forecast evolutionary responses of natural populations to environmental changes resulting from natural and anthropogenic stressors, including climate change\n\nSignificance StatementOur results address the important topic of adaptive evolution in marine species due to climate and environmental change induced due to anthropogenic perturbations. We present a new model system Chaetoceros muelleri which will help us address important evolutionary and long-term adaptation questions across evolutionary timescales. Our study reports (1) the revival of recent (0-80 years) to ancient (7200 years old) resting spores of Chaetoceros (2) the extraction of DNA and amplification of chloroplast and ribosomal genes from recent and ancient ([~]1300 years old) resting spores of Chaetoceros (3) Radiocarbon dating to determine age of sediments (4) Identification of species by reviving the resting spores (5) Baltic sea an excellent ecosystem to study long-term effects of environment on species adaptation