Abstract
Understanding the adaptive capacities of species over long timescales lies in examining the revived recent and millennia-old resting spores buried in sediments. We show for the first time the revival, viability, and germination rate of resting spores of the diatom Chaetoceros deposited in sub-seafloor sediments from three ages (recent: 0 to 80 years; ancient: ~1250 (Medieval Climate Anomaly) and ~6600 (Holocene Thermal Maximum) calendar year before present. Recent and ancient Chaetoceros spores were revived to examine their viability and germination rate. Light and scanning electron microscopy and Sanger sequencing was done to identify the species. We show that ~6600 cal. year BP old Chaetoceros resting spores are still viable and that the vegetative reproduction in recent and ancient resting spores varies. The time taken to germinate is three hours to 2 to 3 days in both recent and ancient spores, but the germination rate of the spores decreased with increasing age. The germination rate of the recent spores was ~41% while that of the ancient spores were ~31% and ~12% for the ~1250 and ~6600 cal. year BP old resting spores, respectively. Based on the morphology of the germinated vegetative cells we identified the species as Chaetoceros muelleri var. subsalsum. Sanger sequences of nuclear and chloroplast markers identified the species as Chaetoceros muelleri. We identify a unique model system, Chaetoceros muelleri var. subsalsum and show that recent and ancient resting spores of the species buried in sediments in the Baltic Sea can be revived and used for long-term evolutionary studies.
Highlights
Phytoplankton forms the basis of the marine food web and if we understand how phytoplankton will respond to environmental and climate change, we will have a better chance to understand ecosystem change
Our study has shown that the resting spore concentrations of the marine diatom Chaetoceros are very high during three ages which correspond to times with high primary production, high sea surface temperatures (HTM, MCA), eutrophication, and result in hypoxia (Zillén et al, 2008; Funkey et al, 2014)
By reviving resting spores deposited in sediments and examining their morphology under LM and SEM we have identified the species as Chaetoceros muelleri var. subsalsum
Summary
Phytoplankton forms the basis of the marine food web and if we understand how phytoplankton will respond to environmental and climate change, we will have a better chance to understand ecosystem change. Nutrient concentration, and anthropogenic disturbances can influence the distribution, composition, abundance, and adaptation of diatom species (Crosta et al, 1997) rendering them a useful probe to study environment induced adaptations over long timescales (Orsini et al, 2013; Burge et al, 2017) In this context, the Baltic Sea is an ideal ecosystem for conducting long‐term evolutionary studies as diatom resting spores are found throughout the brackish sediment stratigraphy in the open Baltic Sea. The history of the Baltic Sea begins after the last glacial maximum about 22000 cal. During and following deglaciation the Baltic Basin went through several different freshwater to brackish water stages due to isostatic rebound and eustatic sea level changes during a geologically and evolutionarily short period of time (Andrén et al, 2011; Snoeijs‐Leijonmalm and Andrén, 2017) These stages are described as Baltic Ice Lake The vertical salinity gradient results in stratification, which together with ongoing eutrophication causes large areas with hypoxic and anoxic (oxygen concentration
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