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Sea ice microalgae DNA fingerprints as proxies in past climate studies

Periodic Reporting for period 1 - ICEPRINT (Sea ice microalgae DNA fingerprints as proxies in past climate studies)

Période du rapport: 2020-01-01 au 2021-12-31

One of the most striking changes observed during the satellite era is the sharp decline in Arctic sea ice extent. According to the latest IPCC projections, the Arctic will transition into a seasonally sea ice free regime within decades. Paleoclimatology relies on proxies or indicators preserved in climate archives such as ice cores, marine and terrestrial sediments to trace past climate changes. Proxies for sea ice include protist microfossils such as diatoms and dinoflagellates and lipid biomarkers. But even though progress has been made, there is a significant gap in our understanding of sea ice variability on longer timescales and the identification of new reliable sea ice proxies is crucial. Although sea ice itself leaves no traces on marine sediments, the sea ice environment harbors an enormously diverse and highly specialized community of sea ice microalgae. Considering that only a fraction of microalgae leave a microfossil in marine sediments, tracing these using DNA is a promising approach, but is still in its infancy. One of the limitations is that for many potential indicator species a molecular reference is lacking. The aim of this project is to develop and test proxies derived from DNA for sea ice, using three objectives: OBJ1: Community-level: Investigating transport and fate of microalgae from the surface waters to the seafloor and their eventual incorporation into marine sediment records. OBJ2: Species-level - identification of sea ice proxies. OBJ3: Application of the identified genetic proxies.
OBJ1, the aim is to describe the diversity of phytoplankton and their export into sediments and for this purpose samples were collected along the north Greenland coast (west and east), Nares strait, Baffin Bay and from the Lincoln Sea. Environmental DNA samples were extracted from sea ice, surface water, the chlorophyll a max, disphotic zone and surface sediments (seabed), and amplicon sequencing of the small subunit 18S rRNA V4 region, was applied. Main achievement the taxonomic biodiversity reference database is established for three data sets. Field work was conducted in summer 2021 to obtain the fourth data set, DNA has been extracted and plans are made for downstream applications.

OBJ2, the aim is to identify species suitable to be sea ice tracers. Based on material available and the results from OBJ1 the target species have been selected: Target 1. Polarella glacialis, a dinoflagellate with circum-polar distribution. The results from this study combine data collected during several large international projects and research expeditions in the Arctic region spanning over a decade, and gathered a team of scientists from Denmark, Greenland, Canada, and Norway. The modern distribution of P. glacialis was examined in sea ice cores samples, ice-loaded waters and material collected using sediment traps from various locations in the Arctic. Validation of this novel proxy was done by quantifying P. glacialis DNA in surface sediment samples from Arctic marine and fjord settings using a quantitative droplet digital PCR (ddPCR) assay and sediment records spanning the Holocene epoch. The results were compared with sea ice concentration data derived from remote sensing and in situ observations, and to the widely used sea ice proxy IP25. Target 2. Six strains of sea ice diatoms were available through collaboration at ULaval. The diatoms are isolated from sea ice in Hudson Strait and include the sea ice biomarker Haslea spicula (3 strains), H. kjellmannii (1 strain), Gyrosigma tenuissimum var. hyperborea. (1 strain), Navicula sp. (1 strain). Additionally, isolated DNA from other biomarker producing Haslea species (H. crucerigoides and H. vitrea) isolated from Svalbard are included in this study.

The results achieved so far, for target 1 we have suggested a new sea ice proxy P. glacialis DNA (Pgla-DNA) and the paper is in review. Pgla-DNA is the first quantitative DNA-based proxy, stemming from a sound ecological knowledge of recorded association to the sea ice. We are confident this new proxy has the potential to improve the representation of sea ice variability in climate and ecosystem models. For target 2 the Sequencing was performed on six cultures and reference building targeted the ribosomal small subunit nuclear 18S rRNA hypervariable gene regions V4, V7, V9, Internal Transcribed Spacer regions, ITS 1 and ITS 2, and the chloroplast gene rbcL.
Arctic sea ice is a crucial tipping element in the Earth system. The results from ICEPRINT will improve future projections of sea ice, with implications for human societies globally. The transport and fate of microalgae from the sea ice to the surface sediments and their eventual incorporation into marine sediment records is poorly understood, ICEPRINT has made progress by applying a metabarcoding approach to study the transport and fate of DNA in from surface water to surface sediments. Sequencing of sedimentary ancient DNA has emerged as a useful tool for assessing biodiversity. However, the source of the molecular fingerprints is often lacking. Molecular references for sea ice and sea ice biomarker producing diatoms have been obtained for the most common metabarcoding region. In this action genetic material preserved in sediment core have been assigned to species-specific taxonomic level. By quantitative PCR methods, gene copies from sympagic microalgae have successfully been quantified in both surface sediments and sediment cores around Greenland, the results can potentially be used as an operational input for climate models.
Tracing sea ice using sedimentary ancient DNA and sea ice biomarkers