Feature article - Past Global Changes Magazine 30(2): Sea Ice in the Polar Regions

"Sedimentary ancient DNA (sedaDNA) as a new paleo proxy to investigate organismal responses to past environmental changes in Antarctica"

Linda Armbrecht

Available in: Past Global Changes Magazine 30(2), 78-79, 2022
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The study of ancient DNA from sediments (sedaDNA) has great potential for paleoclimate research. Using less than a gram of sediment, this new technique allows ecosystem-wide assessments of Antarctic marine biodiversity over hundreds of thousands of years.

SedaDNA: A new paleo proxy

Marine sedimentary ancient DNA (sedaDNA) is DNA from dead organisms that have sunk from the ocean to the seafloor and been preserved there. Over time, layers of sedaDNA accumulate, forming a record of "who" has inhabited the ocean in the past. sedaDNA analysis is an interesting new paleo proxy because the genetic traces of organisms that do not fossilize can be detected, too (Capo and Monchamp et al. 2022). This means that sedaDNA allows us to study past marine biodiversity quite comprehensively across different levels of the food web, including bacterio- and phytoplankton, zooplankton, and potentially even fish, uncovering wide-scale community shifts as a response to past climatic change. Such knowledge is important, as it helps us to better predict the future of marine ecosystems with ongoing climate change and find management strategies to conserve them.

Antarctica: An important location for sedaDNA research

Polar deep-ocean environments are particularly suitable locations for sedaDNA research because they feature favorable conditions for sedaDNA preservation. These include constantly low temperatures and oxygen concentrations (~0°C, ~5 mL/L, respectively, noting that these values vary regionally; Bensi et al. 2022; Garcia et al. 2018; Meredith et al. 2008), and the absence of UV radiation (Karentz 1989). Antarctica and the Southern Ocean are remote and isolated, making them natural climate laboratories to study long-term global change (Barnes et al. 2006).

Sampling logistics in remote Antarctica are difficult, and for sediment studies in particular, large research vessels or platforms are required to have the capacity to drill into the deep seafloor, sometimes several thousands of meters below the ocean surface (Fig. 1). The most suitable coring system to acquire sediments for sedaDNA analysis is piston coring, which "punches a hole" into the seafloor (rather than using active drilling) and thus recovers undisturbed sediments (Armbrecht et al. 2019). The reliance on piston coring means that sedaDNA analyses are restricted to relatively soft sediments, usually found in the upper sediment layers. However, this is not necessarily a limitation – the recovery of sediments of up to ~490 m below the seafloor has been achieved using piston coring (Tada et al. 2015), which, in many Southern Ocean regions, can reach sediments of ages that are far beyond the timescales that allow for detection of ancient DNA.

Deep Southern Ocean sediments have relatively low sedimentation rates compared to coastal areas. For example, in >3,000 m water depth in the Scotia Sea, sedimentation rates have been determined at ~10–40 cm per 1,000 years (in the upper ~430 m; Weber et al. 2021). Thus, even relatively shallow coring can provide access to sediments of considerable age, allowing sedaDNA investigations into changes in marine food web structures over multiple glacial–interglacial cycles.

Consequently, the limitation on how far back in time ancient DNA analyses can be applied to deep ocean sediments remains not a coring capacity question, but rather one of maximum age of sedaDNA preservation. It is expected that ancient DNA can be preserved for up to ~1 million years under the right conditions (although reports exist of non-replicated/authenticated ancient DNA from bacteria reaching several millions of years; Willerslev and Cooper 2005, and references therein). Until recently, the oldest authenticated sedaDNA had been from terrestrial systems (cave sediments) that were ~400,000 years old (Willerslev et al. 2003). In the Arctic environment, eukaryote sedaDNA has been found in up to 140,000-year-old sediments (Pawłowska et al. 2020). In the Antarctic, marine eukaryote sedaDNA has recently been found in ~1 million-year-old sediments in the Scotia Sea (Armbrecht et al. 2022).

Current applications of sedaDNA research in the Antarctic

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Available in: Past Global Changes Magazine 30(2), 78-79, 2022
> Access contents of this issue 
> Download this article as pdf