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Capability of glacial surface ecosystems as refuges for life on the Cryogenian Snowball Earth

Millar, Jasmin L. 2022. Capability of glacial surface ecosystems as refuges for life on the Cryogenian Snowball Earth. PhD Thesis, Cardiff University.
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The Cryogenian Period in Earth’s history extended from approximately 720 to 635 million years ago and was characterized by extensive global glaciations, known as ‘Snowball Earth’. During this time life not only survived but appears to have diversified, staging the conditions for the development of animals in the subsequent Ediacaran period. The mechanisms and habitats that enabled microorganisms to survive and diversify through this time are not known. Cryoconite holes, diverse and robust habitats on glacier surfaces, have been proposed as a candidate refuge habitat as the communities within support both photoautotrophy and heterotrophy, and can survive complete freezing. DNA sequencing, lipid analysis and laboratory incubation of cryoconite communities from both poles identified key eukaryotic crown taxa and provided evidence for how microorganisms could have survived during Snowball Earth. The application of 16S rRNA and 18S rRNA gene high throughput sequencing and intact polar lipid analysis enabled evaluation of the richness, relative abundance and biogeographic distribution of microbial communities across both poles to higher resolution than ever before. The Arctic and Antarctic cryoconite holes harboured distinct microbial communities, but the various biotic niches (grazer, predator, photoautotroph, chemotroph) are filled in every location. Eukaryotic keystone taxa that had emerged prior to the Cryogenian were identified across both polar regions. Laboratory experiments were used to incubate sediments from cryoconite holes under "Snowball Earth" physical conditions. The community responses were measured using oxygen sensors, comparative intact polar lipid analysis and quantification of 3H-Leucine incorporation. Community growth remained the same between Snowball Earth and modern polar conditions, indicating community resilience despite extreme cold and limited access to external resources. For cryoconite communities to have survived during Snowball Earth, the habitats that seed and distribute cryoconite must also have endured, particularly during glaciation and deglaciation. Signy Island of the maritime Antarctic South Orkney Islands was used as a model for these transitional habitats. Communities recovered from ponds, streams, ice margins and cryoconite on Signy Island did not thrive under Snowball Earth temperatures. However, DNA sequencing and intact polar lipid analyses revealed biodiverse and habitat-specific communities. The island presents a useful model for the networks of interconnected meltwater habitats during early glaciations and deglaciations of the Cryogenian. Through this combination of community DNA analysis and laboratory incubations, I present modern analogues for Cryogenian ecosystems, in which glacial surface ecosystems provide "diversity hotspots" that allowed the survival and proliferation of biodiversity during Snowball Earth.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Earth and Environmental Sciences
Subjects: Q Science > QE Geology
Funders: NERC GW4 DTP
Date of First Compliant Deposit: 18 January 2023
Last Modified: 18 Jan 2023 16:45

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