Basalt-Hosted Microbial Communities in the Subsurface of the Young Volcanic Island of Surtsey, Iceland.

Pauline Bergsten,Pauline Vannier,Alexandra María Klonowski,Marie Dolores Jackson,Stephen Knobloch,Viggó Thor Marteinsson,Magnús Tumi Gudmundsson

doi:10.3389/fmicb.2021.728977

Abstract

The island of Surtsey was formed in 1963–1967 on the offshore Icelandic volcanic rift zone. It offers a unique opportunity to study the subsurface biosphere in newly formed oceanic crust and an associated hydrothermal-seawater system, whose maximum temperature is currently above 120°C at about 100m below surface. Here, we present new insights into the diversity, distribution, and abundance of microorganisms in the subsurface of the island, 50years after its creation. Samples, including basaltic tuff drill cores and associated fluids acquired at successive depths as well as surface fumes from fumaroles, were collected during expedition 5059 of the International Continental Scientific Drilling Program specifically designed to collect microbiological samples. Results of this microbial survey are investigated with 16S rRNA gene amplicon sequencing and scanning electron microscopy. To distinguish endemic microbial taxa of subsurface rocks from potential contaminants present in the drilling fluid, we use both methodological and computational strategies. Our 16S rRNA gene analysis results expose diverse and distinct microbial communities in the drill cores and the borehole fluid samples, which harbor thermophiles in high abundance. Whereas some taxonomic lineages detected across these habitats remain uncharacterized (e.g., Acetothermiia, Ammonifexales), our results highlight potential residents of the subsurface that could be identified at lower taxonomic rank such as Thermaerobacter, BRH-c8a (Desulfallas-Sporotomaculum), Thioalkalimicrobium, and Sulfurospirillum. Microscopy images reveal possible biotic structures attached to the basaltic substrate. Finally, microbial colonization of the newly formed basaltic crust and the metabolic potential are discussed on the basis of the data.

Highlights

The subsurface biosphere, defined as an ecosystem encompassing regions beneath soils and sediments, occupies roughly twice the volume of the oceans and holds about 15% of the total biomass on Earth (Pedersen, 2000; Heberling et al, 2010; Bar-On et al, 2018; Orcutt et al, 2019)
No significant correlations in DNA yield were apparent with sampling depth or in situ temperature (Supplementary Figure S2)
The 1979 and 2017 drilling projects at Surtsey volcano provide a rare opportunity to explore the subsurface microbial diversity of a very young basaltic island associated with an active hydrothermal-seawater system in newly formed oceanic crust

Summary

Introduction

The subsurface biosphere, defined as an ecosystem encompassing regions beneath soils and sediments, occupies roughly twice the volume of the oceans and holds about 15% of the total biomass on Earth (Pedersen, 2000; Heberling et al, 2010; Bar-On et al, 2018; Orcutt et al, 2019). Recent estimates suggest that these zones (i.e., continental subsurface, subseafloor sediments, and oceanic crust) contain ~70% of all prokaryotic cells and possibly more than 80% of all bacterial and archaeal species (Magnabosco et al, 2018). This large biome has only recently become the focus of research studies (Gold, 1992), and most microbial surveys have, to date, focused on subseafloor sediments (Parkes et al, 2000; D’Hondt et al, 2004; Inagaki et al, 2015). While microbial communities hosted by young basalt (

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