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

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.

Culturable Bacterial Diversity from the Basaltic Subsurface of the Young Volcanic Island of Surtsey, Iceland

“…Three sample types were collected for cultivation purposes (Table 1): (i) drill core samples collected at successive depths from SE-02a and SE-02b, (ii) borehole fluids from the four drill holes (SE-01, SE-02a, SE-02b, and SE-03), and (iii) condensed steam and biomass from fumarole outlets located on the two tephra cones (63 • 18 15.4" N 20 • 36 07.7" W and 63 • 18 19.9" N 20 • 36 24.7" W). All samples were immediately processed aseptically on-site and kept at 4 • C [30,37].…”

“…Drill cores were sampled during the ICDP SUSTAIN drilling operation at Surtsey in 2017 [30,37]. At the drill site, an 8 cm section was cut from every third 3 m core run at 70 cm from the top and was immediately removed from the liner, put into a sterile plastic bag, oxygen-removed by GasPak™ (BD), and stored at 4 • C. In the laboratory, drill core samples were fragmented with a hammer into an anaerobic chamber (atmosphere: N 2 /CO 2 /H 2 : 80/10/10, Coy Laboratory Inc., Grass Lake, MI, USA), and interior fractions were split into smaller pieces in a mortar.…”

“…The 16S rRNA amplicon datasets used for the comparison with the sequences of the isolated strains were previously published in the European Nucleotide Archive (ENA) at EMBL-EBI (accession number ERP126178) [37]. Briefly, the datasets were obtained by Illumina MiSeq paired-end (2 × 300 base pair) tag sequencing using the universal primers 515f (5 -GTG CCA GCM GCC GCG GTA A-3 ) and 806r (5 -GGA CTA CHV GGG TWT CTA AT-3 ) [50].…”

“…Surtsey thus serves as a unique natural laboratory for investigating fluidrock-microbe interactions, and its boreholes can be thought of as opened windows from the surface, allowing for the examination of subsurface microbial processes at high temperatures associated with oceanic crust. Diverse bacterial and archaeal taxa were detected in the subsurface of Surtsey using high-throughput sequencing [27,37]. Although many of these taxa were previously reported in surface and subsurface habitats in both terrestrial and marine settings, many of the newly discovered clades belonged to previously unknown lineages [37].…”

“…Diverse bacterial and archaeal taxa were detected in the subsurface of Surtsey using high-throughput sequencing [27,37]. Although many of these taxa were previously reported in surface and subsurface habitats in both terrestrial and marine settings, many of the newly discovered clades belonged to previously unknown lineages [37].…”