CO2 and carbonate as substrate for the activation of the microbial community in 180 m deep bedrock fracture fluid of Outokumpu Deep Drill Hole, Finland.

Malin Bomberg,Sirpa Jylhä,Lasse Ahonen,Mari Raulio,Merja Itävaara,Pauliina Rajala,Carmen Höschen,Riikka Kietäväinen,Lotta Purkamo,Carsten W Mueller

doi:10.3934/microbiol.2017.4.846

Abstract

Microbial communities in deep subsurface environments comprise a large portion of Earth's biomass, but the metabolic activities in these habitats are largely unknown. Here the effect of CO2 and carbonate on the microbial community of an isolated groundwater fracture zone at 180 m depth of the Outokumpu Deep Scientific Drill Hole (Finland) was tested. Outokumpu groundwater at 180 m depth contains approximately 0.45 L L−1 dissolved gas of which methane contributes 76%. CO2, on the other hand, is scarce. The number of microbial cells with intracellular activity in the groundwater was low when examined with redox staining. Fluorescence Assisted Cell Sorting (FACS) analyses indicated that only 1% of the microbial community stained active with the redox sensing dye in the untreated groundwater after 4 weeks of starvation. However, carbon substrate and sulfate addition increased the abundance of fluorescent cells up to 7%. CO2 and CO2 + sulfate activated the greatest number of microbes, especially increasing the abundance of Pseudomonas sp., which otherwise was present at only low abundance in Outokumpu. Over longer exposure time (2 months) up to 50% of the bacterial cells in the groundwater were shown to incorporate inorganic carbon from carbonate into biomass. Carbon recapture is an important feature in this ecosystem since it may decrease the rate of carbon loss in form of CO2 released from cellular processes.

Highlights

Precambrian shield areas, such as the Fennoscandian Shield, preserve the oldest geological records of our planet, but have been affected by the continuous evolution of life, atmosphere, and climate
We have previously demonstrated that the microbial community in the AIMS Microbiology fracture zone at 500 m depth in Outokumpu were substantially revived by methanol and methane and that sulfate as electron acceptor together with these carbon sources greatly increased the proportion of metabolically active microbial cells in the fracture fluid [6]
Our results indicate that the Pseudomonas constitutes minority in the Outokumpu deep bedrock environment, they have readily been detected in other Fennoscandian deep subsurface studies [13,38]

Summary

Introduction

Precambrian shield areas, such as the Fennoscandian Shield, preserve the oldest geological records of our planet, but have been affected by the continuous evolution of life, atmosphere, and climate. The Outokumpu Deep Drill Hole provides access to study isolated groundwaters in the Paleoproterozoic part of the Fennoscandian Shield in eastern Finland [2]. Methane is the most abundant hydrocarbon and contributes up to 80% of the total volume of gas released from the groundwaters and is continuously discharging on the drill hole collar [3,4]. The microbial communities in the Outokumpu deep subsurface have been reported to contain genes for different autotrophic carbon fixation pathways [5] and methane cycling [1,6]. One other study exists to date that show specific functional activity of the deep subsurface microbial community in Outokumpu [6]. More indication of actual microbial metabolic activity and ability to assimilate simple carbon compounds in deep subsurface environments is needed

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Conclusion