Microbial Inventory of Deeply Buried Oceanic Crust from a Young Ridge Flank.

Steffen L Jørgensen,Rui Zhao

doi:10.3389/fmicb.2016.00820

Abstract

The deep marine biosphere has over the past decades been exposed as an immense habitat for microorganisms with wide-reaching implications for our understanding of life on Earth. Recent advances in knowledge concerning this biosphere have been achieved mainly through extensive microbial and geochemical studies of deep marine sediments. However, the oceanic crust buried beneath the sediments, is still largely unexplored with respect to even the most fundamental questions related to microbial life. Here, we present quantitative and qualitative data related to the microbial inventory from 33 deeply buried basaltic rocks collected at two different locations, penetrating 300 vertical meters into the upper oceanic crust on the west flank of the Mid-Atlantic spreading ridge. We use quantitative PCR and sequencing of 16S rRNA gene amplicons to estimate cell abundances and to profile the community structure. Our data suggest that the number of cells is relatively stable at ~104 per gram of rock irrespectively of sampling site and depth. Further, we show that Proteobacteria, especially Gammaproteobacteria dominate the microbial assemblage across all investigated samples, with Archaea, in general, represented by < 1% of the community. In addition, we show that the communities within the crust are distinct from the overlying sediment. However, many of their respective microbial inhabitants are shared between the two biomes, but with markedly different relative distributions. Our study provides fundamental information with respect to abundance, distribution, and identity of microorganisms in the upper oceanic crust.

Highlights

Every day ∼100 billion cubic meters of bottom seawater are transported down into the permeable upper oceanic crust
This analysis revealed a unique microbial population dominated by the bacterial phyla Gammaproteobacteria, Actinobacteria, Bacteroidetes, Chloroflexi, and Firmicutes, and different from those in the above sediment and seawater
The total number of sequence reads per sample after filtering and potential contaminants removal (OTUs present in the four controls) varied between 8598 and 25,841 with an average of 17,649 (Table 2)

Summary

Introduction

Every day ∼100 billion cubic meters of bottom seawater are transported down into the permeable upper oceanic crust. The presence and activity of microorganisms were inferred via detection of biosignatures including: (i) microscopic tubular structures in which DNA could be detected by staining (Thorseth et al, 1995; Giovannoni et al, 1996), (ii) targeting and localization of intact and active cells via in situ fluorescent hybridization (FISH) (Torsvik et al, 1998), and (iii) sitespecific nitrogen and carbon enrichment in the altered tubular structures (Giovannoni et al, 1996; Torsvik et al, 1998) These results were later supported by drilling in the Australian Antarctic Discordance (ODP Leg 187) where, in addition to corroborating textural, geochemical, and molecular observations (Furnes et al, 2001a,b; Thorseth et al, 2003), microbial DNA (16S rRNA genes) from subsurface samples was for the first time successfully amplified and sequenced (Lysnes et al, 2004). This analysis revealed a unique microbial population dominated by the bacterial phyla Gammaproteobacteria, Actinobacteria, Bacteroidetes, Chloroflexi, and Firmicutes, and different from those in the above sediment and seawater

Methods

Results

Discussion

Conclusion