Deep-Sourced Fluids From a Convergent Margin Host Distinct Subseafloor Microbial Communities That Change Upon Mud Flow Expulsion.

Scott A Klasek,Thomas Pape,Gerhard Bohrmann,Marta E Torres,Markus Loher,Frederick S Colwell

doi:10.3389/fmicb.2019.01436

Scott A Klasek, Thomas Pape + Show 4 more

Open Access

https://doi.org/10.3389/fmicb.2019.01436

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Abstract

Submarine mud volcanoes (MVs) along continental margins emit mud breccia and globally significant amounts of hydrocarbon-rich fluids from the subsurface, and host distinct chemosynthetic communities of microbes and macrofauna. Venere MV lies at 1,600 m water depth in the Ionian Sea offshore Italy and is located in a forearc basin of the Calabrian accretionary prism. Porewaters of recently extruded mud breccia flowing from its west summit are considerably fresher than seawater (10 PSU), high in Li+ and B (up to 300 and 8,000 μM, respectively), and strongly depleted in K+ (<1 mM) at depths as shallow as 20 cm below seafloor. These properties document upward transport of fluids sourced from >3 km below seafloor. 16S rRNA gene and metagenomic sequencing were used to characterize microbial community composition and gene content within deep-sourced mud breccia flow deposits as they become exposed to seawater along a downslope transect of Venere MV. Summit samples showed consistency in microbial community composition. However, beta-diversity increased markedly in communities from downslope cores, which were dominated by methyl- and methanotrophic genera of Gammaproteobacteria. Methane, sulfate, and chloride concentrations were minor but significant contributors to variation in community composition. Metagenomic analyses revealed differences in relative abundances of predicted protein categories between Venere MV and other subsurface microbial communities, characterizing MVs as windows into distinct deep biosphere habitats.

Highlights

Seafloor mud volcanoes (MVs) release tens of teragrams of methane annually into the overlying water column (Kopf, 2003; Milkov et al, 2004; Sauter et al, 2006; Sahling et al, 2009; Pape et al, 2014; Mazzini and Etiope, 2017)
To investigate microbial community changes during the extrusion and downslope transport of recently exposed, chloride-depleted mud breccia originating from the marine subsurface and the influence of seawater entrainment in such sediments, we collected samples for geochemical and microbial analyses from Venere MV offshore Calabria, Italy (Figure 1A)
We describe results of DNA extractions/quality controls, geochemical trends that characterize Venere MV summit flow deposits, and integrated geochemical and microbial community observations

Summary

Introduction

Seafloor mud volcanoes (MVs) release tens of teragrams of methane annually into the overlying water column (Kopf, 2003; Milkov et al, 2004; Sauter et al, 2006; Sahling et al, 2009; Pape et al, 2014; Mazzini and Etiope, 2017). High fluxes of methane and upward-directed fluid flow at active seafloor MVs can restrict the delivery of relevant electron acceptors, such as oxygen or sulfate, into the seafloor This limits microbial capacity for methane oxidation (Niemann et al, 2006; Boetius and Wenzhöfer, 2013). Anaerobic methanotrophs (ANMEs), the dominant consumers of methane in sediments (Reeburgh, 2007), have doubling times of up to 7 months in vitro (Nauhaus et al, 2007) and take years to dominate microbial communities in situ (Ruff et al, 2018) These limitations on biological methane removal support the argument that MVs represent potent sources of methane emission to the hydrosphere (Etiope and Milkov, 2004)

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