Effectiveness of Fluorescent Viability Assays in Studies of Arctic Cold Seep Foraminifera

Katarzyna Melaniuk

doi:10.3389/fmars.2021.587748

Abstract

Highly negative δ13C values in fossil foraminifera from methane cold seeps have been proposed to reflect episodes of methane release from gas hydrate dissociation or free gas reservoirs triggered by climatic changes in the past. Because most studies on live foraminifera are based on the presence of Rose Bengal staining, that colors the cytoplasm of both live and recently dead individuals it remains unclear if, and to what extent live foraminifera incorporate methane-derived carbon during biomineralization, or whether the isotopic signature is mostly affected by authigenic overgrowth. In this paper, modern foraminiferal assemblages from a gas hydrate province Vestnesa Ridge (∼1,200 m water depth, northeastern Fram Strait) and from Storfjordrenna (∼400 m water depth in the western Barents Sea) is presented. By using the fluorescent viability assays CellTrackerTM Green (CTG) CMFDA and CellHunt Green (CHG) together with conventional Rose Bengal, it was possible to examine live and recently dead foraminifera separately. Metabolically active foraminifera were shown to inhabit methane-enriched sediments at both investigated locations. The benthic foraminiferal faunas were dominated by common Arctic species such as Melonis barleeanus, Cassidulina neoteretis, and Nonionellina labradorica. The combined usage of the fluorescence probe and Rose Bengal revealed only minor shifts in species compositions and differences in ratios between live and recently dead foraminifera from Storfjordrenna. There was no clear evidence that methane significantly affected the δ13C signature of the calcite of living specimens.

Highlights

Due to the present climate warming, the Arctic region is undergoing remarkably rapid environmental changes, termed the Arctic amplification (IPCC, 2013; Box et al, 2019)
This paper presents results of a study of live benthic foraminifera from a gas hydrate province on Vestnesa Ridge (∼1,200 m water depth; western Svalbard margin) and from gas hydrate “pingo” structures from Storfjordrenna (∼400 m water depth) in the western Barents Sea (Figure 1)
Seabed images acquired with a Multicorer-Towed Digital Camera (TowCam) during the CAGE17-2 cruise revealed the presence of white and gray bacterial mats as well as sediments colonized by chemosynthetic Siboglinidae tubeworms, biota well known to indicate active hydrocarbon seepage (Niemann et al, 2006; Treude et al, 2007; Figure 2)

Summary

Introduction

Due to the present climate warming, the Arctic region is undergoing remarkably rapid environmental changes, termed the Arctic amplification (IPCC, 2013; Box et al, 2019). The ocean warming impose a high risk of release of methane from geological reservoirs (IPCC, 2007; Phrampus and Hornbach, 2012) as large amounts of methane are stored on Arctic continental margins in the form of pressure-temperature sensitive gas hydrates (e.g., Maslin et al, 2010; Ruppel and Kessler, 2017). Gas hydrate is a widespread, ice-like substance formed when water and methane or other hydrocarbon gases combine in marine sediments under high pressure (3–5 MPa) and temperatures below ∼25◦C (e.g., Kvenvolden, 1993). It is feared that ongoing climate change can trigger destabilization of gas hydrate reservoirs and methane release into the water column and eventually to atmosphere (Ruppel and Kessler, 2017). It is crucial to understand the fate of methane in marine sediments in order to understand the potential impact of methane release to future climate and Arctic ecosystems

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