Hydrocarbon-related microbial processes in the deep sediments of the Eastern Mediterranean Levantine Basin

Abstract

During the 2011 exploration season of the EV Nautilus in the Mediterranean Sea, we conducted a multidisciplinary study, aimed at exploring the microbial populations below the sediment-water interface (SWI) in the hydrocarbon-rich environments of the Levantine basin. Two c. 1000-m-deep locations were sampled: sediments fueled by methane seepage at the toe of the Palmachim disturbance and a patch of euxinic sediment with high sulfide and methane content offshore Acre, enriched by hydrocarbon from an unknown source. We describe the composition of the microbial population in the top 5 cm of the sediment with 1 cm resolution, accompanied by measurements of methane and sulfate concentrations, and the isotopic composition of this methane and sulfate (δ¹³C(CH₄), δ¹⁸O(SO₄), and δ³⁴S(SO₄)). Our geochemical and microbiological results indicate the presence of the anaerobic methane oxidation (AOM) coupled to bacterial sulfate reduction (BSR). We show that complex methane and sulfur metabolizing microbial populations are present in both locations, although their community structure and metabolic preferences differ due to potential variation in the hydrocarbon source.

Anaerobic oxidation of methane by sulfate in hypersaline groundwater of the Dead Sea aquifer

“…Although these rates were not directly measured but were modeled from the pore fluid sulfate concentration profile, they are much higher than those that have been observed in deep‐sea sediments (up to 7 orders of magnitude higher—Turchyn et al ., ; Wortmann et al ., ; Antler et al ., ). Therefore, even though we were not able to calculate the rate of bacterial sulfate reduction directly in this study due to the poorly constrained physical properties of our system, we suggest, based on the slope of δ 18 O SO4 vs. δ 34 S SO4 , that the rate of bacterial sulfate reduction is comparable to that observed at gas and oil seeps (Aharon & Fu, , ; Rubin‐Blum et al ., ) or estuaries (Antler et al ., ), supporting our link between bacterial sulfate reduction and AOM. In addition, it has recently been shown that during the AOM in seeps or estuaries, a linear correlation between δ 18 O SO4 vs. δ 34 S SO4 is observed (Antler et al ., ).…”

A unique isotopic fingerprint of sulfate-driven anaerobic oxidation of methane

“…The "black patch" was discovered during 2011 E/V Nautilus field season offshore of the city of Arce (Israel) inside a pockmarked field. The black patch has cylindrical symmetry and high methane concentrations (to 1.5 mM, i.e., methane in excess) (Rubin-Blum et al, 2014).…”

“…A gas seep was sampled at the Palmachim disturbance (large slump feature) offshore Israel at a water depth of 1134 m, and has visible methane gas bubbling (i.e., methane in excess) (Rubin-Blum et al, 2014).…”

Geochemical evidence for biogenic methane production and consumption in the shallow sediments of the SE Mediterranean shelf (Israel)

“…Rubin-Blum et al (2014), but here it was found for the first time at the shelf, in shallow sediments within the SMTZ. The AOM process in these sediments at the SMTZ is revealed by the very light isotopic values of the DIC, which reach as low as -40‰.…”

“…Recently, large gas fields from the deep geological layers of the Oligocene-Miocene era were discovered in sediments in the deep water ( 41500 m) of the SE Mediterranean Sea (Levantine basin) (Gardosh and Tannenbaum 2014). Methane seeps were also found in the deep sea sediments (Omoregie et al, 2009(Omoregie et al, , , 2008Rubin-Blum et al (2014). However, methane production in the shallow sediments of the SE Mediterranean continental shelf has not been assumed due to its current oligotrophic conditions (Low nutrient, low chlorophyll; Herut et al, 2000;Kress et al, 2014), which offer a relatively low content of organic carbon in the sediment (o 1%).…”