Temperature limits to deep subseafloor life in the Nankai Trough subduction zone

Microorganisms in marine subsurface sediments substantially contribute to global biomass. Sediments warmer than 40°C account for roughly half the marine sediment volume, but the processes mediated by microbial populations in these hard-to-access environments are poorly understood. We investigated microbial life in up to 1.2-kilometer-deep and up to 120°C hot sediments in the Nankai Trough subduction zone. Above 45°C, concentrations of vegetative cells drop two orders of magnitude and endospores become more than 6000 times more abundant than vegetative cells. Methane is biologically produced and oxidized until sediments reach 80° to 85°C. In 100° to 120°C sediments, isotopic evidence and increased cell concentrations demonstrate the activity of acetate-degrading hyperthermophiles. Above 45°C, populated zones alternate with zones up to 192 meters thick where microbes were undetectable.

Tracing Fe reaction pathways in tephra-rich deep subseafloor sediments from the Nankai Trough offshore Japan by using sequential extractions and stable Fe isotopes

<p>The deep subseafloor biosphere represents one of the Earth’s largest, but also least understood ecosystems with diverse species and mostly uncharacterized microbial communities. International Ocean Discovery Program (IODP) Expedition 370 (Temperature Limit of the Deep Biosphere off Muroto) established Site C0023 down to 1180 mbsf in the Nankai Trough off Japan to explore the upper temperature limit of microbial life in the deep sedimentary biosphere [1]. Site C0023 is characterized by a complex lithostratigraphic and depositional history with strongly changing sedimentation rates. Volcanic ash layers are ubiquitous in all lithological units. However, the highest abundance of ash layers could be observed between 400 and 700 mbsf. Previous studies have shown that volcanic ashes represent hotspots for microbial life [2] and are commonly characterized by high Fe(III) and Mn(IV) contents [3]. Onboard measurements show a release of dissolved Fe in the depth interval associated with the highest abundance of ash layers [1]. Therefore, we hypothesized that the release is related to microbial Fe reduction fueled by the mineralogy of the volcanic ash. In order to identify the source and reaction pathway of the liberated Fe, we applied sequential extractions of differently reactive Fe oxide pools on mud rock and ash layer samples as well as stable iron isotope (δ<sup>56</sup>Fe) analyses on pore-water and solid-phase samples. Microbial Fe reduction leads to Fe isotope fractionation with an enrichment of light isotopes in the released Fe and a respective enrichment of heavy isotopes in the residual ferric substrate. Therefore, the δ<sup>56</sup>Fe signals of different reactive Fe pools and the pore water are used to identify the pools actually involved in microbial respiration processes. Our results show that the total Fe content in mud rock of Site C0023 is relatively constant at ~4.2 wt%. Reactive Fe oxides represent 25% of the total Fe. The bulk Fe content in the ash layers varies between 1.4 and 6.8 wt%. Surprisingly, most ash samples contain less total Fe (3.35 wt% on average) compared to the surrounding mud rock. Similarly, the contents of the reactive Fe oxides are significantly lower. This indicates that either (1) ash layers do not represent the energy substrate for microbial Fe reduction, or (2) reactive Fe in ash samples has already been used up by microbes. The bulk Fe content in recent volcanic material from an active volcano on the Japanese island arc is ~4.4 wt% [4]. The higher Fe content in fresh volcanic material compared to ash samples at Site C0023 might suggest that reactive Fe in ash layers is already reduced. Alternatively, the dissolved Fe release might be related to microbial reduction of structural Fe(III) in smectite promoting the smectite-to-illite transition, which has previously been proposed for Site C0023 [5].</p><p>References:<br>[1] Heuer, V.B. et al., 2017. In <em>Proc. IODP Volume 370</em>.<br>[2] Inagaki, F. et al., 2003. <em>AEM</em> 69: 7224-7235.<br>[3] Torres, M.E. et al., 2015. <em>Geobiology</em> 13: 562-580.<br>[4] Vogel, A. et al., 2017. <em>J. Geophys. Res. Atmos</em>. 122: 9485-9514.<br>[5] Kim, J. et al., 2019. <em>Geology</em> 47: 535-539.</p>

Fossilized Endolithic Microorganisms in Pillow Lavas from the Troodos Ophiolite, Cyprus

The last decade has revealed the igneous oceanic crust to host a more abundant and diverse biota than previously expected. These underexplored rock-hosted deep ecosystems dominated Earth’s biosphere prior to plants colonized land in the Ordovician, thus the fossil record of deep endoliths holds invaluable clues to early life and the work to decrypt them needs to be intensified. Here, we present fossilized microorganisms found in open and sealed pore spaces in pillow lavas from the Troodos Ophiolite (91 Ma) on Cyprus. A fungal interpretation is inferred upon the microorganisms based on characteristic morphological features. Geochemical conditions are reconstructed using data from mineralogy, fluid inclusions and the fossils themselves. Mineralogy indicates at least three hydrothermal events and a continuous increase of temperature and pH. Precipitation of 1) celadonite and saponite together with the microbial introduction was followed by 2) Na and Ca zeolites resulting in clay adherence on the microorganisms as protection, and finally 3) Ca carbonates resulted in final fossilization and preservation of the organisms in-situ. Deciphering the fossil record of the deep subseafloor biosphere is a challenging task, but when successful, can unlock doors to life’s cryptic past.

Divergent methyl-coenzyme M reductase genes in a deep-subseafloor Archaeoglobi

The methyl-coenzyme M reductase (MCR) complex is a key enzyme in archaeal methane generation and has recently been proposed to also be involved in the oxidation of short-chain hydrocarbons including methane, butane and potentially propane. The number of archaeal clades encoding the MCR complex continues to grow, suggesting that this complex was inherited from an ancient ancestor, or has undergone extensive horizontal gene transfer. Expanding the representation of MCR-encoding lineages through metagenomic approaches will help resolve the evolutionary history of this complex. Here, a near-complete Archaeoglobi metagenome-assembled genome (MAG; rG16) was recovered from the deep subseafloor along the Juan de Fuca Ridge flank that encodes two divergent McrABG operons similar to those found inCandidatusBathyarchaeota andCandidatusSyntrophoarchaeum MAGs. rG16 is basal to members of the class Archaeoglobi, and encodes the genes for β-oxidation, potentially allowing an alkanotrophic metabolism similar to that proposed forCa.Syntrophoarchaeum. rG16 also encodes a respiratory electron transport chain that can potentially utilize nitrate, iron, and sulfur compounds as electron acceptors. As the first Archaeoglobi with the MCR complex, rG16 extends our understanding of the evolution and distribution of novel MCR encoding lineages among the Archaea.