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After the discovery of seafloor hydrothermal venting, it became evident that the subseafloor fluid advection system plays an extremely important role in the Earth’s element cycle. We designate these fluid advections as sub-seafloor TAIGAs (which stand for rans-crustal dvection and n-situ biogeochemical processes of lobal sub-seafloor Aquifers. In Japanese, “taiga” refers to “a great river”). This concept emphasizes dynamic signature of subseafloor hydrosphere, especially for a hydrothermal fluid circulation system that might support subseafloor microbial ecosystem. However, the link between the fluid advection and microbial activity has never been clearly demonstrated. We therefore hypothesized four types of sub-seafloor TAIGAs; hydrogen, methane, sulfur, and iron to investigate the relation. Each type of TAIGA is characterized by the most dominant reducing substance available for chemosynthesis. Our trans-disciplinary research between 2008 and 2012 indicates that the hypothesis is valid and the microbial activity within the flow of TAIGAs has strong linkage to chemical characteristics of each TAIGA; that is, the subseafloor TAIGA supplies four different kinds of electron donor for respective chemolithoautotroph ecosystem which is suitable for particular electron donor. It is also shown that the composition of dissolved chemical species in the subseafloor TAIGAs are substantially affected by the geological background of their flow path such as volcanism, surrounding host rocks and tectonic settings. Our research clearly indicates that the chemosynthetic sub-seafloor biosphere is controlled and supported by Earth’s endogenous flux of heat and mass beneath the seafloor.

Since the first discovery of macrofaunal and microbial communities endemic to hydrothermal vents, chemolithoautotrophic microorganisms at and beneath the seafloor have attracted the interest of many researchers. This type of microorganism is known to obtain energy from inorganic substances (e.g., reduced sulfur compounds, molecular hydrogen, and methane) derived from subsurface physical and chemical processes, such as water–rock interactions. As the primary producers, they sustain chemosynthetic ecosystems, which are fundamentally different from terrestrial and shallow marine ecosystems that are sustained by photosynthetic primary production. It is possible that the chemosynthetic ecosystems at and beneath the seafloor are vast and metabolically active, playing an important role in the global geochemical cycles of many bio-essential elements. However, even today, the spatial and temporal distributions of the unseen chemosynthetic biosphere are largely uncertain. Here, we present geochemical constraints on the estimate of the potential biomass in seafloor and subseafloor chemosynthetic ecosystems sustained by high-temperature deep-sea hydrothermal activities and low-temperature alteration/weathering of oceanic crust. The calculations are based on the fluxes of metabolic energy sources (S, H, Fe, and CH), the chemical energy yields for metabolic reactions, and the maintenance energy requirements. The results show that for deep-sea hydrothermal vent ecosystems, most bioavailable energy yields (86 %) are due to oxidation reactions of S. In contrast, for subseafloor oceanic crust ecosystems, oxidation reactions of Fe and S generally yield the same amounts of bioavailable energy (59 % and 41 %, respectively). The estimated biomass potential in the subseafloor oceanic crust ecosystems (0.14 Pg C) is one order of magnitude higher than that in global deep-sea hydrothermal vent ecosystems (0.0074 Pg C), most likely reflecting the greater flux of low-temperature fluids circulating within the oceanic crust than that of high-temperature focused fluids venting at ridge axes. The overall biomass potential of the chemosynthetic ecosystems, estimated to be 0.15 Pg C, corresponds to only 0.02 % of the Earth’s total living biomass. This estimate suggests that chemosynthetic ecosystems comprise a rare biomass fraction of the modern Earth’s biosphere, probably reflecting a significantly smaller energy flux from Earth’s interior compared with that from the sun.

In this report, we compile a study of microbial populations in deep-sea hydrothermal plumes with providing some new data set, and discuss the relationships with geological settings and the type of the hydrothermal system, e.g. ridge or subduction, in the context of the hypothesis “four TAIGAs” (Urabe et al. Chap. ). Deep-sea hydrothermal plumes represent one of the best habitats for chemolithotrophic microbes to drive primary production in hydrothermal systems. Microbial cell densities in hydrothermal plumes are up to several times more elevated than in the general abyssal seawater. Putative sulfur utilizers, e.g. SUP05 and in , SAR324 in , and several , are the dominant microbes that are detected from most of hydrothermal plumes. The microbial community compositions in the plume of an arc-backarc system are different from those of a mid-oceanic ridge hydrothermal system. This is because the cell densities and community composition of the putative sulfur oxidizers may be regulated by reduced sulfur species due to the pH and Eh conditions of the subseafloor and surrounding seawater. Aerobic methanotrophs are found in hydrothermal plumes which contain high concentrations of molecular hydrogen and methane. Quantitative microbial cell analysis by catalyzed reporter deposition based fluorescent in situ hybridization (CARD-FISH) show that the SUP05 populations are 60–100 % responsible for increased microbial cell densities in the hydrothermal plumes of arc-backarc fields. The contribution of the SUP05 cell densities in the plume microbial community is closely connected with the chemical energy from hydrothermal fluids in various types of TAIGA.

Metal oxides including iron oxides, manganese oxides, and ferromanganese oxides have been frequently found at seafloor as a result of the release of dissolved iron and manganese from various sources including hydrothermal activities. These precipitates can adsorb or incorporate various elements, which can affect the behavior of the elements in marine environment. In addition, these precipitates can be resources of rare metals due to their high abundances in ferromanganese oxides. In this review, our aims are (i) to summarize distribution of various trace elements between ferromanganese oxides and seawater, (ii) to understand the distributions based on thermodynamic parameters, and (iii) to show the relationship between the distribution and structural information of the species adsorbed onto the ferromanganese oxides. For this purpose, our original data of chromate adsorption on ferrihydrite was also included. These attempts enable us to obtain systematic explanation of the solid-water distributions of various elements in marine environment, which in turn gives us clearer view on (i) the mechanism of isotopic fractionation during adsorption which is linked to the understanding of paleoenvironment based on the isotope geochemistry and (ii) prediction of abundances of various elements in the ferromanganese oxides that are important from the viewpoint of exploration of marine resources.

To elucidate the evolution of hydrothermal activities, we conducted an interdisciplinary study including geochemistry and biology to develop a method of obtaining reliable age information. As geochemical dating techniques, two methods applicable for hydrothermal ore minerals were developed and improved: electron spin resonance method and uranium–thorium disequilibrium method. Cross checks between the two methods generally showed good agreement for the range of hundreds to thousands of years. As biological analysis, the biodiversity among faunal communities in the targeted areas was analyzed at the species and DNA levels. Species and genetic diversity of the local fauna were not always correlated to geochemical dating, either in the southern Mariana Trough region or in the Okinawa Trough region. Although the results are not simple, comparison of age information obtained from analyses of these two disciplines potentially provides important constraints for discussion of the history and evolution of hydrothermal activities.

The structure of microbial populations near chemosynthetic faunal communities of two geographically and geologically distinct deep-sea hydrothermal vent fields were quantitatively evaluated using catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH). The hydrothermal vent of the Southern Mariana Trough (SMT) was dominated by colonization of gastropods in the low-temperature diffuse hydrothermal fluid, whereas macrofauna in mixing zones of the Mid-Okinawa Trough (MOT) consisted of polychaetes, galatheid crabs, and bivalves. A quantitative comparison revealed that the microbial community of the SMT hydrothermal vent field is significantly different from that of the MOT and is strongly influenced by mixing conditions between reduced hydrothermal fluid and oxygenated seawater. In particular, a high proportion of was found in the SMT hydrothermal fluid, which is composed of approximately 88 % seawater. In contrast, sulfur oxidizers in were most abundant near vent fauna habitats in the MOT. Our results suggest that the SMT hydrothermal environment is distinct from that of the MOT and affects the community structure of macrofauna and microbial flora.

Since the discovery in 1977 of deep-sea hydrothermal vents, they have been shown to host unique but diverse biological communities, despite the dark, barren ocean-floor settings in which they exist. Recent research has indicated that the production by fault systems of abundant reducing agents such as hydrogen possibly sustains the microbial communities in these chemoautotrophic ecosystems. High-pressure and high-temperature hydrothermal experiments, and friction experiments, have resulted in the development of important new experimental apparatuses. A batch-type (closed) experimental system that creates equilibrium conditions has contributed greatly to our understanding of sub-seafloor hydrothermal reactions. Flow-type experimental systems have allowed investigation of natural systems under non-equilibrium conditions. Friction experiments have recently been developed to better understand generation of the hydrogen that makes fault systems habitable by primary producers. These experiments suggest that microbial ecosystems sustained by chemical energy derived from fault systems might be widely distributed within oceanic crust. Moreover, flow-type systems that can be used to simulate natural hydrothermal environments that include crustal aquifers might provide insights into the ecological significance of microorganisms and their global contribution to biogeochemical cycles in the ocean and crust.

Here we describe hydrothermal and friction experiment systems that we developed during our Trans-crustal Advection and In-situ biogeochemical processes of Global sub-seafloor Aquifer (TAIGA) project, and consider the application of some of them to explore the interactions among rocks, fluids, and microbes. For this purpose, our original data obtained in the experiment of interaction between basalt and water in the flow-type system was also included.

Hydrogen generated in hydrothermal and fault systems has recently received considerable attention as a potential energy source for hydrogen-based microbial activity such as methanogenesis. Laboratory experiments that have reproduced conditions for the serpentinization of ultramafic rocks such as peridotite and komatiite have clarified the chemical and petrological processes of H production. In a frictional experimental study, we recently showed that abundant H can also be generated in a simulated fault system. This result suggests that microbial ecosystems might exist in subseafloor fault systems. Here we review the experimental constraints on hydrogen production in hydrothermal and fault systems.

To assess the effects of microbes on the exchange of Cu, Zn, and P between seafloor massive sulfide (SMS) deposits and seawater, we monitored the variation of the concentrations of Cu, Zn, and P in the artificial seawater of reaction systems that did or did not also include slabs and microbes originating from an SMS sample at 4 °C for 71 days. Dissolution of Cu and Zn from the slabs was observed when microbes were present or absent. Zinc from the slabs dissolved 1.4–2.3 fold more rapidly when microbes were present. In the presence of slabs and microbes, the rate of removal of P from the artificial seawater was the sum of the individual removal rates associated with the slabs and microbes. Six bacterial phylotypes including and were present at the end of the experiment as shown by PCR-based analysis targeting 16S rRNA genes. These bacteria probably contribute to the release of Zn from the SMS slab and removal of P from the artificial seawater. Our results provide further insights into the role(s) of microbes on the geochemical interactions between SMS deposits and seawater.

The stable isotopic signatures of biophilic elements, such as carbon, nitrogen, and sulfur, exhibited in animal soft body parts are excellent indicators for evaluating the pathways of energy and food sources. Thioautotrophic and methanotrophic nutrition prevailed in deep-sea hydrothermal vent and methane seep areas results in sulfide-sulfur and methane-carbon isotopic ratios. In this study, we reevaluated the carbon, nitrogen, and sulfur isotope compositions of animals taken from deep-sea hydrothermal vents and methane seep areas in order to understand the detailed pathways of energy and food sources for the habitants. The results showed that most animals collected from sediment-starved hydrothermal areas rely on thioautotrophic nutrition, using hydrogen sulfide dissolved in venting fluids as the sole primary energy source. On the other hand, animals from sediment-covered hydrothermal vent and cold seep fields show some variations in energy sources, of both hydrothermal and microbial origins. Sediment-covered areas tend to be enriched in biomass and diversity relative to sediment-starved areas. The results suggest that fluid discharged through sediments to the seafloor are strongly affected by subsurface microbial processes and result in increased biomass and diversity of the seafloor animal community.