Carbon is one of the most important elements of our planet, and ninety percent of it resides inside Earth's interior. This book summarizes ten years of research by scientists involved in the Deep ...Carbon Observatory, a global community of 1200 scientists. It is a comprehensive guide to carbon inside Earth, including its quantities, movements, forms, origins, changes over time, and impact on planetary processes. Leading experts from a variety of fields, including geoscience, biology, chemistry, and physics, provide exciting new insights into the interconnected nature of the global carbon cycle, and explain why it matters to the past, present, and future of our planet. With end-of-chapter problems, illustrative infographics, full-color images, and access to online models and datasets, it is a valuable reference for graduate students, researchers, and professional scientists interested in carbon cycling and Earth system science. This title is also available as Open Access on Cambridge Core.
The permeability, connectivity, and reactivity of fluid reservoirs in oceanic crust are poorly constrained, yet these reservoirs are pathways for about a quarter of the Earth's heat loss, and ...seawater‐rock exchange within them impact ocean chemical cycles. We present results from the second ever cross‐hole tracer experiment within oceanic crust and the first conducted during a single expedition and in slow‐spreading crust west of the Mid‐Atlantic Ridge at North Pond. Here we employed boreholes that were drilled by the Integrated Ocean Drilling Program (Sites U1382 and U1383) that were instrumented and sealed. A cesium salt solution and bottom seawater tracer experiment provided a measure of the minimum Darcy fluid velocity (2 to 41 m/day) within the upper volcanic crust, constraining the minimum permeability of 10−11 to 10−9 m2. We also document chemical heterogeneities in crustal fluid compositions, rebound from drilling disturbances, and nitrification within the basaltic crust, based on systematic differences in borehole fluid compositions over a 5‐year period. These results also show heterogeneous fluid compositions with depth in the borehole, indicating that hydrothermal circulation is not vigorous enough to homogenize the fluid composition in the upper permeable basaltic basement, at least not on the time scale of 5 years. Our work verifies the potential for future manipulative experiments to characterize hydrologic, biogeochemical, and microbial process within the upper basaltic crust.
Plain Language Summary
Seawater flows within the oceanic crust, much like groundwater flows though permeable aquifers within continental crust. As seawater flows through the oceanic crust it dissolves some minerals while precipitating others. These biogeochemical reactions coupled with subsurface seawater flow affect the distribution of heat, solutes, and microbial populations within the oceanic crust, and such reactions and transport can impact oceanic processes. To assess hydrologic and biogeochemical processes that occur within the oceanic crust west of the Mid‐Atlantic Ridge, we conducted the second ever borehole‐to‐borehole tracer experiment within oceanic crust; however, similar experiments are commonplace in continental settings. On the basis of this tracer experiment and prior sampling of these boreholes, we determined that the upper several hundred meters of volcanic oceanic crust are highly permeable and support microbial nitrification; however, fluids within the upper oceanic crust are not well mixed at this location. Our results provide the foundation to conduct future manipulative experiments within the oceanic crust to characterize hydrologic and biogeochemical processes within this poorly constrained, yet globally significant, environment.
Key Points
The second ever cross‐hole tracer test conducted in oceanic crust confirmed rapid lateral and vertical hydrologic connectivity
The composition of formation fluid is heterogeneous in upper basaltic crust, which has a permeability of 10−11 to 10−9 m2
Consistent with decreases in dissolved oxygen and organic matter, nitrification occurs in basaltic crust, requiring a long residence time
The ocean-atmosphere exchange of CO
largely depends on the balance between marine microbial photosynthesis and respiration. Despite vast taxonomic and metabolic diversity among marine planktonic ...bacteria and archaea (prokaryoplankton)
, their respiration usually is measured in bulk and treated as a 'black box' in global biogeochemical models
; this limits the mechanistic understanding of the global carbon cycle. Here, using a technology for integrated phenotype analyses and genomic sequencing of individual microbial cells, we show that cell-specific respiration rates differ by more than 1,000× among prokaryoplankton genera. The majority of respiration was found to be performed by minority members of prokaryoplankton (including the Roseobacter cluster), whereas cells of the most prevalent lineages (including Pelagibacter and SAR86) had extremely low respiration rates. The decoupling of respiration rates from abundance among lineages, elevated counts of proteorhodopsin transcripts in Pelagibacter and SAR86 cells and elevated respiration of SAR86 at night indicate that proteorhodopsin-based phototrophy
probably constitutes an important source of energy to prokaryoplankton and may increase growth efficiency. These findings suggest that the dependence of prokaryoplankton on respiration and remineralization of phytoplankton-derived organic carbon into CO
for its energy demands and growth may be lower than commonly assumed and variable among lineages.
Bacteria belonging to the newly classified candidate phylum "Atribacteria" (formerly referred to as "OP9" and "JS1") are common in anoxic methane-rich sediments. However, the metabolic functions and ...biogeochemical role of these microorganisms in the subsurface remains unrealized due to the lack of pure culture representatives. In this study of deep sediment from Antarctica's Adélie Basin, collected during Expedition 318 of the Integrated Ocean Drilling Program (IODP), Atribacteria-related sequences of the 16S rRNA gene were abundant (up to 51% of the sequences) and steadily increased in relative abundance with depth throughout the methane-rich zones. To better understand the metabolic potential of Atribacteria within this environment, and to compare with phylogenetically distinct Atribacteria from non-deep-sea environments, individual cells were sorted for single cell genomics from sediment collected from 97.41 m below the seafloor from IODP Hole U1357C. As observed for non-marine Atribacteria, a partial single cell genome suggests a heterotrophic metabolism, with Atribacteria potentially producing fermentation products such as acetate, ethanol, and CO2. These products may in turn support methanogens within the sediment microbial community and explain the frequent occurrence of Atribacteria in anoxic methane-rich sediments. This first report of a single cell genome from deep sediment broadens the known diversity within the Atribacteria phylum and highlights the potential role of Atribacteria in carbon cycling in deep sediment.
Earth's subsurface is often isolated from phototrophic energy sources and characterized by chemotrophic modes of life. These environments are often oligotrophic and limited in electron donors or ...electron acceptors, and include continental crust, subseafloor oceanic crust, and marine sediment as well as subglacial lakes and the subsurface of polar desert soils. These low energy subsurface environments are therefore uniquely positioned for examining minimum energetic requirements and adaptations for chemotrophic life. Current targets for astrobiology investigations of extant life are planetary bodies with largely inhospitable surfaces, such as Mars, Europa, and Enceladus. Subsurface environments on Earth thus serve as analogs to explore possibilities of subsurface life on extraterrestrial bodies. The purpose of this review is to provide an overview of subsurface environments as potential analogs, and the features of microbial communities existing in these low energy environments, with particular emphasis on how they inform the study of energetic limits required for life. The thermodynamic energetic calculations presented here suggest that free energy yields of reactions and energy density of some metabolic redox reactions on Mars, Europa, Enceladus, and Titan could be comparable to analog environments in Earth's low energy subsurface habitats.
Oceanic crust is the largest potential habitat for life on Earth and may contain a significant fraction of Earth's total microbial biomass; yet, empirical analysis of reaction rates in basaltic crust ...is lacking. Here we report the first assessment of oxygen consumption in young (~8 Ma) and cool (<25 °C) basaltic crust, which we calculate from modelling dissolved oxygen and strontium pore water gradients in basal sediments collected during Integrated Ocean Drilling Program Expedition 336 to 'North Pond' on the western flank of the Mid-Atlantic Ridge. Dissolved oxygen is completely consumed within the upper to middle section of the sediment column, with an increase in concentration towards the sediment-basement interface, indicating an upward supply from oxic fluids circulating within the crust. A parametric reaction transport model of oxygen behaviour in upper basement suggests oxygen consumption rates of 1 nmol cm(-3)ROCK d(-1) or less in young and cool basaltic crust.
Oceanic lithosphere exposed at the sea floor undergoes seawater-rock alteration reactions involving the oxidation and hydration of glassy basalt. Basalt alteration reactions are theoretically capable ...of supplying sufficient energy for chemolithoautotrophic growth. Such reactions have been shown to generate microbial biomass in the laboratory, but field-based support for the existence of microbes that are supported by basalt alteration is lacking. Here, using quantitative polymerase chain reaction, in situ hybridization and microscopy, we demonstrate that prokaryotic cell abundances on seafloor-exposed basalts are 3-4 orders of magnitude greater than in overlying deep sea water. Phylogenetic analyses of basaltic lavas from the East Pacific Rise (9° N) and around Hawaii reveal that the basalt-hosted biosphere harbours high bacterial community richness and that community membership is shared between these sites. We hypothesize that alteration reactions fuel chemolithoautotrophic microorganisms, which constitute a trophic base of the basalt habitat, with important implications for deep-sea carbon cycling and chemical exchange between basalt and sea water.
The phyla Nitrospirota and Nitrospinota have received significant research attention due to their unique nitrogen metabolisms important to biogeochemical and industrial processes. These phyla are ...common inhabitants of marine and terrestrial subsurface environments and contain members capable of diverse physiologies in addition to nitrite oxidation and complete ammonia oxidation. Here, we use phylogenomics and gene-based analysis with ancestral state reconstruction and gene-tree-species-tree reconciliation methods to investigate the life histories of these two phyla. We find that basal clades of both phyla primarily inhabit marine and terrestrial subsurface environments. The genomes of basal clades in both phyla appear smaller and more densely coded than the later-branching clades. The extant basal clades of both phyla share many traits inferred to be present in their respective common ancestors, including hydrogen, one-carbon, and sulfur-based metabolisms. Later-branching groups, namely the more frequently studied classes Nitrospiria and Nitrospinia, are both characterized by genome expansions driven by either de novo origination or laterally transferred genes that encode functions expanding their metabolic repertoire. These expansions include gene clusters that perform the unique nitrogen metabolisms that both phyla are most well known for. Our analyses support replicated evolutionary histories of these two bacterial phyla, with modern subsurface environments representing a genomic repository for the coding potential of ancestral metabolic traits.
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