Depressurization and sample processing delays may impact the outcome of shipboard microbial incubations of samples collected from the deep sea. To address this knowledge gap, we developed a remotely operated vehicle (ROV)-powered incubator instrument to carry out and compare results from in situ and shipboard RNA stable isotope probing (RNA-SIP) experiments to identify the key chemolithoautotrophic microbes and metabolisms in diffuse, low-temperature venting fluids from Axial Seamount. All the incubations showed microbial uptake of labeled bicarbonate primarily by thermophilic autotrophic Epsilonbacteraeota that oxidized hydrogen coupled with nitrate reduction. However, the in situ seafloor incubations showed higher abundances of transcripts annotated for aerobic processes, suggesting that oxygen was lost from the hydrothermal fluid samples prior to shipboard analysis. Furthermore, transcripts for thermal stress proteins such as heat shock chaperones and proteases were significantly more abundant in the shipboard incubations, suggesting that depressurization induced thermal stress in the metabolically active microbes in these incubations. Together, the results indicate that while the autotrophic microbial communities in the shipboard and seafloor experiments behaved similarly, there were distinct differences that provide new insight into the activities of natural microbial assemblages under nearly native conditions in the ocean.
The size and biogeochemical impact of the subseafloor biosphere in oceanic crust remain largely unknown due to sampling limitations. We used reactive transport modeling to estimate the size of the subseafloor methanogen population, volume of crust occupied, fluid residence time, and nature of the subsurface mixing zone for two low-temperature hydrothermal vents at Axial Seamount. Monod CH4 production kinetics based on chemostat H2 availability and batch-culture Arrhenius growth kinetics for the hyperthermophile Methanocaldococcus jannaschii and thermophile Methanothermococcus thermolithotrophicus were used to develop and parameterize a reactive transport model, which was constrained by field measurements of H2, CH4, and metagenome methanogen concentration estimates in 20–40 °C hydrothermal fluids. Model results showed that hyperthermophilic methanogens dominate in systems where a narrow flow path geometry is maintained, while thermophilic methanogens dominate in systems where the flow geometry expands. At Axial Seamount, the residence time of fluid below the surface was 29–33 h. Only 1011 methanogenic cells occupying 1.8–18 m3 of ocean crust per m2 of vent seafloor area were needed to produce the observed CH4 anomalies. We show that variations in local geology at diffuse vents can create fluid flow paths that are stable over space and time, harboring persistent and distinct microbial communities.
Determining how microbial communities organize and function at the ecosystem level is essential to understanding and predicting how they will respond to environmental change. Mathematical models can be used to describe these communities, but properly representing all the biological interactions in extremely diverse natural microbial ecosystems in a mathematical model is challenging. We examine a complementary approach based on the maximum entropy production (MEP) principle, which proposes that systems with many degrees of freedom will likely organize to maximize the rate of free energy dissipation. In this study, we develop an MEP model to describe biogeochemistry observed in Siders Pond, a phosphate limited meromictic system located in Falmouth, MA that exhibits steep chemical gradients due to density-driven stratification that supports anaerobic photosynthesis as well as microbial communities that catalyze redox cycles involving O, N, S, Fe, and Mn. The MEP model uses a metabolic network to represent microbial redox reactions, where biomass allocation and reaction rates are determined by solving an optimization problem that maximizes entropy production over time, and a 1D vertical profile constrained by an advection-dispersion-reaction model. We introduce a new approach for modeling phototrophy and explicitly represent oxygenic photoautotrophs, photoheterotrophs and anoxygenic photoautotrophs. The metabolic network also includes reactions for aerobic organoheterotrophic bacteria, sulfate reducing bacteria, sulfide oxidizing bacteria and aerobic and anaerobic grazers. Model results were compared to observations of biogeochemical constituents collected over a 24 h period at 8 depths at a single 15 m deep station in Siders Pond. Maximizing entropy production over long (3 day) intervals produced results more similar to field observations than short (0.25 day) interval optimizations, which support the importance of temporal strategies for maximizing entropy production over time. Furthermore, we found that entropy production must be maximized locally instead of globally where energy potentials are degraded quickly by abiotic processes, such as light absorption by water. This combination of field observations and modeling results indicate that natural microbial systems can be modeled by using the maximum entropy production principle applied over time and space using many fewer parameters than conventional models.
| Project Title | Modeling how virus-microbe interactions influence carbon flow at a deep-sea volcano |
|---|---|
| Acronym | Subseafloor Life |
| URL | http://www.bco-dmo.org/project/711112 |
| Created | August 1, 2017 |
| Modified | August 3, 2017 |
Abstract:
In order to break open the black box of deep-sea hydrothermal vent microbiology and take our understanding of subseafloor microbial processes and the carbon cycle to a new level, we propose to investigate autotrophy in the rocky subseafloor using molecular biological, cultivation, and geochemical techniques at Axial Seamount, an active submarine volcano that will soon be part of a seafloor cabled observatory. In line with the Moore Foundation’s MMI objectives, this project will address the functional roles of various autotrophic subseafloor microbial community members across temperature and metabolism classifications; their relationships with each other, with viruses, and with other sources of syntrophic metabolic energy; and their collective impact on carbon biogeochemistry as dictated by environmental gradients in temperature and geochemistry. Of particular importance is the inclusion of seafloor experimentation at Axial, which will not only yield data on microbial activity and carbon transformations in situ, but will also set the stage for integration of a microbial instrument at the Axial cabled observatory. Our comprehensive suite of land-based, shipboard, and in situ analyses will yield cross-disciplinary advances in our understanding of the microbial ecology and geochemistry of carbon cycling in the subseafloor biosphere at mid-ocean ridges.
| Name | Affiliation | Role |
|---|---|---|
| Julie A. Huber | Woods Hole Oceanographic Institution (WHOI) | Lead Principle Investigator |
| David A. Butterfield | National Oceanic and Atmospheric Administration (NOAA-PMEL) | Co-Principal Investigator |
| James F. Holden | University of Massachusetts Dartmouth (UMass Dartmouth) | Co-Principal Investigator |
| Jullie Zeigler Allen | J. Craig Venter Institute (JCVI) | Co-Principal Investigator |
| Joseph J. Vallino | Marine Biological Laboratory (MBL) | Co-Principal Investigator |
