Microbe-Worm Symbiosis Stabilizes Methane Hydrates in Deep Marine Environments


 Our Planet has a natural ecosystem comprised of living organisms and methane hydrates in deep marine environments. This ecosystem was constructed in the present work to examine the influence that subtle temperature fluctuations could have on the dynamic stability of the hydrate deposits. The coupled mass and energy balance equations that describe the microbial bioreactions, their consumption by feather duster worms, and methane hydrate dissociation confirm that the bioreaction kinetics are dominated by endothermic methanogenic metabolism that stabilizes methane hydrates with a fragile tolerance to 0.001K temperature increases. The feather duster worms also stabilize the hydrates via their selective consumption of methanotrophs that could otherwise overtake the system by their exothermic metabolism. Critical ocean temperature limits exist, beyond which hydrate dissociations would cause underwater eruptions of methane into the sea. Historical ocean temperature records and gas hydrate inventory estimates combined with our model suggests that hydrate deposits as deep as 560-meters below sea level could already be at risk, whereas the methane hydrate stability zone will retreat deeper as ocean temperatures rise. Slowing its retreat could avoid the massive release of greenhouse gas.


Introduction
Oceanic emissions contribute to natural atmospheric methane at a rate of 0.002 to 0.012 gigatons of methane per year. 1 Observations have suggested that a rise in the ocean temperature could induce the dissociation of the marine natural gas hydrates (comprised of >85% methane), and thereby release an enormous amount of methane into our atmosphere. [2][3][4][5][6] Natural gas hydrates are crystalline lattices of hydrogen-bonded water molecules that encapsulate small hydrocarbon (gas) molecules, such as methane 7 . Vital to sustain our planet, hydrates form spontaneously from water and small hydrophobic molecules under specific temperature and pressure conditions 8 , such as in subsurface and suboceanic zones. With the vast quantity of methane (200 to 500 gigatons of CH4) stored in marine hydrates world-wide 9 , its sudden release due to rising ocean temperatures could cause catastrophic environmental impacts that are thought to have had historical significance in large mammal migratory patterns. 10 In deep marine environments, methane is generated by one of two different processes: 'biogenic' methane as a product of methanogenic archaea metabolism, and 'thermogenic' methane from breaking down large organic molecules thermally near hot vents. 11 Isotopic studies of methane sources in natural settings have reported that biogenic methane are generally formed under a lower temperature and methanogenic microbes live near thermodynamic equilibrium. 12 Methane hydrates located along continental margins have drawn increasing attention because that they are a modulator of climate change. 13 In natural sedimentary environments, a variety of microbial communities coexist. [14][15][16][17] Methanotrophs are microbes that oxidize methane aerobically or anaerobically into biomass as their source of energy and carbon dioxide as metabolic waste. Methanogens, on the contrary, are microbes that produce methane as a product of their metabolic activity. 18 Anaerobic oxidation of methane (AOM), mediated by methane-oxidizing archaea, limits the flux of methane from marine environments to the atmosphere. Most methane in oceanic sediment is produced in organic rich areas by microbial methanogenesis. 14 Both carbon dioxide and acetate are believed to be important substrates for methanogenesis. Methanotrophy is generally exothermic, while methanogenesis is endothermic in general. 18 Although microbial metabolism kinetics are considerably slower than that of methane hydrate dissociation, any disruption in the energy balance could affect the stability of methane hydrates. Recently, Goffredi, S. K. et al. have discovered that feather duster worms (Sabellida, Annelida) have established a unique symbiosis with methanotrophic microbes. 19 In their study, worms engulfing methanotrophs as their energy source was captured by transmission electron microscopy. Methanotrophic microbes are a source of fuel for deep sea worm populations and extend the circle of carbon exchange in marine environments beyond microbial communities.
In the present work, we seek answers to the question of how stable methane hydrates are in their natural marine environment. The symbiotic ecosystem consisting of bacterial methanogens, methanotrophs, feather duster worms, and methane hydrates has fragile tolerance to ocean temperature fluctuations that could destabilize our planet's natural methane hydrate ecosystem. 14,[17][18][19] Microbe-worm symbiosis with marine hydrates Herein we considered the symbiosis shown in Figure 1 to demonstrate the equilibrium relationship between methane, methanogens, methanotrophs, and feather duster worms, outlining the carbon exchange routine in the system. As we now know, methane can be aerobically or anaerobically oxidized by methanotrophs into biomass and carbon dioxide. These methanotrophic microbes are constantly being consumed by deep-sea worms as their food resource. Substances produced by worms and from other sources can be reduced by methanogenic microbes back into methane. Under elevated pressure and reduced temperature in deep marine environments, methane forms methane hydrate with water.
The microbial activities can be characterized as first order reactions with steady-state stoichiometry coefficients, whereas the filtration rate of the feather duster worm can be expressed as a suppression effect on methanotroph growth rate. By solving the differential equations involving heat and mass balance of the symbiotic system, one can predict the concentration change of each species involved as well as the net heat exchange. Hence, one can estimate the stabilization effect on methane hydrates offered by the symbiotic system. Figure 2. a, Known (red) and inferred (green) locations of gas hydrate. b, Normalized gas hydrate inventory (average value blue, with standard deviation yellow and red) distribution plotted against depth. c, Sketch of the methane hydrates deposited at different depths on the continental slope with estimated amount of methane carbon stored every 1000 m interval below 500 m. Methane hydrates invade sediments as the stability zone increases as depth increases.

Marine natural gas hydrate inventory (GHI)
The majority of the gas hydrates are deposited along the coast lines on the continental slope according to known and inferred gas hydrate locations data from Waite et al. 20  (500 to 1500 m) is limited by the lack of gas hydrate stability zone due to seawater temperatures above the hydrate phase boundary. The natural habitat of feather duster worms associated with methenotrophic symbionts overlaps with methane hydrate deposits, ranging from 300 m to 1800 m depth on record. 19 Seawater temperature decreases as depth increases, creating larger subcooling respect to methane hydrate phase boundary. Hence, deeper hydrates invade the marine sediments with a thicker gas hydrate stability zone. The deeper distribution (2500 to 3500 m) is limited by the insufficient sedimentation rate. It is within this thicker zone, at a greater depth, that one could expect confinement of the hydrates within nanopores of the minerology, which has been reported to depress their melting point. [22][23][24] In the other hand, hydrates deposited around 500 to 1500 m, also within the feather duster worm's habitat, are more sensitive to temperature fluctuations due to narrower stability zones, whereas deep gas hydrate deposits are more stable but only available at regions with high sedimentation. Our calculation also shows that approximately 43.8% (219 gigatons) of the total gas hydrate inventory (500 gigatons) are deposited at the shallower part of the continental slope (<1500 m) and 0.157% (0.0784 gigatons) are deposited at

Near-equilibrium energy balance simulations
Knowing the natural enviroment where methane hydrates could dissociate, we can calculate their dissociation kinetics based on the established mass and heat balances around the forementioned ecosystem. To estimate the rate of methane formation or consumption, we solved the constructured equations near the methane hydrate phase boundary, realistic of the scenario where the ocean temperature gradually increases. The presence of salts in seawater acts as a thermodynamic inhibitor, depressing the dissociation temperature of methane hydrate by a small amount compared to the freshwater system. 25 Phase boundaries of methane hydrates in fresh water and sea water are plotted using published data 13 in Figure S1, to account for the dissociation Similarly, there would be a net decrease in the substrate concentration, and a net decrease in the oxygen concentration. It is important to note that the net heat is also negative, making the overall bioreaction endothermic. To further address the influence of the endothermic process on methane hydrates, we coupled the dissociation kinetics with the endothermic bioreaction. Based on previous research 26 , methane hydrate dissocation can be de-coupled into two parts, intrinsic dissociation and heat transfer limited dissociation. The heat change can be written as: Thus, the net heat change from methane dissocaition is written as: where, and are heat from heat treansfer and intrinsic methane dissociation, respectively, A is cross-section area, is thermal conductivity of methane hydrate, is the density of methane hydrate, ∆ is the enthalpy of methane hydrate dissociation, ∆ is the temperature difference between system and equilibrium, is intrinsic methane hydrate dissociation rate constant, and is the volume of bulk methane hydrate.
For a gradual increase in the ocean temperature, the heat change rate due to dissocation and the bioreaction are equilibrated near the phase boundary, and the result of which yields a critical temperature. This critical temperature is a function of initial methane hydrate thickness, pressure, and time. The results are plotted in Figure 3c-f. We can clearly see that at the beginning, the dissocation is dominated by intrinsic dissociation, and there is a sharp increase in the critical temperature for approximately tens of seconds. However, soon thereafter the dissociation becomes dominated by heat transfer, and thus the critical temperature reaches a plateau. Depending on the initial thickness of the methane hydrate, heat is transferred throughout the bulk methane hydrate over time, and finally its dissociation becomes dominated once again by intrinsic dissociation leading to a drastic increase in the critical temperature. Obviously, the width of the plateau is a function of the initial bulk methane hydrate thickness as the diffusive heat is a function of the material's thickness. Pressure, and hence ocean depth, as well contributes to the shape of the plateau as the phase boundary of methane hydrate shifts as pressure increases. At the final stage, microbial heat change is no longer comparable with methane hydrate dissociation heat change, causing a drastic increase in the critical temperature. Consequently, a catastrophic dissocation of the entire methane hydrate bulk would occur. It is important to note that the equilibrium between the bioreaction and methane hydrate dissociation is also influenced by biosurfactants secreted by microbes and those naturally dissolved in sea water. 27,28 Their predicted influence on the critical temperature was plotted in Figure 4a assuming a mixture of biosurfactant in sea water, with results at other pressures plotted as Figure S2 in Supplementary Infomation.
The methane hydrate phase boundary becomes dynamic in the presence of microbial activity and feather duster worms, and the recalculated phase boundary is plotted in Figure 4b. In the initial 400 s, the phase boundary of methane hydrate does not change much as the critical temperature is negligible compared to the phase boundary. However, after 400 s the phase Our findings support that if the temperature continues to rise, then despite of the stabilization effect from microbial activity, marine methane hydrates could face catastrophic dissociation. Our results motivate that actions are needed to slow down the retreat of the gas hydrate stability zone due to rising ocean temperatures, to avoid a massive release of greenhouse gas by catastrophic dissociations of marine natural gas hydrates in continental margins. beyond which the catastrophic release of methane would be irreversible due to a heat transfer limited dissociation mechanism. Historical ocean temperature records combined with our model suggest that methane hydrate deposits as deep as 560-meters below sea level could already be at risk, whereas the methane hydrate stability zone would retreat deeper at a rate of 60-meters per degree temperature rise. Global predictions of natural gas hydrate distribution support that the majority of the marine gas hydrates are located in the coastal region on the continental slope, undergirding our concern that shallow gas hydrates are vulnerable to temperature fluctuations.

Implications on the release of methane from marine deposits
The implications of this work further suggest that even if the ocean temperature ceases to rise at depths greater than 560-meters hydrates could still dissociate. Based on our findings, an imbalance in the feather duster worm population could adversely influence the hydrates' stability.
Decreases in worm population could weaken the suppression on methanotroph growth rate, in which scenario the overgrowth of methanotroph would generate excess amount of heat and destabilize the hydrates. Similarly, a change in the microbial activity could impact the hydrates.
An increase in methanogenic microbial activity would make the system more endothermic, thus strengthen the tolerance to temperature fluctuations close to methane hydrate phase boundary.
Whereas an increase in methanotrophic activity would have the opposite effect.
Once the methane is released into the ocean, it is expected to recrystallize into hydrates as the water temperature and pressure in close proximity to the deposits would favor the hydrate formation region. One could expect a phenomenon of underwater snowing, whereby recrystallized hydrates rise in the water column, due to their lower in density than the surrounding sea water.
The concentration of hydrates at shallower depths increases overtime as a product of successive warming and cooling, dissociation and crystallization. 36 Eventually, the crystals would dissociate to release the methane as the ocean temperature increases and the pressure decreases. Methane released at the sea floor by a catastrophic eruption may also be retained by a number of mechanisms. It could recrystallize on geological formations, on suspended particles by heterogeneous nucleation, or by a water-hydrate core-shell mechanism and eventually deposited elsewhere. Molecular dynamic simulations showed the possibility of forming methane nanobubbles suspended in seawater supersaturated with methane from hydrate dissociation. 37 Ocean currents may also result the deposition of the free-flowing hydrate crystals. These discoveries motivate that actions are needed to slow down the retreat of the gas hydrate stability zone due to rising ocean temperatures, to avoid a potential massive release of greenhouse gas by catastrophic dissociations of marine natural gas hydrates.

Methods
Mass and energy balances with the combined bioreaction kinetics were constructed based on the ecosystem described in Figure 1. Methanotrophy and methanogenesis can be expressed by the following bioreactions as 38 : Methanothrophy where the chemistry is described by steady-state stoichiometric coefficients.
As a result, the rate of methane concentration change and bioreaction heat exchange can be described using Monod-like kinetics 38-40 : = 0.52 where, M is concentration of methane, O is oxygen concentration, S is the organic substrate concentration, and ℎ are the specific maximun growth rates of methanogenesis and methanotrophy, respectively, KM, KS, and Ki are Monod-like kinetic constants, is the overall heat change of the bioreactions, ∆ and ∆ ℎ are the enthalpy of methanogenesis and methanotrophy, respectively, and is the filteration rate of methanotrophs by feather duster worms. All the kinetic contants were adopted from published data as listed in Supplementary Infomation to fit in our calculation. 14,19,41 The rate of concentration change of oxygen and organic substrate can be calculated using the stoichiometry of expressions (4) and (5 where, the following fitting parameters are used: = 0.002848 ± 0.00049, = 1.681 ± 0.027, = 24.42 ± 7.2 , = 0.9944 ± 0.10 , = −1.441 ± 0.19 , and = 0.3925 ± 0.032 . Here, is gas hydrate stability zone thickness within marine sediment, which can be calculated by solving the steady state geotherm: where = 1.5 / / , = 63 / 2 , and and are the temperature of methane hydrate boundary and bottom seawater temperature, respectively. Sedimentation rate, , is also a function of depth, z, which can be epressed as: