Possible Tectonic Impact of Biosphere

16 This paper explores the possibility of existence of ultra-deep biosphere (deeper than 17 10 km under the surface) and the biogenic earthquake hypothesis – the idea that sub18 surface microorganisms might be directly related to earthquake activity. The importance 19 of electroautotrophic type of metabolism is underlined, and the role of telluric currents 20 in this process is explored in some detail, as well as the role of subsurface and atmospheric 21 microorganisms in the global electric circuit. 22 It seems that the existing estimates of the adaptability of biological organisms are 23 inconsistent with empirical evidence, and theoretical concepts predict key biochemical 24 processes to fail long before the onset of the temperatures and pressures, at which mi25 croorganisms are actually observed. This implies that life might exist much deeper be26 neath the surface than previously assumed. At the same time the estimates of energy 27 radiated during the strongest earthquakes are consistent with the biochemical energy avail28 able to the subsurface biosphere. 29 Some additional evidence is examined. It is proposed that the ultra-deep biosphere 30 might represent an important factor in resolving the debate on the nature of hydrocar31 bons. At the same time the deep subsurface microorganisms might play a significant evo32 lutionary role, not only providing seismically induced genetic variation and a ”seed bank” 33 for quick recovery after a mass extinction, but also by modulating longer climatic cy34 cles through planetary-wide bio-geo-electrochemistry. 35 Plain Language Summary 36 The depths of the Earth’s crust and layers beneath it are hostile to living organ37 isms due to high temperatures and pressures. Previous estimates have been suggesting 38 that life (even tiny microorganisms) cannot exist in the Earth’s crust deeper than about 39 10 km. Yet recent findings have shown that the limits of heat and pressure that microor40 ganisms can withstand have been underestimated. It is logical to assume that life can 41 exist at greater depths – up to 75 km at least. 42 The energies produced by microbes under the surface (combined) is enough to pro43 duce an earthquake (shaking of the ground). Perhaps it is this previously unrecognized 44 deep microbial collective that is causing the earthquakes. Earthquakes might release the 45 nutrients and other necessary chemical elements from the surrounding rocks, as well as 46 cause exchange of genes between microbial cells, which might drive their evolution. 47 Most of the earthquakes occur at the edges of the Pacific Ocean at large trenches 48 in the Earth’s crust. These trenches allow microorganisms to get deeper into the crust, 49 where they might produce an earthquake. It might also explain the presence of hydro50 carbons (oil and gas) deep beneath the surface – they might be produced by the same 51 microorganisms. 52

3 Detailed analysis 150 3.1 Energy localization 151 Although, as indicated in Section 2.2, the amount of biomass on the planet is more 152 than enough to produce the needed amounts of radiated energy for even the strongest 153 of earthquakes, it is far from being clear how this energy might be localized in the crust 154 through known biological processes. If we abstain from invoking some unknown type of 155 long-range interaction between living cells in the biosphere, it seems that the only op-156 tion would be in situ energy production (or triggering of its release, as e.g. in YN sce-157 nario in Section 2.1). 158 Therefore, in order for the hypothesis to work, we must also assume the presence 159 of biological organisms in the crust and, perhaps, in the layers below. It is currently as- 160 sumed that the conditions in the Earth's interior are unfavorable for life, mostly because 161 the current models imply high temperature and pressure gradients in these areas (Anderson,162 1989). At the same time it is known that the absolute majority of earthquakes happen 163 at fault lines (C. H. Scholz, 1969). 164 Thus, following the initial hypothesis I shall focus on the idea that biological or-165 ganisms connected to earthquake activity might be present beneath the surface in these 166 areas in especially large numbers and/or be more active there for some reason. One ob- high elevation gradients, -these would be the areas, where the crustal interior is most 170 easily accessible for biological organisms from the surface (e.g. subduction zones or mid-171 oceanic ridges). In particular, about 90% of all earthquakes on the planet occur at the 172 "Ring of Fire" (Circum-Pacific belt) (Kious & Tilling, 1996), which topographically rep-173 resents a ribbon of very deep trenches. It is quite natural to assume that the subsurface 174 in this area would be the most accessible for microorganisms. 175 What kind of organisms they might be? It seems reasonable to assume that most 176 likely they would be unicellular -due to the mentioned extreme conditions in the crust 177 and below, not favoring complex multicellular organisms. But beyond that I would not 178 state any hypotheses on their particular taxonomy: they might be represented by one 179 or many species of archaea, bacteria, protozoa, algae, yeasts, fungi or other types of yet 180 unknown organisms (perhaps even of non-cellular nature, such as viruses (also see a com-181 ment in Section 5.1), or some symbiotic arrangement of those. For the purpose of fur-182 ther discussion, in the following sections I shall refer to them simply as "microorganisms" 183 (unless the type of the organism would be known). 184 It is quite obvious that in order to be able to operate in these deep habitats, mi-185 croorganisms would have to overcome at least three significant challenges:  In Section 3.2 we shall consider the potential for solving the first problem (see also 190 Section 5.1), in Section 3.3 we shall concentrate on the second, and in Section 3.4 we shall 191 analyze the third. Let us discuss the environmental conditions that life can withstand, according to 194 the observations. In the recent decades a range of studies has been made on the ability 195 of microorganisms to adapt to the most extreme habitats. It is now known that bacte-

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-5-manuscript submitted to JGR: Biogeosciences ria, for example, might survive and even thrive in the environments with high pressures 197 (barophiles or piezophiles) and high temperatures (thermophiles), and often both. These 198 would be most relevant for us, according to the current models of Earth's crust and lay-199 ers beneath it with their supposedly significant pressure and temperature gradients. 200 In particular, evidence has been found that significant prokaryotic populations are 201 present below the sea floor at least down to the depths of 1.6 km (and temperatures of 202 100 • C) (Roussel et al., 2008). What is perhaps the most interesting is that in this study 203 contrary to all expectations in the deepest examined sample the percentage of dividing 204 cells was more than twice higher than in the layers above. At the same time, methane-205 and sulfur-cycling chemoautotrophes have been found at depths up to 600 m below the 206 mid-ocean ridge, also demonstrating peculiar discrete layering intervals in cycling inten-207 sity (Lever et al., 2013). 208 Barophilic bacteria have been found in the sediment at Mariana Trench at pres-209 sures of 100 MPa (C. Kato et al., 1998). Moreover, even non-barophilic organisms that

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If we just directly assume a moderate temperature gradient of, say, 25 • C (Gholamrezaie 227 et al., 2018) (note that it is considered to be lower for continental crust and higher for 228 oceanic crust), we'd arrive at possible depths for microorganisms to exist of about 16 km 229 beneath the surface. At the same time it is assumed in the current models, that the geother-230 mal gradient in the mantle should be two orders of magnitude lower, otherwise the tem-231 perature would rise too quickly for the rock to remain solid (Monnereau & Yuen, 2002). 232 However, regardless of that the real gradient for most of the planet's surface is un-233 known (except for measurements during isolated drilling operations, which barely got 234 below 12 km beneath the surface (Carr et al., 1996)), and some of the models show that   It appears that our current understanding of key factors making life possible is far from 244 being complete, and the limits of biological adaptability are in general underestimated.

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As an example, some recent theoretical studies have indicated that life cannot exist at 246 temperatures higher than 150-180 • C (Bains et al., 2015), which directly contradicts the 247 -6-manuscript submitted to JGR: Biogeosciences observational evidence given above, some of which has been available for more than a 248 decade prior. 249 Thus, we might conclude that at least some models indicate that the existence of 250 the already known microorganisms (as well as liquid water) might be possible down to 251 the depths of 75 km below the surface of the planet. However, one cannot at the mo-252 ment rule out the existence of some yet unknown microorganisms that might be present 253 even deeper. Additionally, we might suppose that the lack of readily available liquid wa-254 ter at greater depths (if the cited model is correct) can be compensated by the presence 255 of confined water and/or water in the hydrated minerals, assumed to be abundant in the

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In fact, there are indications that these minerals are the primary source of water on the 258 surface in the first place (Pearson et al., 2014), so an assumption of water-depleted man-259 tle does not seem to hold merit at the moment. The analysis given in Section 3.2 shows that microorganisms might tolerate the con-262 ditions present at depths of tens of kilometers beneath the surface or possibly even more.

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Yet, as noted in Section 3.1, it is not enough to make their existence possible: some sources 264 of nutrients and energy would also be required.

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With regards to nutrient production and consumption, I deem reasonable to con-  The second option seems self-evident, and is not going to be discussed here in much 270 detail. We might simply assume that the previous generations have penetrated the lower 271 layers from upper layers, perhaps more favorable for nutrition, and thus provided a cer-272 tain stack of nutrients for next generations; theoretically this process might have con-  With regards to the first option, the current models indicate, for example, that no-281 ticeable amounts of carbon should be present in the mantle (Wood et al., 1996 Xu et al., 2017). Hydrogen seems to also be avail-291 able in mantle minerals, according to the current models (Yang et al., 2016). There is 292 even the evidence of hydrocarbons present in minerals, assumed to be originating from 293 1 and even the very fact of the existence of origin is not proven -7-manuscript submitted to JGR: Biogeosciences the mantle (Sugisaki & Mimura, 1994), which might also serve as an additional source 294 of these elements (see Section 5.2 for additional discussion). And, finally, some studies 295 indicate that nitrogen should be available in the mantle too (Mallik et al., 2018). So it 296 seems that according to the current models of Earth's interior the key elements are read-297 ily present in the surrounding minerals.     Most of the studies usually assume that these observations could be explained by     It is now known that hydrocarbon mining operations using the hydraulic fractur-422 ing techniques can lead to earthquakes (Council, 2013). It is generally assumed that the 423 earthquakes produced during these activities are caused by two different reasons: 1) frack-424 ing itself (fluid injection intended to fracture the hydrocarbon bearing rock) -these are 425 rare and weak earthquakes; 2) disposal of wastewater via injection into the deep stor-426 age wells -this is the primary cause of stronger earthquakes and increased seismicity due 427 to fracking in general (Rubinstein, 2019). 428 We shall not focus our discussion on the earthquakes produced in the first way -429 it is after all understandable that the mechanical shocks, associated with hydraulic frac- croorganisms. Not only does it provide them with water itself, but it is highly conduc-435 tive water, which might play a significant role in the enhancement of extracellular elec-436 tron transport processes and/or telluric currents (see Section 3.4).

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So we might assume that the fracking related induced earthquakes might also be 438 subject to the same mechanisms of biogenic earthquake production. It should be noted  As volcanic activity seems to be related to seismicity, we might also assume that 457 the hypothetical ultra-deep biosphere might play a role in these processes as well. This   One additional hypothesis we might conjure is that the ultra-deep biosphere (con-495 nected to earthquake activity, according to my initial hypothesis) might be partially re-496 leased closer to the surface (e.g. in the groundwater or even the atmosphere) during or 497 after an earthquake. These microorganisms potentially might be pathogenic on their own.

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But more importantly, they might modify the other microorganisms through horizon-  The following considerations are meant to reinforce the points made in Section 3.2 565 -in particular, explore the tools that ultra-deep biosphere members might use in order 566 to withstand the (hypothetically) extremely hostile environment of deep Earth's crust 567 and below. 568 We might assume that in order to better counteract the high pressures and tem-  (Leadbeater, 1990).  There is data that suggests that extreme conditions tend to suppress dormancy, provok-

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On the other hand, perhaps environmental heat might actually be utilized as an 648 energy source. Since the collectives of microorganisms might perform distributed elec-649 tron transport, forming long chains (see Section 3.4), we might assume that they can uti-650 lize the thermal gradients in the crust in order to drive their metabolic processes (and/or 651 the currents associated with them) -in effect, operating as a "biological thermocouple".

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I would also hypothesize that the ultra-deep subsurface might be rich in viruses.

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It seems that at least in the oceans the abundance of viruses is comparable to the abun-  Yet the current models of propagation of seismic signals imply that earthquakes 695 might happen much deeper than that -at the depths of hundreds of kilometers at least 696 (Frohlich, 1989). These are so-called deep focus earthquakes. So their existence seems 697 to be problematic to explain from the standpoint of the hypothesis considered in this pa-

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per. Yet I can see at least four possibilities that would still allow it to be viable:  The second option is self-evident. As we don't fully understand how even the ob-  The fourth option would imply that perhaps a reevaluation of models estimating 720 the depth of earthquake focus is needed. It is worth noting that some debate on this topic 721 has already been going on, indicating serious uncertainties (of about 100 km) in the es-722 timation of depth of certain earthquakes (Rees & Okal, 1987). In absence of real data  It has been recently shown that water microdroplets spontaneously lose electron,

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This might be the case, however we might also assume that these currents might    tentially it might mean that earthquake activity (according to the biogenic hypothesis) 898 might be an evolutionary adaptation mechanism for the deep crustal microorganisms.

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And it would seem that earthquake-related mechanical shocks might not be dis-900 ruptive for their operation, as microorganisms were shown to be able to thrive and re-901 produce even at extreme accelerations (up to 4×10 5 g), which seems to be facilitated  The first option might imply that the biogenic currents would slowly charge the 990 [metaphorical or actual] capacitor, which then for some reason discharges, releasing all 991 the accumulated energy and producing an earthquake. The second option was already 992 partly discussed in previous sections, so I won't consider it here in detail. The third op-993 tion would be discussed in the following paragraphs. Here I wish to note that what seems 994 to be a problem on this level might actually turn out to be a solution for some other ob-995 served peculiar effects. For example, if the large metabolic cycles of microorganisms in 996 ultra-deep biosphere are characterized by timescales of, say, 1-100 kyr, we arrive at the 997 intriguing possibility that perhaps it is this biological factor that might explain some other 998 processes occurring on the planet -e.g. the long climatic cycles. This might be appli-999 cable even to larger geological timescales -for example, it is assumed in some recent stud-