Formation of microtubules in volcanic glass from subsurface environments has been widely attributed to in situ activity of micro-organisms, but evidence directly linking those structures to biological processes remains lacking. Investigations into the alternative possibility of abiotic tubule formation have been limited. A laboratory experiment was conducted to examine whether moderate-temperature hydrothermal alteration of basaltic glass by seawater would produce structures similar to those ascribed to biological processes. Shards of glass were reacted with artificial seawater at 150°C for 48 days. Following reaction, the shards were uniformly covered with a brick-red alteration rind 10–30 μm thick composed primarily of phyllosilicates. Inspection of the margins of reacted shards with light microscopy did not reveal any tubule structures. However, the alteration products did include features containing micron-sized spheroidal structures that resemble granular alteration textures, which some investigators have attributed to biological activity. This result suggests that the granular textures may be at least partially abiotic, and that biological activity may make a smaller contribution to alteration of the oceanic crust than has been previously proposed. Also, while the experimental results do not exclude the possibility that tubules form abiotically, they do place limitations on the conditions under which this may occur.
Active deep-sea hydrothermal vents are hosted by a range of different rock types, including basalt, peridotite, and felsic rocks. The associated hydrothermal fluids exhibit substantial chemical variability, which is largely attributable to compositional differences among the underlying host rocks. Numerical models were used to evaluate the energetics of seven inorganic redox reactions (potential catabolisms of chemolithoautotrophs) and numerous biomolecule synthesis reactions (anabolism) in a representative sampling of these systems, where chemical gradients are established by mixing hydrothermal fluid with seawater. The wide ranging fluid compositions dictate demonstrable differences in Gibbs energies (ΔGr) of these catabolic and anabolic reactions in three peridotite-hosted, six basalt-hosted, one troctolite-basalt hybrid, and two felsic rock-hosted systems. In peridotite-hosted systems at low to moderate temperatures (<∼45 °C) and high seawater:hydrothermal fluid (SW:HF) mixing ratios (>10), hydrogen oxidation yields the most catabolic energy, but the oxidation of methane, ferrous iron, and sulfide can also be moderately exergonic. At higher temperatures, and consequent SW:HF mixing ratios <10, anaerobic processes dominate the energy landscape; sulfate reduction and methanogenesis are more exergonic than any of the aerobic respiration reactions. By comparison, in the basalt-hosted and felsic rock-hosted systems, sulfide oxidation was the predominant catabolic energy source at all temperatures (and SW:HF ratios) considered. The energetics of catabolism at the troctolite-basalt hybrid system were intermediate to these extremes. Reaction energetics for anabolism in chemolithoautotrophs—represented here by the synthesis of amino acids, nucleotides, fatty acids, saccharides, and amines—were generally most favorable at moderate temperatures (22–32 °C) and corresponding SW:HF mixing ratios (∼15). In peridotite-hosted and the troctolite-basalt hybrid systems, ΔGr for primary biomass synthesis yielded up to ∼900 J per g dry cell mass. The energetics of anabolism in basalt- and felsic rock-hosted systems were far less favorable. The results suggest that in peridotite-hosted (and troctolite-basalt hybrid) systems, compared with their basalt (and felsic rock) counterparts, microbial catabolic strategies—and consequently variations in microbial phylotypes—may be far more diverse and some biomass synthesis may yield energy rather than imposing a high energetic cost.
Thermodynamic modelling of organic synthesis has largely been focused on deep-sea hydrothermal systems. When seawater mixes with hydrothermal fluids, redox gradients are established that serve as potential energy sources for the formation of organic compounds and biomolecules from inorganic starting materials. This energetic drive, which varies substantially depending on the type of host rock, is present and available both for abiotic (outside the cell) and biotic (inside the cell) processes. Here, we review and interpret a library of theoretical studies that target organic synthesis energetics. The biogeochemical scenarios evaluated include those in present-day hydrothermal systems and in putative early Earth environments. It is consistently and repeatedly shown in these studies that the formation of relatively simple organic compounds and biomolecules can be energy-yielding (exergonic) at conditions that occur in hydrothermal systems. Expanding on our ability to calculate biomass synthesis energetics, we also present here a new approach for estimating the energetics of polymerization reactions, specifically those associated with polypeptide formation from the requisite amino acids.
The last two decades have seen a steep increase in scientific research focused on serpentinites. One of the key reasons for this growing interest has been the realization that molecular hydrogen (H2) and methane (CH4) produced during serpentinization can be utilized by many types of microorganisms to gain metabolic energy. By converting the energy into biomass, these organisms have the capacity to support entire biological communities based on chemical energy rather than photosynthesis. The prospect that these biological communities exist with little or no photosynthetic input suggests that they might be modern analogs of communities that existed on early Earth. It is also possible that such communities exist on other planetary bodies in our Solar System, such as Mars and Jupiter's moon Europa, whose surfaces appear too hostile to allow the presence of photosynthetic organisms. In recent years, these ideas have stimulated ongoing efforts by geomicrobiologists, astrobiologists, and others to characterize the microbial populations of serpentinites and to understand their relationship with the geochemical environment in which they live.
Here, we provide a brief overview of the geochemical context for serpentinite-hosted biological communities, and we summarize the results of recent investigations into the function and composition of those biological communities. We conclude with a discussion of the potential relationship between serpentinites and the origin and early development of life on Earth, and the possibility that serpentinite-hosted biological ecosystems might exist elsewhere in our Solar System.
The Eastern Lau Spreading Center (ELSC) is the southernmost part of the back-arc spreading axis in the Lau Basin, west of the Tonga trench and the active Tofua volcanic arc. Over its 397-km length it exhibits large and systematic changes in spreading rate, magmatic/tectonic processes, and proximity to the volcanic arc. In 2005, we collected 81 samples of vent water from six hydrothermal fields along the ELSC. The chemistry of these waters varies both within and between vent fields, in response to changes in substrate composition, temperature and pressure, pH, water/rock ratio, and input from magmatic gases and subducted sediment. Hot-spring temperatures range from 229° to 363 °C at the five northernmost fields, with a general decrease to the south that is reversed at the Mariner field. The southernmost field, Vai Lili, emitted water at up to 334 °C in 1989 but had a maximum venting temperature of only 121 °C in 2005, due to waning activity and admixture of bottom seawater into the subseafloor plumbing system. Chloride varies both within fields and from one field to another, from a low of 528 mmol/kg to a high of 656 mmol/kg, and may be enriched by phase separation and/or leaching of Cl from the rock. Concentrations of the soluble elements K, Rb, Cs, and B likewise increase southward as the volcanic substrate becomes more silica-rich, especially on the Valu Fa Ridge. Iodine and δ7Li increase southward, and δ11B decreases as B increases, apparently in response to increased input from subducted sediment as the arc is approached. Species that decrease southward as temperature falls are Si, H2S, Li, Na/Cl, Fe, Mn, and 87Sr/86Sr, whereas pH, alkalinity, Ca, and Sr increase. Oxygen isotopes indicate a higher water/rock ratio in the three systems on Valu Fa Ridge, consistent with higher porosity in more felsic volcanic rocks. Vent waters at the Mariner vent field on the Valu Fa Ridge are significantly hotter, more acid and metal-rich, less saline, and richer in dissolved gases and other volatiles, including H2S, CO2, and F, than the other vent fields, consistent with input of magmatic gases. The large variations in geologic and geophysical parameters produced by back-arc spreading along the ELSC, which exceed those along mid-ocean ridge spreading axes, produce similar large variations in the composition of vent waters, and thus provide new insights into the processes that control the chemistry of submarine hot springs.
Award Dates: September 1, 2013 — February 28, 2015
A laboratory experimental study was initiated to examine whether microtubules might form during low-temperature oxidative alteration of basalt by abiotic processes. Microtubules have been found in basaltic glass in core samples recovered the subseafloor, and these have been widely interpreted as products of biological activity. However, clear evidence for a biological origin of these microtubules has never been reported, and this project was initiated to investigate whether there are non-biological alternatives to the formation of the microtubules. To address this issue, several laboratory experiments were performed heating basalt glass with artificial seawater for periods of up to two months at 150°C. Thus far, however, the results have been inconclusive. Although no tubule formation was observed in the initial set of experiments, the extent of reaction in these experiments was very limited owing to closed-system conditions employed during reaction. Therefore, a new flow-through system that would more effectively simulate the open-system conditions of the natural system was constructed, and resulted in more extensive alteration of the basalt. Analysis of the products for the presence of microtubules is still underway.