Microbial Mats in Antarctica as Models for the Search of Life on the Jovian Moon Europa

Abstract

The discovery of extreme environments with organisms adapted to these conditions has made them useful analogies for the presence of life beyond the Earth, whether autochthonous, or by the transport of microorganisms between different bodies of the solar system. This latter possibility has been known as the hypothesis of panspermia.

Appendices

Appendix A

A1. SIGNIFICANCE OF THE δ³⁴S PARAMETER

Sulfur is one of the key elements of life. Sulfur exists in four stable isotropic forms: ³²S (95.02%), ³³S (0.75%), ³⁴S (4.21%), and ³⁶S (0.02%). Sulfur isotope ratios are typically reported in the delta notation, as deviations with respect to the standard that is the troilite of the Cañon Diablo meteorite (CDT):

$$\delta^x S = \left[^{(x S/^{32} S)_{sample}}\left/_{(^x S/^{32} S)_{std}}\right. -1 \right] \times 10^{3} [0\left/00, CDM\right.]$$

(1)

where x = 33, 34 or 36. As pointed out (Kaplan, 1975; Chela-Flores, 2006), metabolic pathways of sulfur bacteria have enzymes that preferentially select the isotope ³²S over ³⁴S. This implies that where there is an abundance of sulfur bacteria, the value of the δ³⁴S parameter would be negative. Bacterial (dissimilatory) sulfate reduction (BSR) is a naturally occurring process. Under anaerobic conditions, sulfate is used by bacteria as an electron acceptor for oxidation of organic carbon (from pyruvate, lactate, formate, ethanol, methanol, amongst others), according to the following generalized reaction,

$$ 2<CH_2O> +SO_4^{2-} (aq) + 2H^+(aq) + 2CO_2(g) +H_2S(g) + 2H_2O$$

(2)

The above reaction is also called “sulfate respiration.” The product H₂S in the above reaction is highly enriched in ³²S. According to a model (Farquhar and Wing, 2003, Johnston et al., 2007) the parameter\${{\delta }^{\text{34}}}{{\text{S}}_{{\text{SO}}_4^{2 - }}} - {{\delta }}^{\text{34}}{{\text{S}}_{{{\text{H}}_{\text{2}}}{\text{S}}}}$,

$$ {\delta}^{34}{S}_{SO_{4}^{2-}}-{\delta }^{34}{S}_{H{_2}S}= \left[\frac{({^34}S/^{32}S)_{SO_{4}^{2-}}-\left({^34}S/^{32}S \right)_{H_{2}S}}{(^34S/^32S)_{std}}\right] $$

(3)

(The model has been referred to as the “the Rees–Farquhar model”.) The above parameter is initially negative, becomes less negative in the process of sulfate respiration. Indeed, by the metabolic activity of SRB, the quantity ( 34S/ 32S )SO42– ${{(^{34}}{\text{S}}{/^{32}}{\text{S}})_{{\text{SO}}_4^{2 - }}}$ becomes less negative, while ( 34S/ 32S )H2S ${{(^{34}}{\text{S}}{/^{32}}{\text{S}})_{{{\text{H}}_{\text{2}}}{\text{S}}}}$becomes more negative. This basically means that during the process of sulfate respiration the isotope ³² S increases in the product H₂S and is reduced in $_4^{2 - }$. Accelerated sulfate reduction by bacterial communities is known to occur in the presence of organic carbon (Fauville et al., 2004).

The relevance of the δ³⁴S parameter for biogenic sulfur on the icy surface of Europa has been reviewed (Chela-Flores, 2006; Chela-Flores and Kumar, 2008). Life requires an input of energy and must be able to control the flow of energy through redox chemistry, which is a universal concept. As life is based on organic chemistry, such chemistry must be allowed to operate. An extremophile must either live within the extreme environmental parameters, or guard against the outside world in order to maintain these conditions. For example, certain cold-tolerant cyanobacteria (Vincent, 2007) have a variety of strategies to minimize stresses of freeze–up. Like sea-ice microbiota, the mat-forming species in the McMurdo Ice shelf form copious quantities of exopolymeric substances already referred to as EPS in Section 2. This material shows the flow of liquid water during freeze-up and thaw, and may also force crystal formation to occur well away from the cells. Experiments indicate that EPS is critical to surviving desiccation, as well as freeze-up (Tamaru et al., 2005). Also EPS are the source of organic carbon. We now examine the influence on temperature and radiation on biosignatures suitable for Europa and suggest a comparison with their counterparts on Earth.

a2. TEMPERATURE

The magnitude of isotope fractionation by microbial sulfate reduction also depends upon temperature (Kaplan and Rittenberg, 1964; Canfield et al., 2006). The isotope fractionation factor (a) is (Ono, 2008):

$$ {}^x{\alpha_{{\text{so}}_4^{2 - } - {{\text{H}}_{\text{2}}}{\text{S}}}} = \frac{{{{[{}^x{\text{so}}_4^{2 - }]} \mathord{\left/{\vphantom {{[{}^x{\text{so}}_4^{2 - }]} {[{}^{32}{\text{so}}_4^{2 - }]}}} \right.} {[{}^{32}{\text{so}}_4^{2 - }]}}}}{{{{[{}^x{{\text{H}}_{\text{2}}}{\text{S}}]} \mathord{\left/{\vphantom {{[{}^x{{\text{H}}_{\text{2}}}{\text{S}}]} {[{}^{32}{{\text{H}}_{\text{2}}}{\text{S}}]}}} \right.} {[{}^{32}{{\text{H}}_{\text{2}}}{\text{S}}]}}}} $$

(4)

where x = 33, 34, and 36. The isotope fractionation factor at equilibrium can be derived from the ratios of the partition function (Q),

$$ {}^{34}{\alpha_{{\text{so}}_4^{2 - } - {{\text{H}}_{\text{2}}}{\text{S}}}} = \frac{{{}^{34}Q{\text{so}}_4^{2 - }}}{{{}^{32}Q{\text{so}}_4^{2 - }}}\frac{{{}^{32}Q{{\text{H}}_{\text{2}}}{\text{s}}}}{{{}^{34}Q{{\text{H}}_{\text{2}}}{\text{s}}}} $$

(5)

The partition function is

$$ Q = \prod\limits_i {{u_i}} \exp \left[ {{\raise0.7ex\hbox{${ - {u_i}}$} \!\mathord{\left/{\vphantom {{ - {u_i}} {2]}}}\right.}\!\lower0.7ex\hbox{${2]}$}}} \right]\left[ {1 - \exp ( - {u_i})} \right] $$

(6)

where ui = ${{h{\upsilon_i}} \mathord{\left/{\vphantom {{h{\upsilon_i}} {kT}}} \right.} {kT}}$ (h is the Planck constant, k is the Boltzmann constant, T is the temperature in kelvin and ${\upsilon_i}$ is the ith vibrational frequency of the molecule). A plot (cf., Fig. 2) of 1,000 × ln ${}^{34}{\alpha_{{\text{so}}_4^{2 - } - {{\text{H}}_{\text{2}}}{\text{S}}}}$ as a function of temperature shows that as temperature decreases, the value of 1,000 × ln ${}^{34}{\alpha_{{\text{so}}_4^{2 - } - {{\text{H}}_{\text{2}}}{\text{S}}}}$ increases, which is a signature of biological process.

Figure 2.

A plot of 1,000 × ln ${}^{34}{\alpha_{{\text{so}}_{\text{4}}^{{\text{2 - }}}{\text{ - }}{{\text{H}}_{\text{2}}}{\text{S}}}}$ as a function of temperature shows that as temperature decreases the value of 1,000 × ln ${}^{34}{\alpha_{{\text{so}}_{\text{4}}^{{\text{2 - }}} - {{\text{H}}_{\text{2}}}{\text{S}}}}$ increases, which is a signature of biological process. The temperature on the surface of Europa is about 110 K. This corresponds to 1,000 × ln ${}^{34}{\alpha_{{\text{so}}_4^{2 - } - {{\text{H}}_{\text{2}}}{\text{S}}}}$∼15 while the temperature in the sea water below in contact with the ice is 270 K which corresponds to 1,000 × $\times$ln ${}^{34}{\alpha_{{\text{so}}_4^{2 - } - {{\text{H}}_{\text{2}}}{\text{S}}}}$ ∼4.5.

Below the ice, the evidence gathered by the Galileo mission suggests that there is an ocean of liquid water, which could in principle harvest sulfur bacteria. The temperature of such an underlying sea just in contact with the ice is estimated to be near 270 K. This temperature is appropriate for BSR as it is evident from Fig. 2. A surface temperature on the surface of Europa of about 110 K corresponds to 1,000 × ln ${}^{34}{\alpha_{{\text{so}}_4^{2 - } - {{\text{H}}_{\text{2}}}{\text{S}}}}$ ∼ 15, while the temperature in the sea water below in contact with the ice of temperature of the order of 270 K, corresponds to 1,000 × ln${}^{34}{\alpha_{{\text{so}}_4^{2 - } - {{\text{H}}_{\text{2}}}{\text{S}}}}$ ∼ 4.5. In comparison, the average temperature of the dry valley Antarctic lakes is in the range 273–280 K, which means that 1,000 × ln ${}^{34}{\alpha_{{\text{so}}_4^{2 - } - {{\text{H}}_{\text{2}}}{\text{S}}}}$ ∼ 3. The algal mats in these lakes are known to exist about 4 m below the surface of the frozen lakes and are also capable of lifting off, floating, and freezing in ice. These mats selectively remove a huge quantity (40–104 kg) of sulfur annually (discussed in Section 3.1). It may be speculated that similar algal mats, if they exist beneath the icy surface of Europa, may eventually be transported to the surface by surficial ablation from above (produced by the micrometeoroids and refreezing form below the ice layer, thus contributing to the surficial sulfur patches. It is interesting that Europa offers a wide temperature range suitable for a wide variety of microorganisms to exist. Moreover, low temperature favors enhanced biological activity of sulfate respiration. This would be reflected in the extremely high values of 1,000$\times$ln ${}^{34}{\alpha_{{\text{so}}_4^{2 - } - {{\text{H}}_{\text{2}}}{\text{S}}}}$, or by highly negative values of ${\delta^{34}}{{\text{S}}_{{\text{SO}}_4^{2 - }}} - {\delta^{34}}{{\text{S}}_{{{\text{H}}_{\text{2}}}{\text{S}}}}$. These effects are subject to measurement by miniature mass spectrometer in future missions (Blanc and the LAPLACE consortium, 2008). The possibility of highly negative δ³⁴S value due to hydrothermal process is ruled out since extremely high temperatures (>550 K) is required for such a process. The possibility of bacterial life well below the sea underneath with temperatures exceeding 350 K cannot be ruled out since we know examples of sulfur-dependent extremophiles such as Sulfolobus acidocaldarius an archaea that flourishes at pH 3 and > 350 K in Yellowstone National Park (USA) (Rothschild and Mancinelli, 2001).

There are thermophiles among the bacteria (Bacillus, Clostridium, Thiobacillus, Desulfotomaculum), the archaea (Pyrococcus, Thermococcus, Thermoplasma, Sulfolobus), and the methanogens that exist in the temperature range 330–390 K. In contrast, the upper limit for eukaryotes is 330 K, a temperature suitable for some protozoa, algae, and fungi.

A3. SULFUR ION IMPLANTATION ON THE SURFACE OF EUROPA

On Europa, however, there is another source of sulfur, namely, the energetic sulfur ions coming from the nearby Jovian atmosphere. These energetic ions after striking the surface of Europa will penetrate a certain depth into the icy surface. If we assume that there are sulfur bacteria on the surface and below the icy crust of Europa and the initial value of the δ³⁴S parameter is negative then this mechanism of ion implantation will change the value of the δ³⁴S parameter and make it less negative or even positive. A probe on the surface of Europa that tries to measure the δ³⁴S parameter would then wrongly detect the absence of sulfur bacteria. Consequently, it is important that the probe goes well beyond the maximum penetration depth of the ions to measure the δ³⁴S parameter. For this it is essential to know the density distribution of the implanted ions as a function of depth as well as time of implantation. Based on the LSS (Lindhard, Scharff and Schiøt) theory of ion implantation, the implant profile in an amorphous material can be described by the equation (Sze, 1988):

$$ n(x) = {n_{\text{o}}}\exp \left( { - \frac{{{{(x - R{\text{p}})}^2}}}{{2\Delta R_{\text{p}}}}} \right) $$

(7)

where,${n_{\text{o}}}=\frac{\phi}{t}{\sqrt {2 \pi } \Delta R{\text{p}}},\phi$ f is the implanted dose, t is the time of implantation, R _p is the projection range and is equal to the average distance an ion travels before it stops and ΔR _p is the standard deviation of R _p which is roughly 1/5R _p from the known data for different ions and impact surface. The value of R _p for sulfur ion for the Europan surface is 4.8 × 10⁻⁵cm and φ = 9.0× 10⁶ (cm² s)⁻¹ (Cooper et al., 2001) (cf., Fig. 1). It is therefore clear from Fig. 1 that the implanted sulfur is heavily distributed around the maximum depth R _p. This implies that a penetrator has to go beyond R _p to measure biogenic sulfur without any contamination from implanted sulfur.