Introduction
Oxyanions of chlorine have been detected on the surface of Mars (Hecht et al. Reference Hecht2009; Kounaves et al. Reference Kounaves2010; Ming et al. Reference Ming2014; Clark & Kounaves Reference Clark and Kounaves2015). The Phoenix lander at Mars’ polar region measured levels of ~0.4–0.6% in regolith that represent chlorate and perchlorate salts and perhaps other oxychlorines. (Per)Chlorate also has been detected at the Curiosity landing site and in Moon and meteorite samples (Glavin et al. Reference Glavin2013; Jackson et al. Reference Jackson, Davila, Sears, Coates, McKay, Brundrett, Estrada and Böhlke2015b). Common cations include calcium, iron, magnesium and sodium (Kounaves et al. Reference Kounaves, Carrier, O'Neil, Stroble and Claire2014; Ming et al. Reference Ming2014). While their planetary distribution is uneven, chloride and (per)chlorate salts are widespread (Clark & Kounaves Reference Clark and Kounaves2015). The (per)chlorate salts are highly hygroscopic and therefore have relevance to astrobiology studies of Mars. There is reason to believe that enough humidity exists in the atmosphere of Mars to create deliquescent brines through absorption of water by (per)chlorate salts (Chevrier et al. Reference Chevrier, Hanley and Altheide2009; Martín-Torres et al. Reference Martín-Torres2015; Ojha et al. Reference Ojha, Wilhelm, Murchie, McEwen, Wray, Hanley, Massé and Chojnacki2015). Equally as important is the fact that brines of (per)chlorate salts have very low eutectic temperatures, some remaining liquid even to below –70°C (Möhlmann & Thomsen Reference Möhlmann and Thomsen2011; Clark & Kounaves Reference Clark and Kounaves2015).
The intense aridity and extremely low temperatures at the Martian surface create environmental conditions particularly challenging to organisms. It has been suggested that (per)chlorate salts may be important sources of liquid water in the Martian near surface (Zorzano et al. Reference Zorzano, Mateo-Martí, Prieto-Ballesteros, Osuna and Renno2009; Davila et al. Reference Davila2010; Nuding et al. Reference Nuding2014). Spectral evidence is consistent with the idea that recurring slope lineae result from brines of Mg perchlorate, Mg chlorate and Na perchlorate (Ojha et al. Reference Ojha, Wilhelm, Murchie, McEwen, Wray, Hanley, Massé and Chojnacki2015). Strong oxidants, such as oxychlorines, are typically incompatible with living systems. However, due to mechanistic kinetic barriers, (per)chlorate salts are quite stable in aqueous environments and are not easily reduced (Urbansky Reference Urbansky1998; Gu et al. Reference Gu, Dong, Brown and Cole2003).
Perchlorate salts on the Earth tend to result from industrial processes that produce explosives, herbicides, lubricants, paints and paper (Motzer Reference Motzer2001; Urbansky Reference Urbansky2002). These salts can contaminate drinking water and be toxic to humans, through competition with iodine in the thyroid (Hooth et al. Reference Hooth, DeAngelo, George, Gaillard, Travlos, Boorman and Wolf2001; Kalkhoff et al. Reference Kalkhoff, Stetson, Lund, Wanty and Linder2010; EPA 2011). Natural perchlorate salts have been detected in the very arid regions of the Atacama Desert and Antarctica (Kounaves et al. Reference Kounaves2010; Jackson et al. Reference Jackson2015a). This includes Chilean nitrate deposits, which have levels of perchlorate approaching 0.6% (Ericksen Reference Ericksen1981). Trace levels of perchlorate salts have been detected in a variety of other aqueous and soil environments (Smith et al. Reference Smith, Yu, McMurry and Anderson2004; Rajagopalan et al. Reference Rajagopalan, Anderson, Fahlquist, Rainwater, Ridley and Jackson2006, Reference Rajagopalan, Anderson, Cox, Harvey, Cheng and Jackson2009; Rao et al. Reference Rao, Anderson, Orris, Rainwater, Rajagopalan, Sandvig, Scanlon, Stonestrom, Walvoord and Jackson2007). Given the rarity of these salts worldwide, it might be expected that growth tolerances to high concentrations of (per)chlorate would be rare among microorganisms.
An early study by Durand (Reference Durand1938) found appreciable microbial growth tolerance to Na perchlorate. Bacterium coli (Escherichia coli) grew in the presence of 2.5% (~0.25 M) Na perchlorate and Staphylococcus pyogenes aureus (S. aureus), showed slow growth in the presence of 7.5% (~0.75 M), but no growth at 10% (~1.0 M), Na perchlorate. Sterigmatocystis nigra (Aspergillus niger) exhibited strong mycelial growth at 1% (~0.1 M) Na perchlorate and approximately 25% as much growth at 4% (~0.4 M) Na perchlorate. A recent preliminary report suggests that the haloarchaeon Haloarcula argentinensis can grow in the presence of 0.5 M (~5%) perchlorate, even in medium containing 15% (~3.0 M) NaCl (Thombre et al. Reference Thombre, Oke, Dhar and Shouche2015). A study of halotolerant Haloarcula, Haloferax and Halomonas demonstrated strong growth in the presence of 0.4 M Na perchlorate, with weak growth of Haloferax at 0.6 M (Oren et al. Reference Oren, Bardavid and Mana2014). Two other recent studies examine another clade of Archaea. Methanogenesis was demonstrated to proceed in the presence of 1% perchlorate, but no higher, for strains of Methanobacterium, Methanosarcina and Methanothermobacter (Kral et al. Reference Kral, Goodhart, Harpool, Hearnsberger, McCracken and McSpadden2016). However, when adapted to higher concentrations of perchlorate salts, these methanogens appeared to metabolize despite the presence of up to 5% (but not 10%) perchlorate. A similar study with Methanobacterium and Methanosarcina isolates from permafrost observed methanogenesis only at very much lower concentrations of Mg and Na perchlorates, 0.1% (~10 mM) or less (Shcherbakova et al. Reference Shcherbakova, Oshurkova and Yoshimura2015). Another previous study with a facultative anaerobic consortium did not detect growth at 0.4% perchlorate (Bardiya & Bae Reference Bardiya and Bae2005).
There is a substantial body of literature on microbes that can use perchlorate as a terminal electron acceptor for anaerobic respiration (Wallace et al. Reference Wallace, Ward, Breen and Attaway1996, Reference Wallace, Beshear, Williams, Hospadar and Owens1998; Coates et al. Reference Coates, Michaelidou, Bruce, O'Connor, Crespi and Achenbach1999; Herman & Frankenberger Reference Herman and Frankenberger1999; Okeke et al. Reference Okeke, Giblin and Frankenberger2002; Coates & Achenbach Reference Coates and Achenbach2004; Shete et al. Reference Shete, Mukhopadhyaya, Acharya, Aich, Joshi and Gholem2008). Typically perchlorate is added to media at concentrations below 1 mM. The biological reduction of perchlorate is performed by perchlorate reductase, producing a chlorite anion, which is toxic to cells if not further reduced to chloride. It has been suggested that microorganisms with nitrate reductase can convert perchlorate into chlorite, but that this is then cytotoxic. Certainly (per)chlorate respiration has clear astrobiology relevance given that (per)chlorate brines may be present on Mars. However, perchlorate respirers would still need to be tolerant to very high concentrations of (per)chlorate salts.
Microbes exhibiting high tolerance to (per)chlorate anion also need to be tolerant of high concentrations of the accompanying cations. We have examined (per)chlorate growth tolerance in bacteria isolated from hypersaline environments, including terrestrial hyperhaline soils rich in NaCl from the Great Salt Plains of Oklahoma and epsomite lake margins and sediments rich in MgSO4 from Hot Lake in Washington. Our study has relevance to potential forward contamination of (per)chlorate brines on Mars by spacecraft carrying soil particles or the salinotolerant bacteria commonly found in spacecraft assembly facilities (SAF).
Materials and methods
Organisms
Bacterial isolate collections previously generated from hypersaline environments were used to measure growth tolerances. We previously characterized 93 halotolerant aerobic heterotrophic bacteria isolated from the Great Salt Plains of Oklahoma, a wet terrestrial environment saturated with NaCl (Caton et al. Reference Caton, Witte, Ngyuen, Buchheim, Buchheim and Schneegurt2004). Similar work at Hot Lake, WA, an environment saturated with MgSO4 (epsomite), yielded a collection of 64 bacterial isolates from sediments and lake margin samples (Kilmer et al. Reference Kilmer, Eberl, Cunderla, Chen, Clark and Schneegurt2014). A group of 18 isolates from these collections was chosen for further study based on salinotolerance and taxonomy (Table 1).
Table 1. Salinotolerant bacteria used for this study from Hot Lake (Kilmer et al. Reference Kilmer, Eberl, Cunderla, Chen, Clark and Schneegurt 2014 ) and the Great Salt Plains (Caton et al. Reference Caton, Witte, Ngyuen, Buchheim, Buchheim and Schneegurt 2004 )
Media and growth measurements
Bacterial cultures were grown on Salt Plains (SP) medium supplemented with various (per)chlorate salts (Caton et al. Reference Caton, Witte, Ngyuen, Buchheim, Buchheim and Schneegurt2004). Shake tubes (2 ml in 13 × 100 mm tubes) were lightly loop-inoculated in triplicate (below 0.05 OD units) from agar slants and maintained at room temperature on a rotary shaker (150 rpm, 1-in stroke dia). Culture density was determined as A 600 by spectrophotometry (ThermoFisher Genesys 10S) using a medium blank at the time of inoculation and at 2 days intervals for 12 days after inoculation. Error bars of SD were generally smaller than the symbols used for plotting growth curves.
The water activity of each medium (Table 2) was measured using an AqualLab Series 3 water activity meter (Decagon Devices, Inc., Pullman, WA). The instrument was calibrated with standard NaCl solutions and run at room temperature.
Table 2. Water activities of (per)chlorate salt solutions
(Per)chlorate measurements
Chlorate and perchlorate concentrations were measured by ion chromatography as previously described (Carlström et al. Reference Carlström, Loutey, Wang, Engelbrektson, Clark, Lucas, Somasekhar and Coates2015). Briefly, a mobile phase of 36 mM NaOH was used with a Dionex Ion Pac AS 25 column (4 × 250 mm) in a Dionex ICS 500 instrument in recycle mode with a Dionex ASRS 300 (4 mm) and suppressor control at 90 mA. Samples of culture media were clarified by filtration (0.22 µm) before dilution and analysis.
Results
Growth tolerance to perchlorate salts
Growth tolerances to high concentrations of (per)chlorate salts were determined for 18 salinotolerant bacterial isolates. Tests were performed at several concentrations of Mg, K and Na perchlorate salts and with K and Na chlorate salts. Representative growth curves are presented for HL12, a Halomonas venusta isolate from Hot Lake, which was particularly tolerant to (per)chlorate exposure (Figs. 1–3).
Fig. 1. Growth of HL12 in SP medium supplemented with Mg perchlorate. Bacterial growth in shake-tube cultures was measured by turbidity and is presented in OD units. SD of some triplicate cultures were smaller than the point markers. Stars, 0.05 M Mg perchlorate; triangles, 0.25 M Mg perchlorate; squares, 0.5 M Mg perchlorate.
Fig. 2. Growth of HL12 in SP medium supplemented with Na perchlorate. Bacterial growth in shake-tube cultures was measured by turbidity and is presented in OD units. SD of some triplicate cultures were smaller than the point markers. Stars, 0.1 M Na perchlorate; triangles, 0.5 M Na perchlorate; squares, 1.0 M Na perchlorate.
Fig. 3. Growth of HL12 in SP medium supplemented with K perchlorate. Bacterial growth in shake-tube cultures was measured by turbidity and is presented in OD units. SD of triplicate cultures were smaller than the point markers. Stars, 0.1 M K perchlorate; triangles, 0.5 M K perchlorate; squares, 1.0 M K perchlorate.
Magnesium perchlorate is a likely Mars salt and is a major contributor to the ~0.6% (~0.06 M) level of (per)chlorate salts observed by Phoenix (Hecht et al. Reference Hecht2009). Growth of HL12 in the presence of 0.05 M (~1%) Mg perchlorate was rapid and robust (Fig. 1). Growth in the presence of 0.25 M (~5%) Mg perchlorate was slower and the cultures did not become as dense as those cultures grown with 0.05 M Mg perchlorate. The small amount of growth that HL12 may have exhibited in medium supplemented with 0.5 M Mg perchlorate was near the threshold that was chosen for faint growth (0.1 OD unit). All of the salinotolerant isolates grew robustly in the presence of 0.05 M Mg perchlorate, except HL 54 and 82 (Table 3). Half of the isolates exhibited positive growth in the presence 0.25 M Mg perchlorate. While some growth may have occurred at levels as high as 0.5 M (~10%) Mg perchlorate for certain isolates, it was limited and inconsistent.
Table 3. Maximum culture density (OD units) observed for salinotolerant bacterial isolates grown in the presence of perchlorate salts
Growth of HL12 was robust in the presence of 0.1 and 0.5 M Na perchlorate (Fig. 2). At 1.0 M Na perchlorate, growth was slower and the maximum density reached was <30% of that attained in the presence 0.5 M Na perchlorate. Only HL12 showed substantial growth at 1.0 M Na perchlorate. HL11, 12 and 55 grew robustly with 0.5 M Na perchlorate. Overall, 12 of 18 isolates showed growth in 0.5 M Na perchlorate. All showed robust growth at 0.1 M Na perchlorate, except for HL82.
HL12 showed robust growth in the presence of K perchlorate at ≤1.0 M (Fig. 3). The growth tolerance of HL12 to the K salt was substantially greater at 1.0 M than was growth tolerance to the corresponding Mg or Na salts. The salinotolerant isolates overall performed better in the presence of K perchlorate than with Mg or Na perchlorate (Table 3). More than half of the isolates grew well in the presence of 1.0 M K perchlorate, while growth in Mg and Na salts at 1 M perchlorate was barely detectable. All of the isolates grew well at 0.1 M K perchlorate and only HL20 and GSP10 did not grow well at 0.5 M K perchlorate. The solubilities of calcium and iron perchlorate salts in media were too low (<0.1 M) for meaningful growth tolerance experiments.
Growth tolerance to chlorate salts
Growth tolerances to chlorate salts were greater among the isolates than were growth tolerances to perchlorate salts (cf. Tables 3 and 4). All of the isolates grew robustly in the presence of 0.1 M Na chlorate, except HL 54 and 82 (Table 4). This trend continued up to 1.0 M Na chlorate, and in medium with 1.5 M Na chlorate, 13 of 18 isolates grew well. At 2.75 M (~25%) Na chlorate, HL 11 and 12 showed strong growth, while several other isolates exhibited weak growth. Note that none of the isolates grew strongly in 1 M perchlorate salts. Growth curves for HL12 at different concentrations of Na chlorate show that, for the highest concentrations tested, lag phases were longer, growth rates were slower, and maximum culture densities were lower (Fig. 4). However, growth was still substantial even at 2.75 M Na chlorate, the highest concentration at which media could be prepared without visible precipitate. In the presence of 1.0 M K chlorate, growth was strong for all isolates, except HL 80 and 82 (Table 4), with this being the highest concentration that could be added to culture medium without visible precipitate. HL12 grew robustly in the presence of 0.1 or 0.5 M K chlorate, but growth was slowed, although still strong, at 1.0 M K chlorate (Fig. 5). Tolerance to K chlorate was not consistently greater than to Na chlorate, in contrast to the corresponding perchlorate salts. We were unable to obtain Ca or Mg chlorate commercially to complete the iterative matrix of salts. The solubility of iron chlorate in media was too low (<0.1 M) for meaningful growth tolerance experiments.
Fig. 4. Growth of HL12 in SP medium supplemented with Na chlorate. Bacterial growth in shake-tube cultures was measured by turbidity and is presented in OD units. SD of some triplicate cultures were smaller than the point markers. Diamonds, 0.1 M Na chlorate; triangles, 0.5 M Na chlorate; X, 1.0 M Na chlorate; +, 1.5 M Na chlorate; stars, 2.0 M Na chlorate; circles, 2.5 M Na chlorate.
Fig. 5. Growth of HL12 in SP medium supplemented with K chlorate. Bacterial growth in shake-tube cultures was measured by turbidity and is presented in OD units. SD of triplicate cultures were smaller than the point markers. Stars, 0.1 M K chlorate; triangles, 0.5 M K chlorate; squares, 1.0 M K chlorate.
Table 4. Maximum culture density (OD units) observed for salinotolerant bacterial isolates grown in the presence of chlorate salts
(Per)chlorate retention by cultures
While it appears that these salinotolerant bacterial isolates can grow in the presence of high concentrations of (per)chlorate salts, it was possible that the microbes detoxify the anions, removing them before substantially growing. Chlorate and perchlorate levels were measured by ion-exchange chromatography before and after microbial cultivation. Overall there were only small differences between the (per)chlorate concentrations of media before and after cultivation. For example, in medium containing 0.25 M Mg perchlorate, there was less than a 2% difference between the initial concentration of perchlorate and the concentration of perchlorate after the growth of HL12 in batch culture. Similarly, for medium containing 0.5 M Na chlorate, there was less than a 2% change in chlorate concentration after batch culture of HL12. Tests with other cations, isolates and concentrations gave the same general outcome. In addition, no appreciable chlorate was present after bacterial cultivation in any medium initially containing perchlorate. Perchlorate respiration is an anaerobic process and it does not appear that (per)chlorate was metabolized in our aerobic cultures. (Per)Chlorate added to the medium remained at high concentrations throughout the development of the microbial culture.
Discussion
The ability to grow in the presence of 0.1 M (~1%) perchlorate appears to be widespread among bacteria from hypersaline environments. Nearly all of the salinotolerant bacterial isolates examined grew, not only in the presence 0.1 M perchlorate salts, but also at 0.5 M (~5%), and some grew at 1.0 M (~10%). Only Durand (Reference Durand1938) has demonstrated bacterial growth at such high perchlorate concentrations. Furthermore, we have demonstrated bacterial growth at concentrations of chlorate up to 2.75 M (~25%), where previous tolerance studies have not included chlorate salts.
Soils near the Phoenix lander appear to contain ~0.4–0.6% (per)chlorate salts. If this was evenly distributed in the soil and then incorporated into liquid brine, the bacteria in our study could easily tolerate that level of (per)chlorate. However, it is likely that (per)chlorate salts exist as distinct phases within the soil. These hygroscopic salts may form heavy brines through deliquescence. Hence, the effective concentrations of (per)chlorate salts in solution would be much higher than the 0.6% (per)chlorate observed in bulk soil. Strong arguments can be made that (per)chlorate brines are some of the most likely sources of liquid water on Mars (Zorzano et al. Reference Zorzano, Mateo-Martí, Prieto-Ballesteros, Osuna and Renno2009; Davila et al. Reference Davila2010). In our study, the highest perchlorate concentration that allowed bacterial growth was 10%. This is far lower than the eutectic concentrations of perchlorate salts. For instance, Na perchlorate has its eutectic point at 52 wt% at –37°C, while the eutectic point of Mg perchlorate is 44 wt% at –67°C (Chevrier et al. Reference Chevrier, Hanley and Altheide2009). It is possible that certain bacteria, perhaps those adapted to an environment high in perchlorate, are capable of growth at higher concentrations of perchlorate than observed in our study. Survival of viable cells likely occurs at higher (per)chlorate concentrations than growth. The great degree of chlorate tolerance we observed, with growth above 25% chlorate, is closer to what would be needed to grow in a eutectic solution of Na chlorate (39 wt% at –23°C) (Hanley et al. Reference Hanley, Chevrier, Berget and Adams2012). Note that the eutectic point for K chlorate is 3 wt% at –3°C and well within the growth tolerances observed for K chlorate among our isolates.
Tolerance to chlorate salts was greater than tolerance to perchlorate salts by a wide margin. Limited growth in perchlorate salts was not solely due to cation effects, since all of the isolates can grow at concentrations of NaCl and MgSO4 above 1.0 M. At 2.75 M Na chlorate, however, Na concentrations are reaching the Na tolerance limits of even these salinotolerant microbes. Of the isolates in our study, only HL 11, 20 and 54 have been shown to grow above 20% NaCl (Kilmer et al. Reference Kilmer, Eberl, Cunderla, Chen, Clark and Schneegurt2014). Inhibition of the growth of HL91 above 1.5 M (~15%) Na chlorate for example may have been due to cation effects, since this organism tolerates 10% (1.7 M) NaCl, but not 20% (3.4 M) NaCl. While growth of these isolates occurs in ≥50% (~2 M) MgSO4, growth in Mg perchlorate was inhibited at much lower Mg concentrations (Crisler et al. Reference Crisler, Newville, Chen, Clark and Schneegurt2012). Growth in relatively high concentrations of Mg chlorate may be possible, however, that salt could not be obtained commercially. Strong growth was observed with K perchlorate addition, best seen for isolates that were less tolerant overall. For instance, HL 54 and 82 were particularly sensitive to (per)chlorate salts, with HL82 unable to grow at even 0.1 M Na perchlorate. However, these isolates grew strongly in 1.0 M K perchlorate. Isolates most tolerant to perchlorate seemed to mirror those most tolerant to chlorate, suggesting a common mechanism for growth inhibition.
The observation of high (per)chlorate tolerance in salinotolerant bacteria demonstrates that terrestrial microbes are capable of growing at concentrations of (per)chlorate salts found in Mars soils. However, one of the most likely sources of bacteria that contaminate spacecraft is common oligosaline soil from around the SAF (Foster & Winans Reference Foster and Winans1975; Puleo et al. Reference Puelo, Fields, Bergstrom, Oxborrow, Stabekis and Koukol1977). Common soils appear to harbour bacteria capable of growing at relatively high concentrations of salts and this is a concern for forward planetary protection (Echigo et al. Reference Echigo, Hino, Fukushima, Mizuki, Kamekura and Usami2005; Chen et al. Reference Chen, Liu, Peng, Huang, He, Zhang, Li and Chen2010). Initial microbial abundance measurements by most probable number analysis of oligosaline turf soils found that 1.7 × 105 and 7.1 × 103 cells g−1 soil were tolerant to 10 and 20% NaCl, respectively (Kilmer et al. Reference Kilmer, Chambers, Akbar, Bhakta, Beck, Brimmerman, Lundin, DeVries, Kasten, Pringle-Johnson, Ruder and Schneegurt2010; Porazka et al. Reference Porazka, Kilmer and Schneegurt2011). Similarly, preliminary measurements of the abundance of bacteria tolerant to (per)chlorate salts indicate that it is not rare to find microbes that are tolerant to concentrations of (per)chlorate salts relevant to Mars (Crisler et al. Reference Crisler, Mai, Ahmad, Chen, Clark and Schneegurt2013a, Reference Crisler, Mai, Ahmad, Chen, Clark and Schneegurtb; Al Soudi et al. Reference Al Soudi, Farhat, Chen, Clark and Schneegurt2016). While our salinotolerant isolates are from rare extreme environments, some of the bacterial species isolated from SAFs match those from hypersaline environments (Caton et al. Reference Caton, Witte, Ngyuen, Buchheim, Buchheim and Schneegurt2004; Moissl et al. Reference Moissl, Bruckner and Venkateswaran2008; Stieglmeier et al. Reference Stieglmeier, Wirth, Kminek and Moissl-Eichinger2009). The cleanrooms that act as SAFs are dry environments (≤40% relative humidity) and it has been shown that salinotolerant microbes are present on surfaces and perhaps enriched by selection (Venkateswaran et al. Reference Venkateswaran, Satomi, Chung, Koukol, Basic and White2001, Reference Venkateswaran, Kempf, Chen, Satomi, Nicholson and Kern2003a, Reference Venkateswaran, Hattori, La Duc and Kernb; La Duc et al. Reference La Duc, Nicholson, Kern and Venkateswaran2003; Link et al. Reference Link, Sawyer, Venkateswaran and Nicholson2003; Kempf et al. Reference Kempf, Chen, Kern and Venkateswaran2005). It will be interesting to directly test microbial isolates from SAFs and neighbouring soils for (per)chlorate tolerance.
Acknowledgements
The authors are grateful for the contributions of Zonaira Ahmad, Todd Caton, James Crisler, Timothy Eberl, Joshua Fleming, Sascha Khan, Brian Kilmer, Tony Mai, Hieu Nguyen, Anastasiya Nosova and Noah Schneegurt. We thank Fadi Aramouni for determining water activities and Anna Engelbrektson and John Coates for measuring (per)chlorate concentrations. Preliminary accounts of this work have been presented previously and abstracted (Mai et al. Reference Mai, Nosova and Schneegurt2012; Crisler et al. Reference Crisler, Mai, Ahmad, Chen, Clark and Schneegurt2013a, Reference Crisler, Mai, Ahmad, Chen, Clark and Schneegurtb; Al Soudi et al. Reference Al Soudi, Farhat, Chen, Clark and Schneegurt2016). This work was supported by awards from NASA ROSES Planetary Protection Research (09-PPR09-0004 and 14-PPR14-2-0002) and Kansas INBRE IDeA NIGMS NIH (P20 GM103418).
