Astronomical Science

Astronomy Meets Biology: EFOSC2 and the Chirality of Life

Michael Sterzik1 molecule are called enantiomers, and the extreme environment on Earth, and the Stefano Bagnulo 2 two forms are generally referred to as microbial colonisation of subsurface lay­ Armando Azua 3 right-handed and left-handed, or dextro­ ers in halites (rock salt) and quartz rocks Fabiola Salinas 4 rotatory and levorotatory. by specific cyanobacteria. In the most Jorge Alfaro 4 hostile environments (exceptional aridity, Rafael Vicuna 3 The term homochirality is used when a salinity, and extreme temperatures), a molecule (or a crystal) may potentially primitive type of cyanobateria, Chroococ- exist in both forms, but only one is actu­ cidiopsis, can be the sole surviving 1 ESO ally present. Homochirality character- organism. This has interesting implications 2 Armagh Observatory, United Kingdom ises life as we know it: all living material for the potential habitability, and eventual 3 Department of Molecular Genetics and on Earth contains and synthesises ­sugars terraforming, of certain areas on Mars Microbiology, Pontificia Universidad and nucleic acids exclusively in their (Friedmann & Ocampo-Friedmann, 1995). Católica de Chile, Chile right-handed form, while amino acids and 4 Faculty of Physics, Pontificia Universi­ proteins occur only in their left-handed The idea of using the ESO Faint Object dad Católica de Chile, Chile representation. However, in all these Spectrograph and Camera (EFOSC2) cases, both enantiomers are chemically to investigate samples of Chroococcidi- possible and energetically equal. The opsis extracted from the underside of Homochirality, i.e., the exclusive use of ­reasons for homochirality in living mate­ Atacama Desert quartzes and to measure L-amino acids and D-sugar in biologi- rial are unknown, but they must be their circular polarisation in a laboratory cal material, induces circular polarisa- related to the origin of life. It is still dis­ experiment arose at this conference. tion in the diffuse reflectance spectra of puted whether bioactive molecules (and The idea looked appealing, because only biotic material. Polarimetry may there- with them small enantiomeric excesses) limited, and sometimes contradictory, fore become an interesting remote were delivered to Earth (e.g., by mete­or­ reports about circular polarisation meas­ sensing technique in the future search ites) or whether (pre-)biotic chemistry urements of biotic material as a signature for extraterrestrial life. We have explored started on Earth (Bailey et al., 1998). of homochirality are available in the litera­ this technique and performed a labora- Undoubtedly, however, chemical and bio­ ture. Moreover, the successful use of tory experiment making an exotic use logical processes on Earth must have an astronomical instrument for the first of an astronomical instrument. During a favoured the selection of one-handed time for that purpose could serve as a period when EFOSC2 was detached biomolecules leading towards homochi­ benchmark for further applications of this from the Nasmyth focus to host a visitor rality. If similar evolutionary scenarios method in astrophysics. instrument at the NTT, we have ob- naturally occur elsewhere in the Universe, served various samples of biotic and homochirality may be a universal hallmark A detailed feasibility study as well as abiotic material and measured their of all forms of life. the production of significant quantities of ­linear and circular polarisation spectra. Chroococcidiopsis were prepared and Among the various targets, we have in- Chirality induces optical activity: each in­itiated in the following weeks. Since cluded samples of the hypolithic cyano- enantiomer rotates the reflected (or trans­ there is no formal process in place to ob­ bacteria species Chroococcidiopsis mitted) light in opposite directions, and tain “observing time” for laboratory i­solated from the Coastal Range of the homochirality guarantees that there will experiments, the ESO Director General Atacama Desert. To our knowledge, be an excess of circularly polarised light was asked for authorisation. He approved these are the first and highest preci- in one direction. This opens up the inter­ the experiment under the condition that sion measurements of circular polarisa- esting possibility that biosignatures could it did not pose any risk to the instrument. tion using living material and obtained be sensed remotely by means of polari­ This could be achieved by using EFOSC2 with an astronomical instrument. metric techniques. when it was not attached at its nominal Nasmyth focal station (i.e. during an ex­­ This chain of arguments, which had been tended visitor instrument run), but keep­ Motivation raised in various articles in the scientific ing it in a horizontal position (which avoids literature (e.g., Wolstencroft et al., 2004), the possibility of any material falling on The building blocks of life are chiral. Their was also discussed during the workshop the entrance window). Our “laboratory” molecular structure lacks an internal Astrobio 20101 held in Santiago de Chile is shown in Figure 1, left. EFOSC2 is plane of symmetry, and their mirror image last January. For the first time, an inter­ de­tached from the New Technology Tele­ cannot be superimposed on their origi- national and interdisciplinary conference scope (NTT). A microphotograph of nal image. The term chirality is specifi­ that aimed to cover major topics in astro­ our main target, Chroococcidiopsis, is cally used when a molecule (or an object) biology was organised and hosted in displayed on the right. exists in both mirror-symmetric configu­ Chile. The topics covered included the rations. The human hands are the classic origins of life, the chemistry of the Uni­ example that illustrates the concept of verse, extrasolar planetary systems, and Experiment chirality, and the term chiral itself comes the search for life in the Solar System. from the Greek word for hand, χειρ. A prominent topic of discussion was the During one week in June 2010, three In chemistry, the two images of a chiral Atacama Desert as an example of an of us (Pontificia Universidad Católica

The Messenger 142 – December 2010 25 Astronomical Science Sterzik M. et al., Astronomy Meets Biology: EFOSC2 and the Chirality of Life

Figure 1. Left: The EFOSC2 instru- ment is shown detached from the NTT. The instrument attached to the Nasmyth focus (on the left) is ULTRA­ CAM. Right: A microphotograph of the cyanobacteria Chroococcidiopsis, enlarged 100 times.

student Fabiola Salinas, Stefano Bagnulo to measure the polarisation level with an In the Philodendron leaf we detected and Michael Sterzik) were busy using error bar of the order of 10–4 per spectral both broad polarised features (with an EFOSC2 to observe samples of minerals bin. Measurements of the cyanobacte- amplitude of ~ 0.5%), and, superposed and paints (quartz, salt, ­sugar, white ria lasted several hours, but for a total on them, a narrow feature at about ­flat-field screen), leaves (Philodendron, integration time of just about 10 minutes. 680 nm (with an amplitude of ~ 0.05% Ficus, Schefflera) and cyano­bacteria films This exposure allowed us to reach an over the continuum), both well above deposited on filter paper. All samples error bar of 10–6 per spectral bin. These the statistical noise (the green line shows were prepared as thin sheets inserted in figures refer only to the statistical errors the null profile). The behaviour of Stokes the flat-field screen position. An inte­ due to Poisson noise. With our ultra-high V in the continuum closely follows the grating sphere (the usual calibration lamp signal-to-noise ratio measurements, reflectivity, shown with a solid black line mechanism for EFOSC2 when attached we have certainly hit the limits imposed (this is known as the Cotton effect). to the 3.6-metre telescope) was used by the polarimetric optics and the ex­ This behaviour closely resembles the dif­ for diffuse illumination. EFOSC2 covers perimental conditions — we will come fuse reflectance circular dichroism large spectral regions in the spectropola­ back later to this important point. Here we spectra of leaves as seen by Wolstencroft rimetry mode: we used mostly grism note that the reliability of the error bars et al. (2004). The narrow feature around 13 to cover the range 370–930 nm, with (in terms of statistical error) has been vali­ 680 nm is very similar to the results a spectral resolution of ~ 2.3 nm (we dated with the use of so-called null- shown by Gregory & Raps (1974) in their adopted a 1-arcsecond slit width). Both profiles (i.e. the difference between Stokes transmission spectroscopy of chloro­ linear and circular polarisation measure­ profiles obtained from different pairs of plasts and is related to the chlorophyll-a ments of the samples were obtained, exposures). Null profiles were found scat­ pigment response. using the λ/2 and λ/4 retarder wave­ tered around zero within the error bars. plates, respectively (we note that the λ/4 The interpretation of the results for retarder waveplate is a recent addition to Chroococcidiopsis is the most interesting. the instrument, see Saviane et al. [2007]). Results and outlook In the continuum, Stokes V shows a behaviour similar to that observed for the Polarimetric measurements were taken Figures 2 and 3 show the results white screen field, and hence is likely by combining several pairs of expo- ob­tained from our circular polarisation to be of instrumental origin. A number of sures obtained with different position measurements (Stokes V normalised narrow, low amplitude features appear angles of the retarder waveplates (–45º, to the intensity) of a Philodendron leaf, superposed on the continuum. In order to 45º, 135º, 225º for circular polarisation, and a film of Chroococcidiopsis depos­ prove that these features are real (and and 0º, 22.5º, 45º, ... 335.5º for linear ited on filter paper, respectively. We not spurious signals, e.g., caused by the polarisation measurements, measured also obtained calibration measurements CCD readout noise) we also obtained from the principal plane of the Wollaston of a white screen flat-field, produced ultra-high signal-to-noise ratio measure­ prism). This “beam swapping” technique by a barium sulphate based white reflec­ ments with the retarder waveplate at 0º, minimises the instrumental effects — tance coating. The circular polarisation 90º, 180º and 270º. With these retarder for a detailed description of the measur­ measured for the screen flat-field was waveplate settings, we would theoreti­ ing strategies, see, e.g., Bagnulo et found as a negative continuum of about cally expect a null signal. The resulting al. (2009). For inorganic samples and for –0.05%, with the absolute value slowly profile, shown with the red line in Fig­ leaves, the typical observation cycles increasing towards shorter wavelengths. ure 3, and arbitrarily rectified to zero for lasted 20 minutes, dominated in practice No narrow features or signals are pre- display purposes, shows no high fre­ by overheads (readout time and retarder sent in the polarisation spectra of this ref­ quency signatures. This curve, together waveplate setting), and allowed us erence source. with a consistently flat null profile (shown

26 The Messenger 142 – December 2010     Figure 2. The Philoden- dron leaf is shown at left  and its polarised spec­ 

   trum measured with 5 - l  EFOSC2 at middle and right. The blue lines

l   show the circular polari­ sation (V/I). In the right­ most panel, V/I is recti­ l 

( fied to zero. The green l  5 lines show the null pro­ l  file (offset by –0.06% in the rightmost panel, l  l  for display purposes)        6@UDKDMFSGML 6@UDKDMFSGML demonstrating the sta­ tistical errors. The black line is the reflectivity (in arbtirary units).

Figure 3. The sample of     Chroococcidiopsis is shown at left and its   5 polarised spectrum in -   the middle and right   panels. The blue line  shows circular polarisa­ tion and the green line  is the null profile (offset l  ( by + 0.02%). The red 5 line shows the spectrum l  l  obtained with the re- tarder waveplate at      position angles offset by 6@UDKDMFSGML 6@UDKDMFSGML 45º, rectified to zero. In the rightmost panel, V/I is rectified to zero. with the green line offset to + 0.02% in the FORS instrument of the VLT. Some bacteria on other Solar System bodies in the right panel) suggests that the narrow of these effects may also play a role the future with extremely large telescopes features are indeed real. We notice in EFOSC2. We must also remark that to be assessed. again the prominent chlorophyll-a feature the prepared samples are not fully repro­ around 680 nm, but may also infer sig­ ducible in their configuration, and thus We thank all the La Silla staff who sup­ natures of other pigments like carotinoids likely introduce some effects due to their ported the setup and execution of this and phycocyanins. Our measurements different backscattering properties. unusual experiment! Stefano Bagnulo also appear consistent with polarisation acknowledges a grant received as visiting spectra obtained from a different type of In conclusion, the polarised spectra of scientist at ESO/Chile. cyanobacteria by Sparks et al. (2009). Figures 2 and 3 show that the polari­ metric techniques employed by us are While we kept the noise confidently at sensitive to the presence of biotic mate­ References a very low level, the polarimetric mode of rial. We detect chiral signatures of pig­ Bagnulo, S. et al. 2009, PASP, 121, 993 EFOSC2 is lacking the full characterisa­ ments involved in biotic photosynthetic Bailey, J. et al. 1998, Science, 281, 5377, 672 tion needed to reach ultra-high and abso­ reaction chains. Signatures of chlorophyll Bailey, J., Lucas, P. W. & Hough, J. H. 2010, lute precision polarimetric measure- pigments can be captured both in sam­ MNRAS, 405, 2570 ments under the conditions of our experi­ ples of leaves and cyanobacteria of spe­ Gregory, R. P. F. & Raps, S. 1974, Biochem. J., 142, 193 ment. Using special calibration tech­ cies Chroococcidiopsis. More accurate Friedmann, E. I. & Ocampo-Friedmann, R. 1995, niques, in some astronomical observa­ measurements of the polarised signal Adv. Space Res., 15(3), 243 tions it is already possible to achieve a amplitudes require experiments to be Patat, F. & Romaniello, M. 2006, PASP, 118, 146 precision of 10–6 (e.g., Bailey et al., 2010), carried out with samples prepared under Saviane, I. et al. 2007, The Messenger, 129, 14 Sparks, W. B. et al. 2009, PNAS, 106 (19), 7816 but in our experiments we are limited better controlled laboratory conditions. Wolstencroft, R. D., Tranter, G. E. & Le Peleven, D. D. by an instrument setup in which most of It will also be interesting to observe the 2004, Bioastronomy 2002, IAU Symp., 213, 149 the signal comes from off-axis rays. Patat polarimetric signatures of Chroococcidi- & Romaniello (2006) and Bagnulo et al. opsis in their natural habitat beneath (2009) have discussed the spurious polar­ translucent rock surfaces. This will allow Links imetric effects observed off-axis with the feasibility of detecting this type of 1 http://www.astro.puc.cl/astrobio2010

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