11institutetext: UniversitÃ /INFN Cagliari, Italy

# Prospects for the NA60+ experiment at the CERN SPS

Alessandro De Falco for the NA60+ Collaboration 11
<EMAIL_ADDRESS>
###### Abstract

A new heavy-ion experiment on fixed target, NA60+, has been proposed at the
CERN SPS for data taking in the next years. Its main goals will be focused on
precision studies of thermal dimuons, heavy quark and strangeness production
in Pb-Pb collisions at center-of-mass energies ranging from 5 to 17 GeV, which
will provide a unique opportunity to investigate the region of the QCD phase
diagram at high baryochemical potential ($\mu_{B}\sim 200-400$ MeV). The key
points of the NA60+ very broad and ambitious physics program will be
described.

Heavy-ion collisions at low center-of-mass (CM) energies provide a unique tool
to investigate the QCD phase diagram at large values of the baryochemical
potential $\mu_{B}$. An energy scan at the SPS, with CM energies ranging from
5 to 17 GeV per nucleon, will allow the study of the phase diagram in the
range $\mu_{B}\sim 200-400$ MeV (fig. 1), where key information can be
obtained about the presence of a critical point, the order of the phase
transition at large $\mu_{B}$, the chiral symmetry restoration effects and the
temperature at which the onset of the deconfinement takes place.

Figure 1: The QCD phase diagram (courtesy of Thomas Ullrich)

The NA60+ experiment has been proposed eoi  to study hard and electromagnetic
probes in this $\mu_{B}$ region. Such measurements would address many yet
uncovered points in the understanding of the QGP phase. Hard probes may bring
information on the onset of deconfinement via the measurement of $J/\psi$
suppression as a function of the CM energy, while open charm states would
allow to determine the transport properties of the Quark-Gluon Plasma (QGP) at
large values of $\mu_{B}$. Electromagnetic probes, and in particular
dileptons, would give insight on the temperature of the system via the
measurement of the thermal dimuon mass spectrum, on chiral symmetry
restoration effects and on the order of the phase transition thermal ; chiral
.

Figure 2: Layout of the proposed NA60+ apparatus in the low energy setup.

The detector concept of NA60+ inherits the design from its predecessor NA60,
and is based on a muon spectrometer coupled with a vertex telescope. Muons are
detected by a magnetic spectrometer, composed of a toroidal magnet and four
tracking stations, placed after a hadron absorber. A new magnet, which should
provide a field $B\cdot R\sim 0.2-0.5$ Tm is being designed and a small-scale
prototype has been constructed. The tracking stations will be equipped with
about 330 GEM modules, each one measuring $50\times 110$ cm2, overlapping in
both horizontal and vertical coordinates by 10 cm. The GEM module design
profits from the experience developed for the CMS muon system and the ALICE
TPC upgrades. A trigger system based on two RPC stations is placed after an
additional graphite absorber. The width of the front absorber, as well as the
position of the tracking stations will be changed depending on the beam
energy, such that the detector keeps a good acceptance around mid-rapidity.
The low-energy ($E_{\mathrm{beam}}/A=20-40$ GeV) setup is reported in fig. 2.
At high energy, $E_{beam}/A=158$ GeV, the muon spectrometer will be moved on
rails by 3.3 m and the absorber thickness will be increased to 4.6 m. The
presence of the hadron absorber provides the muon identification. However, it
introduces a deterioration in the muon momentum resolution, due to multiple
scattering and fluctuations in the energy loss. This loss in resolution can be
recovered by matching the muons reconstructed in the muon spectrometer with
the charged tracks measured in the vertex telescope (VT), both in coordinates
and momentum space. The VT can also be used for multiplicity measurements and
for the study of hadronic decay channels of charm and strange hadrons, as a
stand-alone detector. The VT consists of a dipole magnet and a set of pixel
planes for tracking. The choice considered for the magnet is the CERN MEP48
dipole, that provides a field $B=1.47$ T and an angular coverage of $21^{0}$.
The tracking system will consist of a set of (5 or 10) planes made of four
large area Monolithic Active Pixel Sensors (MAPS) obtained by means of the
stitching technology. The pixel dimensions are of about $15\times
15\leavevmode\nobreak\ \mu\mathrm{m}^{2}$. The sensor thickness is of the
order of $20\leavevmode\nobreak\ \mu$m. Moreover, the mechanical supports and
cooling are moved to the border. Therefore, the material budget in the
sensitive area is very small, lower than $0.1\%X_{0}$, reducing the effect of
multiple scattering in the VT. The expected spatial resolution is $5\mu$m or
better, with a gain of a factor of $\leavevmode\nobreak\ 2$ with respect to
the hybrid technology.

A high intensity beam is required for the measurement of hard and
electromagnetic probes. The experiment is foreseen to use the H8 beam line at
the CERN SPS, which can provide an intensity of $10^{7}$ Pb ions per 20 s
spill. The interaction rate is thus expected to be one order of magnitude
higher with respect to other experiments in the same $\mu_{B}$ range. With a
typical run time of one month per energy, a very significant statistics can be
reached for each of the proposed measurements.

Figure 3: Left: simulated dimuon mass spectrum in central Pb-Pb collisions at
$E=40$ GeV per nucleon. Right: medium temperature evolution vs $\sqrt{s_{NN}}$
in central Pb-Pb collisions using thermal . Measurements from NA60 NA60thermal
and HADES hades .

The physics performances of NA60+ were studied performing fast simulations of
the signals with semi-analytical tracking based on the Kalman filter, while
the simulation of the background was performed with FLUKA. The obtained
opposite sign dimuon mass spectrum is shown in the left panel of fig. 3. The
mass resolution for $M=M_{\omega}$ is $\sim 7$ MeV/$c^{2}$, about one third of
the NA60 resolution at the same mass. Besides the signal, composed of muon
pairs, two sources of background are visible: the combinatorial background and
the fake matches contribution. The former is due to uncorrelated pairs of
muons mainly originated from pion and kaon decays. The latter is due to
incorrect matches between muons reconstructed in the muon spectrometer and
tracks reconstructed in the vertex telescope. Both contributions can be
evaluated using event mixing techniques and subtracted to the opposite sign
mass spectrum. The signal mass spectrum resulting after background subtraction
is dominated by the hadronic cocktail for $M<1.5$ GeV/$c^{2}$. In this region,
a precision measurement of the $\rho$ spectral function can be performed,
complementing the NA60 measurement in In-In at top SPS energy with results at
lower energy. In the region $1<M<1.5$ GeV/$c^{2}$, a dimuon enhancement due to
the chiral mixing between the $\rho$ and its chiral partner $a_{1}$ via $4\pi$
states can be observed chiral . For $M>2$ GeV/$c^{2}$, after subtracting the
Drell-Yan and open charm contributions, the mass spectrum is dominated by
thermal dimuons, that can be fitted to a thermal distribution of the form
$dN/dM\propto M^{-3/2}\exp(-M/T_{S})$. The parameter $T_{S}$ represents a
space-time average of the thermal temperature over the fireball evolution and
can be determined with a precision of the order of 10 MeV. Measurements of
$T_{S}$ vs $\sqrt{s_{NN}}$ at low energy ($<10$ GeV) may allow to discover a
plateau in the caloric curve (fig. 3, right) that would be present in case of
a phase transition of the first order thermal .

Charmonium suppression was extensively studied at the SPS, where the NA50
collaboration observed a suppression of $\sim 30\%$ of the $J/\psi$ meson
yield in central Pb-Pb collisions at the top SPS energy ($\sqrt{s_{NN}}=17.3$
GeV) that could not be ascribed to cold nuclear matter effects alone NA50 .
The suppression is qualitatively consistent with the melting of the less bound
$\chi_{c}$ and $\psi(2S)$ states in a deconfined medium, which would lead to a
suppression of the $J/\psi$ produced from the decays of these particles. At
present, no measurement is available at lower energy. NA60+ aims to extend the
measurements down to $E_{\mathrm{lab}}/A=40$ GeV in order to search for the
onset of the deconfinement. By correlating the measurement with the
corresponding determination of the temperature obtained from thermal dimuons,
one could obtain the temperature at which the $\chi_{c}$ and $\psi(2S)$ melt.
With one month data taking at an intensity of $5\times 10^{5}$ Pb ions per
second, a number of $J/\psi$ in the range $1.5-20\times 10^{4}$, depending on
the beam energy, may be reconstructed. Also cold nuclear matter effects have
to be determined precisely, therefore, data taking with p-A collisions are
mandatory at each CM energy. A 15-days long data taking with a beam intensity
of $3\cdot 10^{8}$ protons/s on 7 nuclear targets is planned to measure the
$J/\psi$ production cross section as a function of the mass number.

Open charm measurements can be performed reconstructing the decays of charmed
hadrons into two or three charged hadrons, using the vertex telescope as a
stand-alone detector. The huge combinatorial background can be reduced by
applying geometrical selections on the displaced decay vertex topology,
profiting from the fact that the $c\tau$ for these decays is about
$60-310\mu$m, and therefore the decay vertices are displaced by a few hundred
$\mu$m from the interaction point. A high resolution on the vertex
reconstruction is needed. The MAPS technology can provide a signal to
background ratio $\sim 10$ times higher than the corresponding value
obtainable with hybrid pixel sensors. As of today, no open charm measurement
has been performed below the top SPS energy. In one month data taking, more
than $3\cdot 10^{6}\leavevmode\nobreak\ D^{0}$ can be reconstructed in central
collisions at $\sqrt{s_{NN}}=17.3$ GeV, allowing for a precise determination
of the yield and elliptic flow as a function of $p_{T}$, rapidity and
centrality. At $\sqrt{s_{NN}}=10.6$ GeV due to the lower production cross
section, the number of reconstructed $D^{0}$ is expected to be lower by about
an order of magnitude. Still, the measurement will be feasible with a
statistical precision at the level of the percent.

Finally, strangeness measurements in the hadronic decay channels will allow to
explore the low multiplicity region for the understanding of strangeness
enhancement with multiplicity. The high statistics expected to be collected
will guarantee a high $p_{T}$-reach, the study in narrow centrality bins and
an extention to multistrange hadrons of the elliptic flow measurements.

The broad physics program of the NA60+ experiment provides a strong motivation
for its realization. The goal is to start data taking in 2027, with LHC run 4.

## References

  * (1) T. Dahms, E. Scomparin, G. Usai (NA60+ Collaboration), Tech. rep., CERN, Geneva (2019), https://cds.cern.ch/record/2673280
  * (2) R. Rapp, H. van Hees, Phys. Lett. B 753, 586 (2016), 1411.4612
  * (3) M. Dey, V.L. Eletsky, B.L. Ioffe, Phys. Lett. B 252, 620 (1990)
  * (4) R. Arnaldi et al. (NA60), Eur. Phys. J. C 59, 607 (2009), 0810.3204
  * (5) S. Harabasz (HADES), Nucl. Phys. A 982, 771 (2019)
  * (6) B. Alessandro et al. (NA50), Eur. Phys. J. C 39, 335 (2005), hep-ex/0412036


# Purple is the new green: biopigments and spectra of Earth-like purple worlds

Lígia Fonseca Coelho,1,2 Lisa Kaltenegger,1,2 Stephen Zinder,2,3 William
Philpot,2,4 Taylor L. Price,5 and Trinity L. Hamilton 5,6
1Department of Astronomy, Cornell University, Space Sciences Building 404,
Ithaca, NY 14850, USA
2Carl Sagan Institute, Cornell University, Space Science Building 311, Ithaca,
NY 14850, USA
3Department of Microbiology, Cornell University, Ithaca, NY 14853, USA
4School of Civil and Environmental Engineering, Cornell University, Ithaca, NY
14853, USA
5Department of Plant and Microbial Biology, University of Minnesota, St. Paul,
MN 55108, USA
6The Biotechnology Institute, University of Minnesota, St. Paul, MN 55108, USA
E-mail<EMAIL_ADDRESS>
(Accepted 2024 February 16. Received 2024 February 15; in original form 2023
September 30)

###### Abstract

With more than 5500 detected exoplanets, the search for life is entering a new
era. Using life on Earth as our guide, we look beyond green landscapes to
expand our ability to detect signs of surface life on other worlds. While
oxygenic photosynthesis gives rise to modern green landscapes,
bacteriochlorophyll-based anoxygenic phototrophs can also colour their
habitats and could dominate a much wider range of environments on Earth-like
exoplanets. Here, we characterize the reflectance spectra of a collection of
purple sulfur and purple non-sulfur bacteria from a variety of anoxic and oxic
environments. We present models for Earth-like planets where purple bacteria
dominate the surface and show the impact of their signatures on the
reflectance spectra of terrestrial exoplanets. Our research provides a new
resource to guide the detection of purple bacteria and impro ves our chances
of detecting life on exoplanets with upcoming telescopes. Our biological
pigment data base for purple bacteria and the high-resolution spectra of
Earth-like planets, including ocean worlds, snowball planets, frozen worlds,
and Earth analogues, are available online, providing a tool for modellers and
observers to train retrie v al algorithms, optimize search strategies, and
inform models of Earth-like planets, where purple is the new green.

###### keywords:

astrobiology – techniques: spectroscopic – planets and satellites: oceans –
planets and satellites: surface – planets and satellites: terrestrial planets.

††pubyear: 2024††pagerange: Purple is the new green: biopigments and spectra
of Earth-like purple worlds–Supporting Information

##  1 Introduction

With more than 5500 confirmed exoplanets and more than 30 potential Earth-like
planets (exoplanets.nasa.gov, 2024 February) our search for life in the cosmos
is entering a new stage that requires including a wider diversity of life on
Earth (Hegde et al., 2015; O’Malley-James & Kaltenegger, 2018; Coelho et al.,
2022) and its evolution (O’Malley-James & Kaltenegger, 2018, 2019) to generate
our key to finding it elsewhere.

Before the emergence of chlorophyll-based oxygenic photosynthesis (Lyons et
al., 2014; Hamilton et al., 2016), anoxygenic photosynthesis prevailed,
relying on bacteriochlorophylls (Hohmann-Marriott & Blankenship, 2011).
Oxygenic photosynthesis on modern Earth is often linked to lush green
landscapes. Anoxygenic phototrophs, which can inhabit both aquatic and land
environments, can also colour their environments. Early Earth may have been
dominated by purple-pigmented bacteria, including purple non-sulfur bacteria
(PNSB) and purple sulfur bacteria (PSB) (Brocks et al., 2005; Sanromá et al.,
2013).

PSB are phototrophic anoxygenic bacteria that prefer reduced sulfur compounds
as electron donors instead of water, thus not producing oxygen as a byproduct
of photosynthesis. PNSB are photoheterotrophic bacteria meaning they use their
photosystems to mainly produce energy and electric potential while still
requiring an external organic carbon source. PNSB are versatile organisms that
can use some inorganic compounds (e.g. H2 ) as electron donors, and that are
less sensitive to oxygen and darkness than PSB, although more sensitive to
sulfide (Madigan et al., 2009).

Figure 1: Filters showing the measured purple non-sulfur bacteria (E01, E02,
E11, E18, E23, E26, E33, E45, BV, and RV), purple sulfur bacteria (E03, E05,
E06, E07, E10, E13, E28, E35, E41, E50, E51, E53), and purple cyanobacteria
(Glo). For details on the biota see Table S1.

PNSB and PSB use biopigments called carotenoids as antennas to harvest light
energy in the visible range (Magdaong et al., 2014; Ma & Cui, 2022). These
bacteria also use bacteriochlorophylls (BChl) a and b, which serve as both
antenna and reaction centre pigments and can harness visible to infrared (IR)
radiation, absorbing and reflecting from 750 to 1100 nm (Kimura et al., 2023).
This spectral versatility arguably provides a competitive edge, allowing
organisms with BChls to thrive in diverse and often challenging ecological
niches across our planet, even deep-sea hydrothermal vents (Beatty et al.,
2005). In deep-sea conditions, they rely on IR energy to drive anoxygenic
phototrophy, where radiation longer than 850 nm can serve as their standard
energy source. These characteristics could allow PNSB and PSB to thrive on
exoplanets orbiting M-type stars because the spectral energy distribution of
M-stars overlaps with the optimal spectral range for absorption and
reflectance by BChl-a and -b (Wolstencroft & Raven, 2002; Tinetti et al.,
2006; Kiang et al., 2007; Schwieterman et al., 2018; Lehmer et al., 2021;
Duffy et al., 2023).

Even Earth-analogue planets orbiting Sun-analogue stars can promote purple
bacteria. Sanromá et al. (2013) measured the reflectivity of a single non-
sulfur bacterium, Rhodobacter sphaeroides, and modelled the spectrum of a
purple Archean Earth from it. On modern Earth, purple cyanobacteria, PNSB, and
PSB, colour surface landscapes in shallow waters, coasts, and marshes
(Guglielmi et al., 1981; Wilbanks et al., 2014). Their colour intensifies when
light becomes scarce, and pigment production increases (Magdaong et al.,
2014), making purple bacteria particularly intriguing candidates for
exoplanetary habitability biopigment assessments. However, to date, there is
no reference data base for the reflectance spectra of the diverse biopigments
of purple biota.

Here we grow and measure the reflectance spectra of a wide range of PNSB, PSB,
and a purple cyanobacterium (hereafter we refer to our collection as ‘purple
bacteria’). We show bacteria closely related to R. sphaeroides for reference
in Figs 1 and 2 and expand on the measurement by Sanromá et al. (2013) by
measuring the reflectance for a wide range of purple bacteria, including
Blastochloris viridis, which broadens the range of photosynthesis to the near-
IR. Until now, most models for terrestrial exoplanets that incorporate surface
biota exhibit a bias towards green pigments (Seager et al., 2005; O’Malley-
James & Kaltenegger, 2018, 2019; Schwieterman et al., 2018). This bias is
primarily due to the limited reflectance spectra data for purple bacteria,
which have not been extensively measured.

In this study, we present a catalogue of reflectance spectra of biopigments
for a range of purple bacteria in fresh and dry states (following Coelho et
al., 2022), measured with an integrating sphere to mimic exoplanet observing
conditions (see Hegde et al., 2015). We used those measurements to model the
spectra of a range of Earth-like planets dominated by diverse purple bacteria
for Earth analogue planets, ocean worlds, snowball planets, as well as frozen
worlds, with and without clouds. Our goal was to explore how their biopigments
can shape the reflectance spectra of habitable worlds.

We created a data base for the reflectance spectra of biopigments of a wide
range of purple bacteria from 400 to 2500 nm. This data base is available
online (https://doi.org/10.5281/zenodo.10697546) as a resource for observers
and modellers to inform design decisions and optimize observations. We use
those measurements to simulate high-resolution reflectance spectra of Earth-
like exoplanets dominated by purple bacteria providing a data set to train
retrieval algorithms and inform the observing strategies for telescopes like
the European Extremely Large Telescope (Gilmozzi & Spyromilio, 2007) and the
design of upcoming telescopes like the Habitable World Observatory (HWO)
(Vaughan et al., 2023). These data bases provide a critical resource for
observers and modellers searching for life in the cosmos.

##  2 Methods

We grew a diverse set of purple bacteria in the laboratory, measured the
reflectivity of their biopigments, and then used that data to simulate the
reflectance spectra of Earth-like exoplanets dominated by purple biota.

We present enrichment samples (E01 to E53) and pure cultures (RV, BV, and Glo)
– please see Fig.1. The difference between an enrichment and a pure culture is
that a pure culture is a single species while an enrichment can contain many
species but is typically dominated by a species of interest. Life as we know
it exists in complex communities and not in pure cultures and those
communities will not always be dominated by one species. Thus, our sampling
relates to an exoplanet surface dominated by an ecosystem enriched in purple
bacteria (a similar phenomenon to algae blooms and watermelon snow, where a
single group of microbes dominates a whole landscape on modern Earth).

###  2.1 Biological measurements

All cultures and enrichments were sampled from aquatic biomes, and the
specifics of their location of origin can be found in the supplementary
materials. Briefly, RV and BV were obtained from S. Zinder’s laboratory at
Cornell University (NY, US). PSB E53 was sampled by L.F. Coelho and S. Zinder
from Beebe Lake (NY, US) and enriched using a modular medium for PSB with 3 mM
of sulfide. PSB E51 and Gloeobacter violaceus PCC 7421 were obtained from T.
Hamilton’s laboratory at the University of Minnesota (MN, US). PSB E51 was
enriched from a sulfidic spring in NY. PSB and PNSB enrichments (E01-E50) were
courtesy of the Microbial Diversity 2023 Course (Marine Biological Laboratory,
Woods Hole, MA, US).

We grew PSB and PNSB in a modular mineral basal medium based on Pfennig’s SL9
medium (Tschech & Pfennig, 1984). The PNSB grew under an N2/CO2 atmosphere
with 1 g L-1 sodium succinate as the carbon source, and the PSB grew under
autotrophic conditions in a 20 per cent CO2 atmosphere and 1 per cent
bicarbonate. The purple cyanobacterium  G. violaceus PCC 7421 and blue-green
cyanobacterium Anabaena sp. UTEX 2576 were grown in BG11 medium. All cultures
were maintained at room temperature. Differences in the media and light
requirements for each culture or enrichment can be found in the supplementary
material – Table S1. Depending on the culture, the time required for growth
varied from about 24 h to 1 week. A volume of 20 mL of the
cultures/enrichments was transferred to 50 mL Falcon centrifuge tubes until
filtration.

###  2.2 Sample data collection

We deposited cultures on to a 0.45 $\micron$ 25 mm white mixed cellulose ester
filter using a 10 mL syringe until saturation was reached (as previously
described in Hegde et al., 2015; Coelho et al., 2022). Subsequently, we
measured hemispherical reflectance from 400–2500 nm at intervals of 1 nm,
using three detectors: a 512-element silicon photodiode array (up to 1000 nm)
and two indium–gallium–arsenide photodiodes (1000–1800 and 1800–2500 nm) using
an ASD FieldSpec 4 Spectrometer coupled with an ASD integrating sphere (as
described in detail in Hegde et al., 2015; Coelho et al., 2022). Measuring the
reflectance of the biopigments using an integrating sphere mimics the
observing geometry of an exoplanet (as previously described in Hegde et al.,
2015).

Dry biopigments show stronger reflectance than fresh samples (see also Coelho
et al., 2022) and thus we measured the same sample twice: immediately after
deposition on the filter (referred to as ‘fresh’) and after a week at the
benchtop in the dark (referred to as ‘dry’). Filters with only culture media
served as controls.

###  2.3 Data generation and simulations

We used the established Exo-Prime II model (see e.g. Kaltenegger et al., 2020;
Madden & Kaltenegger, 2020) to generate high-resolution atmospheric spectra
with a minimum resolution of $\lambda/\Delta\lambda=100\,000$ We chose a
slightly colder modern Earth for our analysis with an average surface
temperature of 273 K to allow for a wide range of surface conditions, using
modern Earth outgassing rates (e.g. Rugheimer et al., 2013; Kaltenegger & Lin,
2021) and a 10 per cent decreased solar irradiation to account for a planet
that could be covered with liquid or frozen water. This slightly colder
atmosphere does not show significant changes in spectral features compared to
modern Earth, except for slightly reduced water features due to the lower
humidity of the atmosphere. However, these slight differences do not influence
our analysis of the effect of surface biota on the spectra.

Ocean and snow surface albedo have been selected from the United States
Geological Survey (USGS) Spectra Library (Clark et al., 2007), and the single-
layer cloud albedo is based on the 20 $\micron$ MODIS model (King et al.,
1997; Rossow & Schiffer, 1999) following previous work (see e.g. Madden &
Kaltenegger, 2020; Pham & Kaltenegger, 2021, 2022).

We resampled the high resolution of the reflectance spectra of Earth-like
exoplanets and the low-resolution biota (Fig. 3), cloud, ocean, and snow
albedos using the Python SPECTRES 2.2.0 package (Carnall, 2017). We resampled
the low-resolution biota, cloud, ocean, and snow albedos to R $=140$
(comparable to the HWO concept, Fig. S2) and R $=100\,000$, comparable to
high-resolution instruments such as ELTs (Fig. S3). Our exoplanet models
contain reflectance spectra of frozen worlds and snowball planets, Earth-
analogues, and ocean worlds with a different surface coverage of purple
bacteria for atmospheres with and without clouds.

##  3 Results and Discussion

To expand our baseline for finding life in the cosmos, we have measured the
reflectance of purple bacteria that thrive across a range of anoxic and oxic
environments. We present the samples in Fig. 1, the reflectance of the
biopigments in Fig. 2, and the model reflectance spectra of Earth-like
exoplanets dominated by a range of such purple biota in Fig. 3.

Figure 2: Reflectance spectra of purple and blue-green cyanobacteria (top),
PNSB (N $=10$) (middle), and, PSB (N $=12$) (bottom) where N denotes the
number of samples. Labels of main pigments identified: (a) Chl-a Soret band,
(b) Carotenoids, (c) Chl-a Qx band, (d) Phycobilins, (e) Chl-a Qy band, (f)
BChl-a Soret band, (g) BChl-a, b Qx band, (h) BChl-a Qy band, and (i) BChl-b
Qy band. Dry samples (right) show a stronger reflectance than fresh samples
(left), water absorption features at 1490 and 1900 nm are stronger in the
fresh samples. The reflectance of the culture medium alone is shown as a
dotted line for control. Figure 3: Modelled reflectance spectra of Earth-like
planets (top) for a frozen planet covered with 100 per cent dry biota (i) and
a snowball planet covered with 50 per cent dry biota and 50 per cent snow
(ii), which increases the overall reflectivity slightly. Adding a 50 per cent
cloud cover (2nd row, iii and iv) reduces the overall reflectivity of these
cold worlds due to the lower albedo of clouds compared to snow and the dry
biota (see Fig. S1). The models of an ocean world (100 per cent fresh biota)
and an Earth-analogue planet (70 per cent ocean and 30 per cent fresh biota)
(3rd row, v and vi) show that the overall reflectivity of the planet decreases
due to the lower reflectivity of the fresh biota as well as the low
reflectivity of oceans compared to frozen worlds. Adding clouds to the planet
(bottom, vii and viii) increases the overall reflectivity of the models
because clouds reflect more light than oceans or fresh biota. Labels: (a)
Chl-a Soret band, (b) Carotenoids, (c) Chl-a Qx band, (d) Phycobilins, (e)
Chl-a Qy band, (f) BChl-a Soret band, (g) BChl-a, b Qx band, (h) BChl-a Qy
band, and (i) BChl-b Qy band (Table S2). Atmospheric absorption features: O3
(450 to 740 nm), O2 (690, 760, and 1260 nm), and H2O (950, 1150, 1490, and
1900 nm).

In Fig. 1, we show 23 purple bacteria samples, originating from oxic to anoxic
conditions. Despite being generally referred to as ‘purple’, these organisms
can exhibit a wide range of colours as shown here, including yellow, orange,
brown, or even red due to the presence of different carotenoids. Fig. 2 shows
the distinct reflectivity of the 23 purple bacteria measured in both fresh and
dry conditions. The main features can be identified in the spectra range
between 400–1100 nm. Our spectra show a minimum resolution of 125 comparable
to the intended resolution of the Habitable World Observatory (R $=140$). We
resampled the data to R $=140$ (Fig. S2) showing no significant changes in the
biota signatures. Fig. 2 (top) shows the reflectance spectra of the purple
cyanobacterium G. violaceus compared to that of Anabaena sp., a typical blue-
green cyanobacterium, both of which thrive in oxygen-rich environments.
Despite inhabiting similar biomes, the purple-coloured cyanobacterium G.
violaceus exhibits a distinct reflectance spectrum. The Anabaena sp.
reflectance spectrum is characterized by a prominent Chlorophyll a (Chl-a)
peak at 548 nm (see Table S2), a typical reflectance signature of organisms
with a high Chl-a content (see Coelho et al., 2022, Fig. 2). Both
cyanobacteria have Chl-a absorption features at 437–439 and 675–677 nm and a
phycobilin absorption feature at 620–624 nm, typical of cyanobacteria. Purple
cyanobacteria produce distinct carotenoids (440–550 nm), G. violaceus spectrum
(Fig. 2 top) displays distinct carotenoid absorption features at 500 and 560
nm (visible in dry samples) in the spectrum. Fig. 2 (top) also shows some
overlaps of biopigment features between the biota: echinenone, a carotenoid
usually present in Gloeobacter absorbs at 440 nm and is similar to the Soret
band of Chl-a. Similarly, Anabaena sp. presents an absorption feature at 488
nm that agrees with the reference peak of canthaxanthin, another carotenoid
(Takaichi et al., 2005).

In addition to oxic environments (Fig. 2 top), we show the reflectivity of
purple bacteria that grow under different conditions: PNSB adapted to
environments with limited oxygen (Fig. 2 middle) and PSB in anoxic conditions
(Fig. 2 bottom). In both panels part of the BChl Soret bands can be seen at
400–420 nm. In PSB and PNSB, BChl-a and -b Qy bands are present in the near-IR
(800–1100 nm). The BChl-a spectral Q-band features exhibit minimal variability
among organisms, with absorption features at 800–805 nm and 850–855 nm typical
of PNSB and PSB (Kimura et al., 2023). BChl b displays a considerably
different Q band, with the position of the peaks ranging from 820 to 1100 nm.
Note that Fig. 2 (middle) displays a PNSB (B. viridis) that absorbs at 1010 nm
and can use this energy for phototrophy, suggesting that ecosystems dominated
by purple biota may be more likely to exist on planets orbiting cooler M-stars
that provide stellar energy distribution peaking at longer, redder,
wavelengths (as suggested previously by Wolstencroft & Raven, 2002; Tinetti et
al., 2006; Kiang et al., 2007; Lehmer et al., 2021). See Table S2 for detailed
listing of the biopigments spectral signatures in purple bacteria.

We then simulated the reflectance spectra of Earth-like planets shown in Fig.
3 for Earth-analogue planets (70 per cent ocean and 30 per cent land covered
with fresh biota), ocean worlds (covered with 100 per cent fresh biota),
frozen worlds (covered with 100 per cent dry biota) and snowball
worlds(covered with 50 per cent dry biota and 50 per cent snow). We then added
50 per cent cloud cover to show the effect of clouds on the reflectance
spectra (see Fig. S1 for the albedos of snow, oceans, clouds, and the selected
fresh and dry biota). Models of the reflectance spectra of Earth-like
exoplanets show a strong reflectivity for planetary models covered in dry
biota and snow, but lower reflectivity for planets with open oceans and fresh
biota (see Fig. 3). A snowball planet with 50 per cent dry biota and 50 per
cent snow (Fig. 3–ii) shows the highest overall reflectivity due to the high
reflectivity of snow and dry biota (see Fig. S1).

The frozen planet models, covered with 100 per cent dry biota, show the second
highest reflectivity, only slightly lower than the snowball planet. Adding a
50 per cent cloud cover (Fig. 3–iii and –iv) reduces the overall reflectivity
of both models slightly due to the lower albedo of clouds compared to snow and
the dry biota. For Earth-analog models of a planet (70 per cent ocean and 30
per cent land covered by fresh biota) and ocean worlds (100 per cent fresh
biota) (Fig. 3–v and –vi) the overall reflectivity is lower than for the
previous cases due to the lower reflectivity of the fresh biota compared to
the dry biota and the low reflectivity of oceans compared to snow. Adding
clouds to the models (bottom) increases the overall reflectivity slightly
(Fig. 3–vii and –viii).

There is an overlap between biopigments and atmospheric absorption features.
B(Chl)-a Qy absorption features (680 nm for Chl-a and 800–850 nm for BChl-a)
are too close, in the lower and upper limit of O2 (760 nm). Note that BChl-a
Qy of PSB (anoxic surfaces) has been reported to also absorb at 900–963 nm
(Kimura et al., 2023), which does not overlap with O2, but may overlap with an
intense H2O feature at 950 nm. Similarly, BChl-b Qy absorption features far in
the 1000 nm may overlap with the same H2O feature. Carotenoids overlap with O3
(450–740 nm) although in Fig. 3 that does not significantly mask the
biopigments features.

Barrientos et al. (2023) showed changes in surface albedos provide vital and
usable information for observations, even with a low signal-to-noise ratio.
Their quantitative work was applied to a general ‘edge feature’ in the
spectrum, based on the specific biological surface albedo caused by green
photosynthetic organisms – the ‘red edge’ (Barrientos et al., 2023).

Traditionally, the ‘red edge’ (e.g. Sagan et al., 1993; Seager et al., 2005;
Arnold, 2008; O’Malley-James & Kaltenegger, 2018, 2019) has been used as a
surface biosignature for modern Earth (see Fig. 2, top panel). However, a
photosynthesis ‘red edge’ is an advanced surface biosignature for life on
Earth that may only have become detectable between 0.75 and 1.2 billion years
ago (O’Malley-James & Kaltenegger, 2018). Sharp changes in the surface albedo
in the visible range could have a similar observation promise (Barrientos et
al., 2023). These results stress the importance of applying wavelength-
dependent surface features to retrieval models to estimate atmospheric
features such as H2O, O2, and O3 correctly. Future studies applying a similar
quantitative method will determine the detectability of purple bacteria
features.

Here we extend our search for surface biosignatures to a much wider range of
terrestrial exoplanet environments using biopigments of purple bacteria.
Further, we expand the type of habitats such biota could dominate including
exoplanets with anoxic or microaerobic environments. Some spectral features of
(B)Chl and carotenoids lie close to atmospheric features like H2O and O3;
thus, the community needs to consider a diverse set of surface biopigments in
the retrieval models (Barrientos et al., 2023), not only to detect signs of
life but also to retrieved atmospheric features co-existing in the same
wavelengths as surface features.

In this study, we focused on terrestrial Earth-like planets. Other types of
proposed exoplanet environments, like Hycean planets, covered by oceans under
substantial H2 atmospheres (Madhusudhan et al., 2021) could also be promising
candidates for purple bacterial ecosystems. Some purple bacteria organisms can
use H2 as a primary electron donor. Thus, the abundance of H2 on these
proposed planets may facilitate biomass production.

The distinct reflectivity of the biopigments of purple bacteria offers surface
biosignatures for a wide range of conditions – from oxic to anoxic conditions
– to the point of detection with upcoming telescopes like ELTs or the
Habitable World Telescope.

##  4 Conclusions

Using life on Earth as our guide we identified purple bacteria as biota that
could dominate a wide range of environments on exoplanets. Especially on
exoplanets orbiting M-stars, purple bacteria could arguably be dominant
because they can use red to infrared light to fuel photosynthesis. Where
chlorophyll-based photosynthesis thrives under the Sun’s spectral energy
distribution, a range of purple bacteria could thrive under a red star’s
light.

We provide a reflectance spectra data base of the biopigments of purple
bacteria as a resource for modelers and observers to optimize search
strategies for life with upcoming telescopes. We expand the range of surface
biosignatures on potentially habitable exoplanets beyond the ‘red edge’
traditionally used to search for vegetation. Despite being generally referred
to as purple bacteria, these organisms can exhibit a wide range of colours,
including yellow, orange, brown, or red due to the presence of different
carotenoids, and could colour the surface of exoplanets in many shades.

Based on these reflectance measurements we created an exoplanet data base of
high-resolution reflectance spectra for Earth-like exoplanets including ocean
worlds, Earth-analogues, frozen worlds, and snowball planets to explore the
effects of purple bacteria on their overall reflectance spectra.

Our models show that depending on the surface coverage of the biota and the
cloud coverage, a wide variety of terrestrial planets could show signs of
purple bacteria surface biopigments. While it is unknown whether life – or
purple bacteria – can evolve on other worlds, purple might just be the new
green in the search for surface life.

## Acknowledgements

LFC acknowledges the funding from Fulbright Schuman. LFC and LK acknowledge
the funding from the Binson Foundation. TLH acknowledges funding from NSF
award 1939303. The authors acknowledge the 2023 Microbial Diversity course
from the Marine Biological Laboratory with funding from the Moore Foundation,
Simons Foundation, US Department of Energy, and NSF award 22055876 (PIs:
RachelWitaker, Scott Dawson, and L. Hyman) for the samples of PNSB and PSB
(E01 to E50). The authors expressly acknowledge the Microbial Diversity class
of 2023, including Isabelle (Izzy) Lakis, Priyanka Chatterjee, Amaranta Khan,
Etan Dieppa, Nicole Dames, Maria Jose Rodriguez, (Mache), Lucy Androsuik,
McKenzie Powers, Becky Parales, Huda Usman, Marwa Baloza, and Allison Higgins
for the enrichments E02-E50, respectively. The authors acknowledge Jeff Havig
for his assistance in collecting sample E51. The authors acknowledge Adam
Langeveld and Rebecca Payne for the very useful modelling insights.

## Data Availability

These data bases are available online
(https://doi.org/10.5281/zenodo.10697546).

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