1 Introduction
Cave environments on Earth have long provided shelter to a variety of
organisms, from microbes to humans. Though their scales of interest
differ vastly, these two examples have sought the same comfort from
caves: a stable and sheltered environment, protected from the woes of
the surface world. Since the discovery of lava caves on Mars (see (Sauro
et al., 2020) for a review), they have become of renewed interest as
targets for human shelter in future missions, as well as areas of
astrobiological interest, with the potential of harboring traces of
extant or extinct extraterrestrial life.
Recent years have seen a huge advancement in the development and
miniaturization of autonomous mobility systems and exploration
technologies for robotic missions to planetary caves, including
instrument suites for in situ astrobiological studies (summarized
in (Blank et al., 2020)). All aspects of cave mission preparation, from
robotics development to astronaut training and scientific advancement,
are currently being tested in terrestrial analogue sites, namely lava
tubes in the Azores, Hawai’i, Iceland, Lanzarote, the western
continental United States, and other volcanic areas.
Astrobiological studies in terrestrial lava tubes focus on the
characterization of the microbe-mineral continuum and the identification
of biosignatures in the form of biologically mediated speleothems
(secondary cave minerals) and other geochemical fingerprints that may
remain preserved on geological timescales (Boston et al., 2001; Léveillé
& Datta, 2010; Northup et al., 2011; Northup & Lavoie, 2001). A suite
of analyses is required to distinguish a biologically mediated secondary
mineral from one that is abiotic (Uckert et al., 2017), presenting a
difficult challenge. Nonetheless, a variety of biologically
mediated speleothems has been reported in the literature, including
filamentous manganese ”snow”, ”crisco” moonmilk, lithified U-loops and
living sulfuric acid ”snotties”, and pool fingers in limestone caves in
New Mexico and Mexico (Boston et al., 2001).
Microbes in caves on Earth attach to minerals on cave walls, ceilings,
or floors and initiate biomineralization reactions, creating biofilms.
Biofilm formation is controlled by several processes, starting by
initial microbe adhesion to the surface, governed by fluid flow and
charging of the substratum. The initial colonizers excrete
exopolysaccharides, which increase the surface irregularity and allow
the biofilm to grow. Thus, the location of biofilm growth in caves
depends not only on where particles and microbes can be transported to,
but also be allowed to accumulate. A detailed review of the dynamics of
biofilm formation on mineral surfaces and their spatial distribution is
given by Little & Wagner (2018).
As the biofilm grows in layers away from the surface, it becomes a
microbial mat of great complexity. Within its structure, chemical
environments can exist that are radically different from that of the
surrounding, allowing for the growth of minerals and microbes that would
otherwise not be expected. Concentration of organic and inorganic
particles can sustain a consortium of microorganisms of different
nutritional modes. The putatively high microbial diversity within the
biofilm may create local changes in pH or redox conditions, which can
facilitate the precipitation of minerals that are unstable outside of
the biofilm. Microbes can control the precipitation of these minerals
either passively, where microbial cells act as nucleation sites, or
actively, where bacterially produced enzymes control mineralization
(Northup & Lavoie, 2001).
The majority of life in the Universe is thought to be unicellular
(Schulze-Makuch & Irwin, 2018). Moreover, the tendency to form biofilms
and mats may well be an adaptation to be expected on other planets, with
other biologies, and perhaps other fundamental chemistries (Boston et
al., 2001). Regardless of their specific chemistries, metabolisms
relying on differences in redox potentials in elements present in the
lava rock can be expected for any initial colonizers in Martian lava
caves, due to the absence of light and the oligotrophic (low nutrient)
quality of caves on Mars. The chemolithoautotrophic nature of this
hypothetical life may eventuate the production of similar speleothems as
found in terrestrial caves. Basaltic terrestrial lava tubes are most
similar in mineralogy to those posited on Mars and may thus provide
analogous potential for chemolithoautotrophy, resulting in similar,
recognizable molecular markers and biologically mediated speleothems.
To prepare a multidisciplinary sampling protocol for future
astrobiological missions to caves on Mars, we set up the Planetary
Analogues and Exobiology Lava Tube Expedition (PELE). Since 2017, we
have explored lava fields and lava tubes on Hawai’i, the Azores, and
Iceland. In this paper we focus on data from three lava tubes in two
distinct regions of Iceland during a PELE field expedition in the summer
of 2018, during which we collected samples of microbial mats and their
geological substrates. We obtained in situ elemental data with a
portable X-ray fluorescence (XRF) spectrometer, analyzed biological
samples with 16S rRNA gene sequencing methods, biogeochemical samples
with Raman spectroscopy, and geological samples with scanning electron
microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). In
synergizing these data sets, we attempted to describe the biogeochemical
fingerprints of microbial life in lava tubes and define their validity
as biosignatures. While the microbial mats collected in the caves we
visited came in a variety of colors, the blue samples are used here as a
case study to exemplify our protocols, describe our workflow, and show
the data that can be gleaned from such a study.