The alien characteristics of sea ice microbes : the potential for life on icy ocean worlds & beyond

The characterisation of extraterrestrial environments has evolved over the past decades as more discoveries about astronomical objects in our solar system and the detection of exoplanets continues to enhance our understanding for the potential of life. These discoveries have broadened the definition of habitability, widening the concepts on what sort of extraterrestrial environments could potentially sustain life. The sea ice environment is analogous to some extraterrestrial habitats, such as the icy ocean moons Europa and Enceladus, and is populated by a variety of microbial organisms. These microorganisms must tolerate large variations in physiochemical parameters (e.g temperature, light, salinity, pH, nutrients, anoxia and dissolved gases), making them prime candidates to test the physiochemical limits to life. The primary aim of this thesis was to define the habitability potential of icy ocean worlds and other extraterrestrial environments. This was done using multiple physiological, molecular, and ecological techniques - culminating in a series of in situ laboratory experiments with polar microbes in simulated extraterrestrial conditions. The first chapter details the history, development and central philosophies of astrobiological research. A particular focus is given to the analogous nature of polar microbial environments to certain extraterrestrial habitats and why polar microbes are legitimate proxies for investigating the potential for extraterrestrial life. A central tenant of determining whether extraterrestrial sites are of astrobiological interest has been to ‚ÄövÑv¿follow the water‚ÄövÑvp ‚ÄövÑv¨ that is finding planetary bodies with reservoirs of liquid water. However, the presence of water alone does not equate to life and to constrain the habitability potential of a water-laden world the chemical and physical characteristics also need to be investigated. The second chapter of this thesis explores the capacity for sea ice bacteria to adapt and grow under the different salinity constraints of Europa‚ÄövÑv¥s subsurface ocean and icy exterior shell. Here, the study demonstrates that bacterial survival and growth was strongly influenced by salinity and salt type although the response may be species specific. The subsequent experimental chapters pivot to explore the ‚ÄövÑv¿follow the energy‚ÄövÑvp philosophy in the search for extraterrestrial life. Along with water, life also has a universal need for energy to power biological metabolisms for cell metabolism, growth and reproduction. The third chapter of this thesis examines the minimal amount of PAR required for oxygenic photosynthesis and the potential for photosynthesis to occur on an icy ocean world. Using Enceladus as a case study, evidence is provided for the first time of oxygenic photosynthetic activity and growth occurring at 0.01 ¬¨¬µmol photon m\\(_{-2}\\) s\\(_{-1}\\) ‚ÄövÑv¨ the theoretical limit of photosynthesis - and shows that oxygenic photosynthesis is theoretically possible in discrete microhabitats on icy ocean worlds. The fourth chapter of this thesis investigates heterotrophic bacterial growth in an asteroid-like environment to answer whether the presence of organic compounds are a good indicator for habitability. Ceres is used as a case study to demonstrate that known earth-based lifeforms have the capability of surviving and growing in asteroid like conditions and utilising the organic substrates on offer. This suggests that microbial life on Ceres and other small-bodied celestial objects will not be limited by the chemical composition of the substrate but by other abiotic stressors. In the fifth chapter of this thesis, a proteomic analysis is conducted on a polar cyanobacterial strain exposed to far-red light wavelengths similar to those emitted by Red Dwarf stars. This was used to investigate the likelihood for oxygenic photosynthesis occurring beyond the solar system. The results demonstrate the first example of infrared utilisation for photosynthesis by an Antarctic cyanobacterium and suggests that infrared photosynthesis capability may be more prevalent than expected. In summary, sea ice microbial survival and growth were examined in simulated extraterrestrial conditions. In an effort to assay their tolerance to multiple extremes which enhances our understanding of the potential for life elsewhere in our universe. The results illustrate sea ice microbes have a remarkable capacity and plasticity to adapt to a range of alien conditions suggesting that a number of habitable abodes could exist within the galaxy. These habitats could be populated with a diverse microbial community structure with microorganisms utilising a number of metabolic pathways and ecological function.