Habitability of ammoniacal waters on icy moons and Earth

Abstract

The search for life has expanded to include the icy moons of Jupiter (Europa, Ganymede, and Callisto) and Saturn’s moons Titan and Enceladus. These moons feature surfaces encrusted in ice ranging from several to hundreds of kilometres thick, beneath which substantial subsurface oceans of liquid water are thought to exist. Liquid water is a prerequisite for life, and thus the habitability prospects of these oceans is speculated. Preservation of this liquid water is thought possible by freezing point depressants, such as ammonia.

Indeed, the Cassini-Huygens mission revealed not only active cryovolcanism on Enceladus but also the presence of ammonia. On Earth, ammonia facilitates biotic chemistry at low concentrations and is a common pollutant from agricultural and industrial processes. As a proton acceptor, elevated concentrations of ammonia are known to disrupt biological chemistries. The presence of ammonia in extraterrestrial oceans, as well as terrestrial ecosystems, could therefore constrain the habitability prospects of these environments.

In this thesis, I explore whether the presence of ammonia could impact the potential for habitability in icy moon oceans, with additional implications for the habitability of Earth environments. I use growth dynamics and cellular viability assays to establish the growth response and cultivation limits of the extremophile Halomonas meridiana Slthf1 in concentrations of aqueous ammonia relevant to the oceans of Enceladus, Titan, and Europa. Through these approaches, I also examine the growth impacts of indirect ammonia exposure occurring by volatilized ammonia gas. The morphological and physiological changes exerted by ammonia on H. meridiana are additionally examined by transmission electron microscopy and metabolomics. Through this research I show that aqueous ammonia exposure, either as dissolved ammonia gas (NH3) or as a salt (NH4)2SO4), can extend lag phase duration and doubling time, slow growth rate, diminish cell density and reduce cell viability, even in cultures indirectly exposed to NH3 by volatilization. I elucidate that exposure to ammonia can disfigure cell morphology and elevate the occurrence of cell lysis events. I present evidence that ammonia toxicity is distinct from external pH toxicity and could be encouraged by internal and potentially destructive NH3-driven reactions. Toxicity of ammonia may also be driven by modulation to essential nitrogen, carbon, and energy metabolism.

Possible survival strategies, such as cell wall remodelling, were indicated by metabolomics. The results demonstrate that at specified molar thresholds, ammonia can impose constraints on growth, viability, and the metabolism of H. meridiana. This data cannot suggest whether icy moons oceans are or have been inhabited but can provide a foundation for which to assess the potential for habitability. The molar concentrations at which the outlined effects occur exceed the putative ammonia concentrations in the oceans of Enceladus and Europa. Based on this evidence, it is plausible dissolved or volatilized ammonia in these environments may not pose as a limiting factor for habitability. For Titan, the ammonia content of the interior ocean ranges to as high as 15%.

High accumulations of ammonia from agricultural and industrial sources are also possible on Earth. In the case of these higher concentration thresholds, the results of this research indicate ammonia could constrain the habitability potential of both Titan’s ocean and certain Earth environments. These findings advance the current understanding of bacterial life in ammonia and demonstrate the importance of ammonia concentration when assessing conditions that could support life in extra-terrestrial and terrestrial environments.