Iron-Based Metabolisms at Icy Moons
What pathways could life use to survive in the oceans of moons like Enceladus?
IJA 2023 Paper đź“„ 
Enceladus, Saturn’s sixth largest moon, has gained profound interest amongst astrobiologists in the search for life beyond Earth owing to its thick liquid water ocean located beneath its icy crust. Chemical reactions between its rocky seafloor and the ocean above are thought to create hydrothermal vent systems akin to those made by similar processes on Earth, such as the famed Lost City discovered in 2000. On Earth, these hydrothermal vent systems harbour vast and diverse communities of life and have been proposed as a possible setting for the origin of life. If this happened on Earth, why not elsewhere in the solar system too?
Enceladus’ ocean is being spewed out into space from giant cryovolcanic plumes at its south pole. The Cassini spacecraft flew through these plumes during its exploration of the Saturn system in the 2000s and 2010s and was able to measure the composition of Enceladus’ ocean, allowing us to gain a clearer picture of the processes operating in its interior. All necessary ingredients for life have been detected at Enceladus in sufficient amounts. The detection of H2 and CO2 in the plumes – ingredients for a form of microbial metabolism known as methanogensis – sparked debate as to whether microbial life could survive and flourish around the hydrothermal vent systems expected at the moon’s seafloor.
On Earth, microbes can use a suite of different redox-sensitive elements for their metabolism, yet whether any of these other pathways could be viable at Enceladus as well as the proposed methanogens has received limited attention. Microbial respiration of iron is the primary control on the mobility and reactivity of iron in the environment on Earth, and can be biotically transformed between its soluble ferrous – Fe(II) – and insoluble ferric – Fe(III) – states via a variety of different pathways, one of which involves H2. Some of the earliest forms of life on Earth are thought to have used iron for their metabolism. Iron is expected to be present inside minerals that make up Enceladus’ rocky seafloor, and since H2 has been detected in its plumes, could microbial life use these ingredients for its metabolism?
During my final years master’s project working with Dr James Byrne, I aimed to answer this question. I performed a series of experiments to investigate: (a) could Fe(III)-reducing bacteria access iron in iron-bearing minerals for their metabolism?; (b) does Enceladus’ ocean contain the necessary ingredients in sufficient amounts to allow Fe(III)-reducing bacteria to grow?; and (c) can microbial Fe(III) reduction operate efficiently enough under the temperature and pH of Enceladus’ ocean to allow life to survive?
To cut a long story short, the answer that I found to these questions was yes, yes, and yes! I showed experimentally, for the first time, that microbial life could use a different metabolic pathway to survive and flourish in Enceladus’ ocean. Since other icy bodies across the solar system are thought to have subsurface oceans beneath their icy crusts – such as Jupiter’s moon Europa, Neptune’s moon triton, and even Pluto – meaning it is possible that iron reduction could be a viable, and potentially common, metabolic pathway for life in other places as well.
Project image credit: NASA/JPL-Caltech
