Deep Life The Hunt for the Hidden Biology of Earth, Mars, and Beyond

One of the more exciting developments in biology in recent decades is the full-blown realisation that there is an enormous biosphere underneath our feet that rivals the surface world in biomass. With the recent publication of Karen G. Lloyd's Intraterrestrials, I decided this was a great opportunity to finally read Deep Life that I bought over seven years ago. In it, the late Tullis C. Onstott gives an in-depth account of research into the subsurface biosphere over the 23 years from 1986 to 2009, with his own entry into this field happening in 1992. It is equal parts geomicrobiology and science history, told as a chronological narrative of the research he was involved in.

Let me delve into the geomicrobiology first, as that is really why I wanted to read this book. As was true of early research on ancient DNA, the concern of contamination was a major stumbling block and an important hurdle to clear for this field to really take off. The first few chapters thus spill quite some ink walking you through the so-called quality assurance and quality control protocols they developed. After convincing critics and themselves that the microbes they were finding were indigenous to the subsurface and not introduced during retrieval and handling, the book can pivot to describing these weird and wonderful extremophiles. One question that occupies Onstott and his colleagues throughout this book is where these microbes get their energy from, as it cannot be from photosynthesis. Over time, evidence starts stacking up in favour of radiolysis. This is the process whereby ionising radiation produced by decaying radioactive material cleaves the bonds of nearby molecules. This could produce some of the hydrogen, oxygen, sulfate, and other nutrients these microbes feed on.

Equally remarkable is that these microbes live life in the slow lane, dividing once every few centuries or much less, potentially resulting in lifespans of a million years or more. This is where you start approaching life's edge, with some of the microbes they are retrieving chronically starved and much reduced in size, or in stasis with metabolisms ground to a halt. Movement similarly happens slowly as infiltrating groundwater moves through rock pores on geological time scales.

One thing to keep in mind, which Onstott does not make explicit as they search for microbial life in the ultra-deep gold mines in South Africa, is that this research often relies on indirect evidence. The water they gather when the miners breach water-bearing geological fractures is checked for genetic material using an approach known as metagenomics. The advantage is that you do not have to grow microbes in culture, meaning that many species that were previously missed or hard to grow in the lab have now been identified. The disadvantage is that you do not have to grow microbes in culture, meaning that we know very little about them beyond their identity: we have effectively found their calling card, but most have never been observed under the microscope.

A final notable aspect is what these microbes are capable of collectively. Some of the work Onstott initially did under the banner of the US Department of Energy focuses on bioremediation of e.g. radioactive and chemical waste. In South Africa, there similarly is the hope to use microbes to clean up the toxic legacy of a century of heavy-metal mining. And then there is the astrobiology angle. NASA hopes to learn from our attempts at exploring Earth's subsurface biosphere for finding life elsewhere in the universe. The last chapter discusses how some microbes might be responsible for cave formation, with limestone being corroded from below by sulfuric acid that is a byproduct of sulfur-oxidising bacteria.

At 486 pages, Deep Life is a big book, and the gotcha is that you have to wade through a lot of background material. The above insights are not explicitly summarised and presented to you, but surface in bits and pieces as you follow Onstott on his research journey, with the book written as a blow-by-blow, year-by-year eyewitness account. He goes into great detail on the practical obstacles and research questions he and his collaborators wrestle with and how they find solutions, or fail to. I hope you like engineering details on drilling and mining, because there is plenty of both here! He talks at length about his colleagues, the conferences and workshops he attends, the organisations he works for, the companies he works with, and the places he works in. Though not intended as a history of the discipline, elements of this are woven throughout with two appendices giving chronologies. There are numerous abbreviations for chemicals, methodologies, research programmes, and organisations sprinkled throughout the text, and you will want to bookmark the 3-page list at the beginning. Additional details on methods, people, and places are provided in 81 pages of endnotes, with individual notes sometimes running to two pages.

In short, Deep Life provides an insider's perspective of this kind of research, showing you how the sausage is made. Conceivably, some readers might get frustrated and wonder where the editor was. I am a pretty tolerant and patient reader and instead found myself fascinated by some of the engineering details. Notably, Onstott is unapologetically candid about his own ignorance. Given he was trained as a geologist, getting to grips with microbiology and genetics was a long and hard intellectual journey, and he openly writes about the moments where he felt clueless. If I have to criticise him, it is for a certain naivety I noticed. Much of this research rides on the coattails of extractive industries which are in the costly business of venturing underground. It does feel a little incongruent when in Nunavut, he wistfully observes that "it was reassuring that parts of the planet still existed that had not been completely overrun by the resource extractors" (p. 311) when he is literally working in a mining concession. I do not think this makes such research morally "wrong", but do not expect much in the way of Onstott reckoning with how the two are entwined.

Overall, though I found Deep Life< interesting, it will likely test the patience of some readers. This is less a book about geomicrobiology and more a book about *doing* geomicrobiology. Onstott is a knowledgeable guide who was clearly deeply embedded in the research community while helping to advance this field by leaps and bounds, but the intense focus on the daily realities of such research threatens to smother the otherwise fascinating subject matter. With Deep Life charting research up to 2009, I am next turning to Intraterrestrials to fill me in on more recent developments.