Introduction

Christophe THOMAZO1 and Karim BENZERARA2

1 Laboratoire Biogéosciences, University of Burgundy, Dijon, France

2 IMPMC, UMR 7590, CNRS, Sorbonne Université, Paris, France

The division of the Earth into compartments, each with their own dynamics but still interacting with one another, is a key principle in biogeochemistry, which aims to understand how the Earth functions and how it has evolved over geological time, as well as to predict how it will evolve in the future. These compartments are called "spheres" and can be delimited differently and at different scales depending on the questions asked (Figure I.1). We regularly see the terms atmosphere, hydrosphere, lithosphere or biosphere as constituents of the Earth system1. At smaller scales, we can also consider the troposphere and the stratosphere when we are more specifically interested in the functioning of the atmosphere, or the lithosphere and the asthenosphere for the study of plate tectonics. At other scales, we can define the pedosphere when studying soils. Reference to the anthroposphere designates the compartment influenced by anthropogenic activities. In this book, we understand the geosphere as the set of solid, gaseous and liquid compartments composing the Earth. Thus, the geosphere brings together the oceanic and continental lithospheres, the even more internal parts, namely the mantle and the core as well as the most superficial parts constituting the hydrosphere and the atmosphere. The term biosphere, a concept introduced for the first time in the natural sciences in 1875 by Eduard Süß, professor of paleontology at the University of Vienna, designates all living beings.

Figure I.1. A representation of a biogeochemical cycle (e.g. of carbon but it could be for many other chemical elements) involving different reservoirs and in particular the geosphere and the biosphere, all making up the Earth system. The geosphere is itself made up of different reservoirs such as the atmosphere, the hydrosphere or the lithosphere. The biosphere is a reservoir in itself, but living organisms also catalyze transfers between reservoirs.

The existence of interactions between these two compartments has been suspected since the 16th century, when debates on the origin of oil, used for more than 3,000 years, began. It was Andreas Libavius, a German chemist and doctor, who, in 1597, was one of the first to suggest an organic origin of petroleum corresponding to ancient tree resin. In 1757, Mikhail Lomonosov clarified this theory and proposed that liquid petroleum and bitumen came from the transformation of plant organism remains at depth, under the effect of increasing temperature and pressure. Joint observations using geological (oil found in the center of sedimentary basins) and geochemical approaches (the isotopic composition of oil is the same as that of the surface biosphere, and all oils contain very specific molecules called porphyrins, which derive from chlorophyll) confirmed four centuries later the hypotheses of Libavius and Lomnosov on its biological origin. However, it was Vladimir Verdnasky, an illustrious Russian scientist in his time (1863-1945), who theorized these interactions between the geosphere and biosphere by emphasizing the role of life (and human beings) as a geological force (Verdnasky 1986). These ideas formed the foundations of the discipline, biogeochemistry, which has since continued to develop.

When the two editors of this book were studying at French universities, courses combined life sciences and Earth sciences, with the particular objective of training secondary school teachers. This coupling seemed to come directly from the traditional alliance of the natural sciences, which had generated much knowledge in both biology and geology in the era of Jean-Baptiste Lamarck, Georges-Louis Leclerc de Buffon, Georges Cuvier or Charles Darwin. However, specialization in scientific research further separated biology and geology and a few rare lessons in ecology and paleontology made it possible to note the obvious links between these two scientific fields.

The influence of the geosphere on living organisms generally seems the most obvious. In ecology, the concept of fundamental ecological niche is associated with the surrounding abiotic conditions, themselves partly imposed by the geosphere. The biotope is a geographical area defined by relatively homogeneous physical and chemical properties. Associated biotope and biocenosis form an ecosystem. There is a roughly latitudinal distribution of large vegetation covers on the Earth's surface, from tundra and taiga at high latitudes to tropical forests, including temperate forests and savannahs. This distribution is controlled by the climate, itself controlling the properties of the soil, which imposes local living conditions. Few places seem uninhabitable on Earth. Indeed, since the 1960s, we have known about ecosystems developing in hydrothermal environments at the bottom of the ocean in the absence of light. The temperature limit of life appears to be approximately 120°C, which can be reached in environments under a relatively high pressure such as at the bottom of the oceans. We also find traces of life in the coldest deserts in Antarctica or the hottest and driest deserts such as the Atacama. Only a few hypersaline, hyperacidic and (hyper)hot lakes in Ethiopia do not seem to support life.

On the other hand, the influence of the biosphere on the geosphere may seem less obvious at first glance. Indeed, the difference in scale (in size and over time) between organisms such as bacteria and a mountain makes it difficult to imagine how the former could influence the latter. And yet, in the 1960s, James Lovelock and Lynn Margulis jointly developed a revolutionary idea, outlined in the Gaia hypothesis (Lovelock and Margulis 1974). This hypothesis suggests that Earth's living organisms collectively regulate the planet's conditions, including the chemical composition of the atmosphere, creating a stable and hospitable environment for life. They thus envisaged the Earth as a self-regulating system, in which life itself influences the geosphere, and in particular, climate.

Although such a global interaction between the biosphere and geosphere remains debated, we will see in the first chapter that the influence of the biosphere on the geosphere can be studied on a more local scale, notably through mineral dissolution processes. This process ensures the cycling of chemical elements, including those essential to life, releasing them into the soil, whose fertility is thus impacted. Despite all of our knowledge, predicting the dissolution rate of minerals is still difficult and although the influence of life on these rates has been determined in certain cases, quantifying it on a global scale is a current issue. A large quantity of these chemical elements released by minerals will, after having transited or not through the biosphere, reach the hydrosphere (rivers, lakes, streams and oceans). There, they can be re-trapped in de novo minerals and participate in redistributing matter, sometimes by creating resources of interest to humans, such as iron or phosphorus deposits, for example.

The second chapter addresses another action of the biosphere on the geosphere: the formation of minerals by living beings or biomineralization. We will see that from a very early point in the Earth's history, organisms have impacted the formation of minerals. On the one hand, this has had repercussions on the functioning of living organisms and the history of life. But in return, biomineralization also seems to have played a role in the evolution of the chemical functioning of the Earth's surface by notably lowering the concentration of certain chemical species in the oceans.

Stromatolites are emblematic examples of rocks formed by life. These "living stones" have been observed in geological records as old as 3.5 billion years. The third chapter will explain the mechanisms involved in their formation, and the information obtained from the study of ancient stromatolites will provide knowledge about the history of life. Stromatolites are mainly formed by diverse microbial ecosystems that transform their immediate chemical environment. The identification of these microbes and their geochemical functions through their metabolisms requires the use of molecular biology methods, whose methodological progress over the past decades has revolutionized our knowledge of the diversity of living organisms. The fourth chapter will explain this approach as well as how we can obtain valuable information about the history of life from studying the diversity of living beings today.

The most obvious manifestation of the role of life in the transformation of the Earth throughout its history is the protracted oxygenation of the atmosphere, which remarkably expanded during the Great Oxidation Event, nearly 2.3 billion years ago and has allowed over the last 600 million years for the development of sulfate-rich oceans as well as the development of eukaryotes, including plants and animals. This chemical revolution followed a biological innovation: the appearance of oxygenic photosynthesis. The fifth chapter will detail how this event became engrained in the geological record, how it was dated and will specify the links between this biological innovation and the chemical evolution of the geosphere. Beyond the provision of a new molecule abundant in the atmosphere (the dioxygen molecule), this event profoundly disrupted the chemical functioning of the Earth's...