ELEMENTS OF LIFE

In the past 25 years there has been a revolution in our knowledge of the extremes to which life can adapt. By expanding our perceptions of what constitutes a habitable environment, such knowledge has transformed the search for life on the modern and ancient Earth and on other planets. Optimistically, we “follow the water”, carbon and sources of energy.

However, all organisms are comprised of a highly non-random selection from the menu of chemical elements beyond the macronutrients C, H, and O. While these elements comprise the greatest fraction of living biomass, the elements of life also include N and P, as well as a suite of metallic and non-metallic elements, such as Na, Ca, Mn, Fe, Cu, Zn and even Mo, which play critical structural and catalytic roles in living things. Collectively, these inorganic ingredients of life are called the “metallome”. Some of their chemical properties are so unique that it is a challenge to us to imagine biology without them.

This challenge is not easily solved by evolution, as evidenced by the fact that the distribution of life at the Earth's surface is closely tied to the distribution of these elements. Most spectacularly, vast areas of the Earth's oceans are biological deserts despite abundant water, carbon and energy; in these regions, life is limited by the availability of P, N or even Fe. There are some examples of alternative biochemistries that evolved to cope with the Fe scarcity that followed the rise of atmospheric oxygen, such as substitution of Cu for Fe in some photosynthetic pathways, but strategies to scavenge, store and recycle this scarce nutrient are at least as important.

A deeper understanding of the elemental requirements of life requires a quantitative understanding of the metallome. However, we know surprisingly little about the stoichiometric elemental requirements of biochemistry, the variability of these requirements among different types of organisms, and the “plasticity” of the metallome in the face of environmental pressures. Insights into the relative importance of key elements come from a combination of biological and geochemical field studies, laboratory experiments, and investigations into the coevolution of element availability and life in Earth history. Some of these studies and their implications will be reviewed.