Where are all the microbes?

 

This week, our Research Associate, Dr. Haley Sapers, introduces us to the enormous hidden world of microbes all around us. Studying these organisms, the niches they inhabit, and the strategies they use to survive provide clues to the adaptability of life writ large. That, in turn, helps us to understand what kinds of planetary environments might be clement to some form of life. Above, microbes from 2.8 km below the surface of our world. (image credit: Luc Riolon, https://commons.wikimedia.org/wiki/File:Candidatus_Desulforudis_audaxviator.jpg CC-BY-SA-2.5)

by Dr. Haley Sapers

If I asked you where most of the life on Earth was, you would probably tell me it’s all around us. On the surface in forests and jungles, in the oceans around coral reefs and out there swimming around as whales and sharks. And you wouldn’t be wrong.

Macroflora and fauna – that is the large plants and animals that we can see with the unaided eye – have a lot of mass. Plants alone are massive. The cumulative weight of plant life on Earth accounts for a whopping 450 Gt (450 billion tons) of carbon. To put that in perspective, all of the cars in the world only weigh in at about 2.5 billion tons. And because of their large mass, plants and animals comprise most of the biomass on Earth. But mass isn’t the whole story –  the total number of all living organisms that we can see pales in comparison to the extraordinarily high numbers of microbes that inhabit the Earth.

A Phylogenetic Tree of all life on Earth showing relationships between large groups of organisms. Bacteria are in Blue, Archaea in Green and Eucaryotes appear in red.
(image by TimVickers https://commons.wikimedia.org/wiki/File:Collapsed_tree_labels_simplified.png)

There are 3 domains of life; the domain we, and all plants, animals, fungi, and insects are part of is called Eukarya. Prokaryotes, or “Microbes”, as they are colloquially known, form the Bacterial and Archaeal domains. Although bacteria and archaea are both microscopic, they are as different from each other as E. coli is from us! There are about 1030 individual bacterial and archaeal cells on Earth (that’s 1 nonillion or 1 thousand billion billion billion!) To throw a few more astonishingly large numbers out there, there are only an estimated 10 to the power of 24 or 1 quadrillion stars in the Universe or a measly 10 to the power of 21 (one thousand trillion) grains of sand on all the beaches and deserts of the Earth. All those thousand billion billion billion cells weigh in at approximately 77 Gt of carbon. The ~10 to the power of 10 (10 billion) people on Earth only comprise 0.06 Gt of carbon or less than 0.1% of the weight of the microbes.

So, where are all those microbial cells?

You might be surprised to learn that you’re only half human. Of all the cells that are part of your body, about half of them are microbial. They live in our mouths, stomachs, intestines, and skin (among other places…).  Don’t be alarmed – we need all these microbes – in fact, we wouldn’t be able to get any nutrients from our food without them. But if each of the 10 billion people on Earth are home to 10 to the power of 14 microbial cells – we still only end up with 10 to the power of 24 microbes, 6 entire orders of magnitude short! In 2018, a group of scientists decided to count up all the life on Earth and figure out where most of it is. There’s a great (free) publicly available book that looks at a bunch of biological statistics authored by the same group (http://book.bionumbers.org/). 

But back to where the microbes are.  

All that macroflora and fauna that we see around us every day (us included) is in some way dependent on the sun for energy. Life on the surface of the Earth is fueled by the sun, and all life needs energy. So where else could life be? It turns out almost all of those bacterial and archaeal cells (over 95% of them) are actually living deep in the Earth’s subsurface, far, far away from the energy of sun. How is that even possible? That’s actually a really good question, and one many scientists are still trying to figure out. 

There are many different metabolic strategies, or ways for life to get energy. Getting energy from the sun, or consuming other organic matter are only two – perhaps the most common to us, but by far not the most common considering the vast diversity of life on Earth. There are bacteria, for example, that ‘breathe’ iron the same way that we breath oxygen. The iron provides a different electron dumping ground instead of oxygen in anaerobic (or oxygen-free) environments. Many of the microbial subsurface dwellers use strategies like this, gaining energy directly from rocks in a metabolic process known as chemoautolithotrophy (chemo = chemical, auto = self, litho = rock or the self production of chemical energy from rocks). They’re literally living geo-electrical circuits! 

In fact, these seemingly strange metabolisms may have been the first to evolve on Earth (much before photosynthesis, or the ability to harvest energy from sunlight). The very first life on Earth may have been similar to the microbes that now live deep in the Earth’s surface. Because of the diverse energy harvesting strategies that subsurface microbes use and the evidence to suggest that these are some of the earliest metabolisms to have evolved, it’s possible that the subsurface of other planets such as Mars are also habitable in the same way. Who knows – maybe there are even microorganisms living off rocks in the deep subsurface of Mars today!