The invisible world

by Alizée Le Moigne 

Right here, in front of your eyes live billions of organisms that you can’t see: The micro-organisms. The invention of the microscope allowed us to dive into this invisible world and scientists are exploring this continent that still abounds with surprises. Let me take you there for a short trip.

One drop of lake or sea water contains an important variety of micro-organisms, from viruses, bacteria, to unicellular algae and animals. Although we don’t see them, they transform the matter around us. The phytoplankton, analogous to plants, recycle the CO2 and produce oxygen. Some bacteria decompose the dead organic matter. The latest are eaten by other micro-organisms, unicellular animals called protists which are then eaten by bigger organisms to be finally ingested by fish. When fish die, they are decomposed by bacteria and the cycle starts again.

 

A group of Lake Zürich micro-organisms

Credit: Limnological station UZH

Just to give you an idea of how many micro-organisms surrounds us, there are 520 times more bacteria living in Lake Zürich than humans living on earth.

Microbial ecologists – scientists that study micro-organisms in their environment - want to find out why there are so many different micro-organisms and how do they live together.

Micro-organisms are organised in communities. Just like you can’t find tigers in the desert, you can’t find all micro-organisms everywhere. For instance some oceanic bacteria will not survive in the soil. Far to live in an isolated way, micro-organisms interact with each other. On the one hand, they can fight for the same resource, i.e. food or sunlight. Predation is also common in the invisible world, a paramecium can ingest more than 1000 bacteria a day! On the other hand, micro-organisms can help each other by sharing and trading food.

If you can’t find everything everywhere, it’s also because some micro-organisms can’t reach the site you’re looking at. Some bacteria are mobile, they actively swim toward what interests them, while some other are passive, just floating with the current.

Chance is also to be considered when you want to understand which micro-organisms live in a specific place. The random death of an organism can influence the ability of the whole species to reproduce. The importance of chance in the organisation of microbial communities is still not yet well understood.

Like all living form, micro-organisms evolve, and they do it faster than humans. They also appeared long before Human on earth. According to our current knowledge, prokaryotes, i.e. microorganisms without nucleus were the first living organisms to colonize our planet. Their long presence and fast evolution rate might explain the tremendous diversity of micro-organisms!

 Since the world has been entirely mapped and can be observed from space, some people think there is nothing left to discover on earth. Like the sailors of the 15th to 17th century, microbial ecologists explore an invisible world still rich in secrets.