Except for bacteria, survival basically involves two processes that a creature made up of eukaryotic cells must entrust itself to: either breathing or capturing sunlight.
Ironically, it is thanks to metabolic innovations initiated by bacteria that we have air to breathe .
All animals based on eukaryotic cells (plants, animals, fungi) survive thanks to one of the two metabolic strategies mentioned: photosynthesis or aerobic respiration. But that is not what happens with bacteria, which are much more diverse than us, as Steven Johnson explains in his book The Ghost Map :
They consume nitrogen directly from the air, extract energy from sulfur, are capable of living at extremely high temperatures in the water of underwater volcanoes, and are concentrated by the millions in a single human colon.
Bacteria are the most widespread life form in terms of biomass, and they are also the most efficient organisms. Without them there would be no air we could breathe :
With the exception of a few unusual compounds (including snake venom), bacteria can process all the molecules of life, a quality that makes them an essential source of energy for the planet and its main recycling mechanism.
However, most microorganisms are heterotrophs (or more accurately chemoorganoheterotrophs), with organic compounds as sources of carbon and energy. Heterotrophic microorganisms live on food that they steal from living hosts (such as diners or parasites) or on dead organic matter of all kinds (saprophagi). Most pathogenic bacteria are heterotrophic parasites of humans or other eukaryotic species .
Bacteria have been around since the beginning . The rest of the more complex organisms only exist thanks to them. We are apprentices in the field of survival, and that is why we are less diverse, less adapted, less numerous, easier to eradicate if something changes in the ecosystem.
And despite that, not even our eyes are designed to be able to see them with the naked eye . In order to know of its existence we need technology, or that bacteria are concentrated in the order of ten million per milliliter of water (0.4% of a cup).