Photosynthesis on a planetary scale

Feeding the biosphere and oxygenating the atmosphere

Photosynthesis does not only feed the plant: it feeds, directly or not, almost all living things.

The entry point of energy

Every food chain begins with a producer:

   SUN
     |
     v
  [ producers ]  grass, algae, phytoplankton         <- photosynthesis
     |
     v
  [ consumers I ]  herbivores
     |
     v
  [ consumers II ]  carnivores
     |
     v
  [ decomposers ]  bacteria, fungi

Each arrow loses about 90 % of the energy, dissipated as heat. This is why food chains rarely exceed four or five links: beyond that, not enough energy is left to feed another level.

It is also what makes a meat-based diet costly in farmland: producing one calorie of beef takes roughly ten times more plant matter than eating one directly.

Atmospheric oxygen

Earth's early atmosphere contained no free oxygen. About 2.4 billion years ago, cyanobacteria — the first photosynthesisers — began releasing it in bulk.

Before:  atmosphere without O₂, anaerobic life only
         |
         |  cyanobacteria -> photosynthesis -> O₂
         v
After:   Great Oxidation      -> mass extinction of anaerobes
                              -> rise of aerobic respiration
                              -> formation of the ozone layer

This episode, the Great Oxidation, was the first planetary pollution in history: a metabolic waste product, toxic to the life of the time, that transformed the planet. It also made aerobic respiration possible — far more efficient — and hence, eventually, complex organisms.

Fossil fuels

Coal, oil and gas are organic matter produced by photosynthesis hundreds of millions of years ago, buried before it could decompose.

photosynthesis (300 Ma ago)  ->  organic matter
                             ->  burial, pressure, time
                             ->  coal, oil, gas

combustion (today)           ->  CO₂ returned to the atmosphere

Burning a fossil fuel means releasing in an instant carbon that photosynthesis removed from the atmosphere over millions of years. It is the most direct physical description of current climate disruption: a mismatch in speed between very slow storage and very fast release.

A carbon sink still at work

Even today, photosynthesis removes about 120 billion tonnes of carbon from the atmosphere each year. The oceans contribute as much as forests, via phytoplankton — often forgotten, it accounts for nearly half of global photosynthesis.

This sink absorbs roughly a quarter of human emissions. It is not enough to offset them, but without it atmospheric CO₂ would rise far faster.

Summary