Global carbon cycle

Back to main index

The most important cycle in the biosphere is that of carbon, because it is linked to the way in which organisms use energy and make, break down or recompose the carbon compounds that constitute them. Classically, a distinction is made between primary producers or prototrophs that use light or chemical energy to fix carbon dioxide in the form of organic molecules, and consumers or heterotrophs that use organic carbon compounds produced by prototrophs as their source of energy and carbon. Primary eukaryotic producers are unable to utilize the variety of materials and energy sources that prokaryotes are able to. Eukaryotes only use light as a direct primary source of energy, since there is no known chemo-autotrophic eukaryote that uses geochemical energy directly (Box 24).

Eukaryotic protists participate in carbon fixation either directly through the photosynthetic activity of algae, or by interacting positively or negatively with primary producers, in particular with plants. Carbon is released in the form of carbon dioxide by all the organisms that breathe in aerobic ecosystems or that ferment in anaerobic ecosystems. The role of eukaryotic microbes in this restitution is crucial in terrestrial ecosystems because plants there manufacture lignocellulose, a complex mixture of polymers, which is only efficiently degraded by Dikarya fungi, even if eubacteria, Streptomycetes, and protists also participate in its enzymatic degradation.

An estimate of the carbon levels in various terrestrial environments, as well as the flow between environments are given in Figure 353. In this diagram, it is simple to see that the terrestrial and oceanic ecosystems participate in an equivalent way in the carbon cycle: about 43% of trade takes place in the oceans and 57% in terrestrial ecosystems. It should also be noted that the flows are important with regard to the available carbon reserves in the form of CO₂, which amount to 800 gigatonnes of carbon. Indeed, with a total flow of around 210 gigatons of carbon per year and a flow of 120 gigatons in terrestrial ecosystems alone, they would disappear in a few years if the plant biomass accumulated in the form of lignocellulose was not recycled! Note that the human contribution seems minor, 9 GtC against more than 210 GtC for the rest of the biosphere; mushrooms alone would release 85 of the 120 GtC from terrestrial ecosystems! However, humans introduce additional carbon into the atmosphere from fossil reserves and therefore upset flows that were in equilibrium. What will be the new point of equilibrium and with what consequences is still debated. Nevertheless, the entire scientific community agrees that the global warming currently observed is partly due to the modification of these carbon flows, in particular the accumulation of carbon dioxide in the atmosphere. Note that our time is not the only one in the history of the earth that has seen global ecological upheavals linked to biological inventions that have made it possible to use energy in different ways (Box 25).

Back to chapter index