ABSTRACT
Besides being fundamental for the growth of plants, algae, and related food webs, photosynthesis is a key ecological process for another reason: it can release oxygen. As detailed below, photosynthesis requires an electron donor to take place; in the most common type of photosynthesis, such electron donor is water, and this leads to the production of oxygen according to the following overall reaction:
6 CO 6 H O Photon energy C H O 6 O2 2 6 12 6 2+ + → + (13.1)
Equation 13.1 shows how photosynthesis modified the chemical composition of the earth’s atmosphere, over billions of years, by producing free oxygen whose concentration allows the survival of aerobic organisms representing the most common life forms on earth. It also highlights that photosynthesis is tightly connected to the biogeochemical cycles of oxygen and carbon (Chapter 12) and thus becomes a key process to understand
the ongoing climate change. Besides the oxygenic (aerobic) photosynthesis exemplified by Equation 13.1, anoxygenic (anaerobic) photosynthesis is also possible, although it is less common and can be inhibited by the presence of oxygen. For example, green-purple sulfur-oxidizing bacteria can use H2S as an electron donor instead of H2O, and thus they do not release oxygen during photosynthesis.
