Phototrophy is the use of light energy by various organisms with CO2 as the focus of carbon energy source. Oxygenic photosynthesis with the help of chlorophylls is one of the most important phototrophic processes. In oxygenic phototrophy, electrons flow through photosystem I and photosystem II. The process is often represented by the Z-scheme where the reduction potential of P680 in PS II is strongly electropositive that allows for the splitting of water into oxygen and electrons. The organism harvests light energy and uses it to convert P680 into a strong reductant to reduce pheophytin alpha. An electron from water is donated to the oxidized P680 to return it to a ground-state potential. The electron continues to increase positive potential and is eventually accepted by P700 to generate a proton motive force. Later on, the process is terminated with the reduction of NADP+ to NADPH. Of note, ATP can be produced in cyclic or noncyclic photophosphorylation.

In anoxygenic photosynthesis, PS I is utilized to use cyclic photophosphorylation and CO2 reduction from sources other than water. Cyanobacteria can use H2S as an electron donor and green algae can use H2. Of note, these processes are often less efficient than that of their oxygenic counterparts. However, they are useful for environments with limited or specific resources.

Phycobolins are light harvesting bilins often found in cyanobacteria, red algae, and glaucophytes. They are marked by colors and are bonded to certain water-soluble proteins. These bilins then pass the harvested light energy to chlorophylls for photosynthesis. The phycobilin is known for efficient absorption of the colors red, yellow, orange, and green which allows for supplementation of the chlorophyll’s weaknesses.