Algal Mechanisms

In over 85% of plant species, the carbon dioxide (CO2) that enters a leaf is converted by the enzyme Rubisco into a carbohydrate made up of three carbon atoms, known as 3-phosphoglycerate (PGA).  These plants are called C3 plants, and include rice, cassava, all legumes and wheat.  Rubisco is an inefficient enzyme because it cannot distinguish between CO2 and oxygen molecules.  Around 35% of the time, Rubisco binds with oxygen instead of carbon dioxide, resulting in wasted energy and reduced photosynthesis.

Many algae have overcome the limitation of Rubisco with the formation of an organelle called a pyrenoid, which concentrates carbon dioxide around Rubisco.  Concentrations of carbon dioxide near the site of Rubisco are so high that oxygen cannot bind with the enzyme, thereby suppressing photorespiration.  Photosynthesis in our crops takes place in small organelles within the cells of leaves, called chloroplasts, which evolved from algae.  Our mathematical modeling suggests that a large increase in photosynthesis could be achieved by re-engineering the pyrenoid apparatus back into modern plant chloroplasts.  RIPE is attempting this re-engineering. 

Obstacles to implementation: because many proteins are required to form a pyrenoid, this is a particularly high risk strategy, but one that could pay maximum dividends.