<P> Early photosynthetic systems, such as those in green and purple sulfur and green and purple nonsulfur bacteria, are thought to have been anoxygenic, and used various other molecules as electron donors rather than water . Green and purple sulfur bacteria are thought to have used hydrogen and sulfur as electron donors . Green nonsulfur bacteria used various amino and other organic acids as an electron donor . Purple nonsulfur bacteria used a variety of nonspecific organic molecules . The use of these molecules is consistent with the geological evidence that Earth's early atmosphere was highly reducing at that time . </P> <P> Fossils of what are thought to be filamentous photosynthetic organisms have been dated at 3.4 billion years old . </P> <P> The main source of oxygen in the Earth's atmosphere derives from oxygenic photosynthesis, and its first appearance is sometimes referred to as the oxygen catastrophe . Geological evidence suggests that oxygenic photosynthesis, such as that in cyanobacteria, became important during the Paleoproterozoic era around 2 billion years ago . Modern photosynthesis in plants and most photosynthetic prokaryotes is oxygenic . Oxygenic photosynthesis uses water as an electron donor, which is oxidized to molecular oxygen (O 2) in the photosynthetic reaction center . </P> <P> Several groups of animals have formed symbiotic relationships with photosynthetic algae . These are most common in corals, sponges and sea anemones . It is presumed that this is due to the particularly simple body plans and large surface areas of these animals compared to their volumes . In addition, a few marine mollusks Elysia viridis and Elysia chlorotica also maintain a symbiotic relationship with chloroplasts they capture from the algae in their diet and then store in their bodies . This allows the mollusks to survive solely by photosynthesis for several months at a time . Some of the genes from the plant cell nucleus have even been transferred to the slugs, so that the chloroplasts can be supplied with proteins that they need to survive . </P>

What two substances are also taken in by the plant for photosynthesis to occur