Photosynthesis is an incredibly complex process, but luckily for most aquarists only a basic understanding is required, but the time should be taken to understand that basic definition. Photosynthesis is the “synthesizing by organisms of organic chemical compounds (Oxford Dictionary of Biochemistry and Molecular Biology ), mainly carbohydrates, from carbon dioxide using energy obtained by light rather than the oxidation of chemical compounds”. Instead of consuming carbohydrates like we do for energy, photosynthesis can create carbohydrates for invertebrates and plants. More simply, this means that plant cells can use energy gathered from light to produce cellular chemical energy (ATP) and carbon products (carbohydrates) when combined with carbon dioxide.
Photosynthetic efficiency peaks at around a wavelength of 430 nanometers and 680 nanometers.
In order for the photosynthetic process to take place, the chloroplast (the organelle of the cell where the light energy to chemical energy conversion occurs) must receive sufficient PAR and/or PUR. If the saturation or compensation point of the chloroplast isn’t met, the organelle will not produce the optimum amount of carbon bi-products (carbohydrates), and this excess energy will not be transferred to the host invertebrate. Obviously, the compensation point is something every aquarist will want to meet at all depths in their aquarium.
In attempting to meet the compensation point, however, the aquarist must avoid photoinhibition. Photoinhibition is the result of an excess amount of light energy reaching the chloroplast, causing photosynthesis to stop completely! As we know, photosynthetic invertebrates have a host of light inhibiting pigments to protect themselves from tissue damage and their symbiotic Zooxanthellae sp. algae from photoinhibition, where is where all the pretty colors in corals and clams come from; because photoinhibition occurs much more frequently than insufficient light energy both in nature and in our aquariums.
Some aquarists believe that some corals only need light to survive, but this is absolutely untrue. No animal known to science can survive solely on light energy! There must always be a phosphate and nitrogen source for a living cell to create the compounds needed to function, even if these organic chemicals are simply dissolved in the water column. Photosynthesis is the process used to gain the necessary energy to convert these phosphate and nitrogen sources into ATP from light, which can be used directly by the cell. Photosynthesis does not ‘invent’ something from nothing – it is simply an energy conversion process, using light energy to turn compounds in the water column into more advanced and usable compounds for the algae, and in turn, the host invertebrate.