In the aquarium, plants are responsible for preforming several action which are beneficial to the overall water quality and in turn to our fish. In order to accomplish nutrient uptake, oxygen production and carbon fixation plant first have to harness and convert the energy coming from the light above. This process is called the light dependent reaction which happens only during the photo period, or the time in which your aquarium lights are on. 

 

          Plant cells located on epithelial layer of leaves contain several small organelles called chloroplasts which have been determined to be evolutionarily related to cyanobacteria. Inside each chloroplast is yet another structure called a thylakoid. It’s another membrane enclosed system that houses all the machinery necessary for this first step in photosynthesis. Let’s take a journey closer to the thylakoid membrane where we can get a better understanding of how the light reaction works.

 

          Embedded within the membrane of the thylakoid are two distinct photosystems that work simultaneously, but with their own agenda. Both photosystem 2 and 1 house chlorophyll a and b molecules whose job is to harvest and pass light energy to a special reaction center chlorophyll. Now, the type of available light is important. Both chlorophylls absorb light in the blue and red portion of the photosynthetically active range that falls within the visible light spectrum. This photosynthetically active radiation or PAR light spans from about 400 to 700 nm and again the chlorophyll found in plants will primarily absorb the blue and red light present in this spectrum. This is why full spectrum and or 6500K bulbs are most common on planted tanks.

 

          Photosystem 2 uses light to excite a pair of electrons that are passed on to an electron transport chain.  h20 molecules are split into free H+ ions and oxygen effectively replacing those electrons, continuing the cycle and creating the essential proton gradient or (proton motive force) across the membrane of the thylakoid. Because the protons on the inside will be in a higher concentration than the outside, they will naturally diffuse out by passing through a molecular machine called ATP synthase which converts ADP into ATP, the energy currency of the cell. 

 

          Remember those electrons that went into the transport chain? They’re passed thought the membrane by various carrier proteins, ending up at photosystem 1. Light is used yet again here by chlorophyll molecules as a way of re-exciting those electrons so they can be utilized in reducing NADP+ to NADPH a compound that plays a vital role in many biosynthetic processes including carbon fixation. 

 

          At the end of this process the plant has generated ATP, NADPH and oxygen all from the power of light energy. The Oxygen is released into the environment and is also shuttled down the plant to the roots where it’s used to prevent necrosis that would otherwise take place in the anaerobic substrate. The NADPH and ATP will be used by the plant in the Calvin Cycle where CO2 is converted into sugars which end up driving cellular respiration and even more ATP generation as well as carbon for assimilation. This will allow the plant to growth and function the way we want in our aquarium.

 

          Optimizing the lighting can be really important in your planted aquariums because without the energy produced as a result of the light reaction, your plants won’t be able to assimilate as much carbon and will therefore consume less of the left over nutrients that cause algae problems. Of course the other half of the photosynthesis equation is often more crucial in the home aquarium so Next time we’ll go over the Calvin Cycle in detail and discuss why having available CO2 can be extremely effective in heavily planted aquariums. 

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