Algae CO2 Capture Mechanism
Week 1 Algae Update
TLDR:
The enzyme ribulose bisphosphate carboxylase (RuBisCO)takes the CO2 coming into the plant and activates a reaction with to ribulose bisphosphate (RuBP) to start the Calvin Cycle (second stage of photosynthesis).
Algae capture CO2 through photosynthesis. The explanation below is based on resources similar ideas are in the paper, and found a similarly high-level explanation on Khan Academy.
High-Level Definition
Photo — refers to light
synthesis — the process of building compounds from simple substances — in this case, the process of making sugars — glucose — for plants.
Essentially plants are taking light and creating their food (glucose) to help grow and strengthen the plant.
How Plants Get Their Colour and Store Energy:
Chlorophylls and carotenoids are the two major classes of photosynthetic pigments found in plants and algae. Chlorophyll’s role is to absorb light for photosynthesis, while carotenoids dispose of excess energy, otherwise, it would damage the plant.
Based on this paper (+ the ones cited within it) it seems that red light is the best for maximizing the creation of biomass + protein. More biomass = more carbon need = more carbon captured.
More protein = more stuff to sell.
Light Dependent Reactions (Photosystem ll)
Inside the chlorophyll (what absorbs light energy) there is a chloroplast, which is where photosynthesis happens.
When Photosystem 2 (PSII) —named #2 because it was discovered second— receives the photons from the light energy from the sun (which can be replaced by red LED lights). To activate the PSII and PSI the energy needs to be within red light (625-740 nano-meters) because that is the light that produces the most biomass and protein which are the 2 main products that chlorella Vulgaris produces.
Energy levels than those represented by red light are sometimes insufficient to activate the electrons to start the process of photosynthesis — leading to slower growth and less protein biomass, and less carbon captured.
The light transfers through the pigment molecules until it reaches the reaction center, then it activates the electron — which later transfers to Photosystem II (explained later). When photosystem II loses its electron, P680 turns into P680+ —known as the strongest oxidizing agent in biology. An oxidizing agent refers to removing from an atom or molecule. In this case, the molecule being oxidized is water (H2O). This results in the water losing one electron and ending with 1/2 an O2 (the same as 1 oxygen) and 2H+ (which refers to 2 hydrogens with 1 less electron — which was the one lost to P680+.
Once this electron is in PSII it goes into two special chlorophyll molecules in the reaction center that have the electron interact with the light energy. This creates the excited electron which then goes into the primary electron accepter which acts as a place holder. As the electron leaves the primary electron acceptor. Then P680 turns into P680+ and then you get low energy electrons from the water bonds like described above.
In the image below you can see the electron transport chain (ETC) from PSII to PSI. You can see the transport chain of the electron from plastoquinone (Pq) to cytochrome complex (CYT) to plastocyanin (Pc). Throughout the ETC energy is lost. That lost energy takes the H+ ions in the stroma (outside layer) to the Thylakoid lumen (inside layer).
Light Dependent Reactions (Photosystem ll and Photosystem l)
Once the electron goes PSI, it is in a state of low energy. Like PSII, it uses the light energy to activate the electron.
P700 is oxidized (loses the electron) and sends a high-energy electron to the ferredoxin (fd) which are proteins that mediate electron transfer to the NADP+ reductase, an enzyme that reduces both the NADP+ and H+ ions— meaning NADP+ reductase provides both of ions with an electron, creating NADPH which is then used to make ATP.
Creating ATP (Adenosine triphosphate)
ATP is the principal molecule for storing and transferring energy in cells. It is often referred to as the energy currency of the cell and can be compared to storing money in a bank.
Initially, there is a buildup of hydrogen ions that are moving from the stroma to the thylakoid lumen plus the ones produced by splitting water (to create the electron for PSII) accumulate in the thylakoid lumen. The ions build up energy because of diffusion and because they all have the same electrical charge, repelling each other.
To release this energy, hydrogen ions will rush through any opening, similar to water jetting through a hole in a dam. The energy released by the hydrogen ion stream allows ATP synthase to attach a third phosphate group to ADP, which forms a molecule of ATP. The flow of hydrogen ions through ATP synthase is called chemiosmosis because the ions move from an area of high to an area of low concentration through a semi-permeable structure.
In the thylakoid lumen, that opening is a passage through a specialized protein channel called the ATP synthase. The ATP synthase is a mitochondrial enzyme localized in the inner membrane which is responsible for the activation of the reaction to create ATP from ADP and phosphate.
Light Independent Reactions (Calvin Cycle)
After the energy from the sun is converted into chemical energy and temporarily stored in ATP and NADPH molecules, the cell has the fuel needed to build carbohydrate (sugar) molecules for long-term energy storage. The products of the light-dependent reactions, ATP and NADPH, have lifespans in the range of millionths of seconds, whereas the products of the light-independent reactions (carbohydrates and other forms of reduced carbon) can survive for hundreds of millions of years.
In plants, carbon dioxide (CO2) enters the leaves through stomata (plural of stoma) and reaches the mesophyll cells. Once in the mesophyll cells, CO2 diffuses into the stroma of the chloroplast—the site of light-independent reactions of photosynthesis. These reactions are called the Calvin cycles — named after the man who discovered it: Dr. Melvin Calvin.
For algae, peak growth is reached within 14 days, which is why it captures more carbon than other plants. For perennials, including most bushes and trees, take about two years to mature, which have a much lower capacity.
Generally there are 3 stages of the Calvin cycle: fixation, reduction, and regeneration.
High Level of all 3 stages
In stage 1, the enzyme RuBisCO incorporates carbon dioxide into an organic molecule, 3-PGA. In stage 2, the organic molecule is reduced using electrons supplied by NADPH. In stage 3, RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue. Only one carbon dioxide molecule is incorporated at a time, so the cycle must be completed three times to produce a single three-carbon GA3P molecule, and six times to produce a six-carbon glucose molecule
Stage 1: Fixation
In the stroma, in addition to CO2, two other components are present to initiate the light-independent reactions: an enzyme called ribulose bisphosphate carboxylase (RuBisCO) which includes 5 carbon atoms, and three molecules of ribulose bisphosphate (RuBP). RuBisCO catalyzes a reaction between CO2 and RuBP.
For each CO2 molecule that reacts with one RuBP, two molecules of another compound (3-PGA) form. 3-PGA has three carbons and one phosphate. Each turn of the cycle involves only one RuBP and one carbon dioxide and forms two molecules of 3-PGA. The number of carbon atoms remains the same, as the atoms move to form new bonds during the reactions (3 atoms from 3CO2 + 15 atoms from 3RuBP = 18 atoms in 3 atoms of 3-PGA). This process is called carbon fixation because CO2 is “fixed” from an inorganic form into organic molecules. When carbon is fixated it turned into a form that can be used by plants.
Stage 2: Reduction
ATP and NADPH are used to convert the six molecules of 3-PGA into six molecules of a chemical called glyceraldehyde 3-phosphate (G3P). That is a reduction reaction because it involves the gain of electrons by 3-PGA. Recall that a reduction in the gain of an electron by an atom or molecule. Six molecules of both ATP and NADPH are used. For ATP, energy is released with the loss of the terminal phosphate atom, converting it into ADP; for NADPH, both energy and a hydrogen atom are lost, converting it into NADP+. Both of these molecules return to the nearby light-dependent reactions to be reused and reenergized.
Stage 3: Regeneration
Interestingly, at this point, only one of the G3P molecules leaves the Calvin cycle and is sent to the cytoplasm to contribute to the formation of other compounds needed by the plant. Because the G3P exported from the chloroplast has three carbon atoms, it takes three “turns” of the Calvin cycle to fix enough net carbon to export one G3P. But each turn makes two G3Ps, thus three turns make six G3Ps. One is exported while the remaining five G3P molecules remain in the cycle and are used to regenerate RuBP (since it requires 5 carbon atoms), which enables the system to prepare for more CO2 to be fixed. Three more molecules of ATP are used in these regeneration reactions.