These microbes have adapted to dim light conditions, and they carry out photosynthesis both for themselves and for the benefit of other living things.
Cyanobacteria are ancient microbes that have been living on our earth for billions of years. Cyanobacteria are said to be responsible for creating the oxygen-filled atmosphere we live in [ 1 ]. For carrying out photosynthesis in low light conditions, cyanobacteria have the help of proteins called phycobiliproteins , which are found buried in the cell membranes the outer covering of the cyanobacteria. Phycobiliproteins play the role of assistants to Chl in aquatic water environments.
Since light has a difficult time penetrating into the oceans, phycobiliproteins make this job easier by absorbing whatever light is available; they absorb the green portion of the light and turn it to red light, which is the color of light required by Chl [ 2 ]. However, changing the color of light is not as easy as it seems. The green light has to pass through different phycobiliprotein molecules, which absorb light of one color and give out light of another color. The color that is given out is then taken up by a second phycobiliprotein, which turns it into a third color.
This process continues until the emitted light is red, which can finally be taken up by Chl. For this whole process to take place, we have three different kinds of phycobiliprotein molecules arranged as a sort of a hat over the Chl molecule, as you can see in Figure 3. These three kinds of phycobiliproteins are:. The reason phycobiliproteins absorb light of different colors is that they contain chemical molecules called bilins inside them, which give them their bright colors.
These bilins are responsible for absorbing light of one color and emitting light of another color, thus causing a change in the color of light. Advanced instruments have let us analyze the arrangement of these molecules and proteins in the cyanobacteria.
We know that phycobiliproteins are shaped like disks [ 3 ], and the disks are stacked on top of each other to form the hat-like structure. This assembly joins to the core, made of APC. This entire structure is linked to Chl, which accepts the red light emitted by APC.
The arrangement of the hat-like structure has been shown in Figure 3. The change in light color from green to red takes place through a process known as fluorescence. Let us see what fluorescence is. Imagine a transparent container filled with a pink-colored liquid that, when illuminated with a flashlight, shines a bright orange! That is exactly what CPE does Figure 4. In the meantime, water is oxidized on the surface of another semiconductor to release oxygen.
After several hours or several days of this process, the chemists can collect the product. This PBS achieved a solar-to-chemical conversion efficiency of 0. A diagram of the first-generation artificial photosynthesis, with its four main steps. Since artificial photosynthesis would absorb and reduce carbon dioxide in order to create fuels, we could continue to use liquid fuel without destroying the environment or warming the planet. However, in order to ensure that artificial photosynthesis can reliably produce our fuels in the future, it has to be better than nature, as Ciamician foresaw.
Since the major breakthrough in April , Yang has continued to improve his system in hopes of eventually producing fuels that are commercially viable, efficient, and durable. In August , Yang and his team tested his system with a different type of bacteria. The method is the same, except instead of electrons, the bacteria use molecular hydrogen from water molecules to reduce carbon dioxide and create methane, the primary component of natural gas.
A diagram of this second-generation PBS that produces methane. In December , Yang advanced his system further by making the remarkable discovery that certain bacteria could grow the semiconductors by themselves. This development short-circuited the two-step process of growing the nanowires and then culturing the bacteria in the nanowires. The improved semiconductor-bacteria interface could potentially be more efficient in producing acetate, as well as other chemicals and fuels, according to Yang.
And in terms of scaling up, it has the greatest potential. A diagram of this third-generation PBS that produces acetate. In the past few weeks, Yang made yet another important breakthrough in elucidating the electron transfer mechanism between the semiconductor-bacteria interface. This sort of fundamental understanding of the charge transfer at the interface will provide critical insights for the designing of the next generation PBS with better efficiency and durability.
He will be releasing the details of this breakthrough shortly. But it creates a more useable source of energy than solar panels, which are currently the most popular and commercially viable form of solar power.
This difference is crucial. The electricity generated from solar panels simply cannot meet our diverse energy needs, but these renewable liquid fuels and natural gases can. With artificial photosynthesis creating our fuels, driving cars and operating machinery becomes much less harmful. But both cases are controversial, and the details of what the pigments are actually doing are unclear.
And neither example is true photosynthesis, which also involves transforming carbon dioxide into sugars and other such compounds. Using solar energy is just part of the full conversion process. There are, however, animals that photosynthesise in the fullest sense of the word. All of them do so by forming partnerships. Corals are the classic example.
They depend upon microscopic algae called dinoflagellates that live in special compartments within their cells. These residents, or endosymbionts, can photosynthesise and they provide the corals with nutrients. Its green-tinged eggs are loaded with algae, which actually invade the cells of the embryos within, turning them into solar-powered animals. The algae die as the salamanders turn into adults, but not before providing them with a useful source of energy in the earliest parts of their lives.
Despite these varied examples, photosynthetic symbionts are again the exception rather than the rule. In a classic paper , botanist David Smith and entomologist Elizabeth Bernays explain why: such partnerships are more complicated than they seem. They need ways of persuading the symbionts to release their manufactured nutrients, rather than hoarding it for themselves.
They need to transfer their partners to the next generation corals do it by releasing the symbionts into the surrounding water. In , Christina Agapakis , a synthetic biologist from the University of California, Los Angeles got baby zebrafish to accept photosynthetic bacteria, simply by injecting them into the fish when they were embryos.
And with a little tweak, she even persuaded the bacteria to invade mammalian cells. There is another option to adding entire symbionts: steal their factories instead. Within the cells of plants and algae, photosynthesis takes place within tiny structures called chloroplasts. Chloroplasts are the remnants of a free-living photosynthetic bacterium that was swallowed by a larger microbe billions of years ago.
Instead, the two cells formed a permanent partnership that fuels the cells of plants and algae to this day. So rather than teaming up with a symbiont, why not cut out the middle-man and take its chloroplasts for yourself?
At least one group of animals has done this — the Elysia sea slugs.
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