How Does Photosynthesis Take Place in Our Oceans?
The food we eat ultimately comes from plants, either directly or indirectly. The importance of plants as the global kitchen can never be underestimated. Plants “eat” sunlight and carbon dioxide to produce their own food and food for the millions of other organisms dependent on them. A molecule, chlorophyll (Chl), is crucial for this process, since it absorbs sunlight. However, the way land plants produce their food is very different from the way plants in the oceans produce their food. Since it is difficult for light to reach underneath the water in the oceans, food production, scientifically called photosynthesis, becomes very slow. Phycobiliproteins are proteins that make this job easier, by absorbing the available light and passing it on to Chl. These phycobiliproteins are found in tiny, invisible organisms called cyanobacteria. Their “food-producing” reactions are critical for the survival of many living organisms like fish, birds, and other sea life. It is, therefore, very important for everyone to understand how cyanobacteria make their food, and what important roles the phycobiliproteins play in the process.
How Do Living Things Obtain Their Food?
When you think of food, do you usually come up with images of your favorite food? This is a natural process, since food is important for every living thing. To fulfill this basic need, all living things either make their own food or get it from some other source. Humans can eat both plants and animals. Some animals consume other animals, while some animals eat plants as their food. Ultimately, we see that everybody on this planet is dependent on plants for their food. But then, what do plants eat? Actually, plants “eat” sunlight and a gas called carbon dioxide, both of which are easily available right here on earth. The process by which land plants produce their own food using sunlight and carbon dioxide is known as photosynthesis (Figure 1). While carbon dioxide is absorbed by the leaves, the sunlight is captured by a chemical molecule in the plant, called chlorophyll (Chl). All photosynthetic organisms contain Chl.
However, the way land plants perform photosynthesis does not help the organisms living in the oceans, which cover nearly 70% of our earth. Plants in the oceans face problems with light availability. The blue and green portions of light penetrate into the water more than the yellow and red portions of light do (Figure 2). Luckily, ocean plants get help in producing food from such limited light and carbon dioxide, from tiny microscopic microbes called cyanobacteria (also known as blue-green algae). 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 . 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.
What are Phycobiliproteins?
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 . 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:
(a) C-phycoerythrin (CPE), pinkish-red in color and responsible for absorbing the green portion of sunlight.
(b) C-phycocyanin (CPC), deep blue in color and responsible for absorbing the orange-red portion of sunlight.
(c) Allophycocyanin (APC), light blue in color and responsible for absorbing the red portion of sunlight.
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 , and the disks are stacked on top of each other to form the hat-like structure. One end of the stack is made of CPE, whereas the other end is made of CPC. 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.
How Does the Light Energy Transfer Take Place in Phycobiliproteins?
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). All phycobiliproteins possess this exciting property of giving off visible light of a color different from the color of light that is shone on them. After CPE changes green light to yellow-orange, CPC takes up the yellow-orange light and changes it to light red. APC takes up this light-red light and changes it to a deep red light for Chl. So, now we have the green light changed to red, which is the color of light that nature intended Chl to absorb. The entire process is a sort of a relay race, where each participant picks up where the previous one left off (Figure 5). These phycobiliproteins are an important part of the tiny microscopic organisms called cyanobacteria, which carry out photosynthesis in much the same way as land plants do. The only difference is that they use a different set of chemical molecules—cyanobacteria use phycobiliproteins while land plants use Chl.
What Did We Learn?
So, we now know that photosynthesis is the process by which plants produce their food, using Chl. We also know that the reduced amount of light available in the oceans decreases this photosynthetic process. Nature has evolved some helper chemical molecules known as phycobiliproteins, which are able to absorb the colors of light available in the oceans and turn this light into a color that Chl molecules can use. These phycobiliproteins are found in tiny, invisible-to-the-naked-eye cyanobacteria, whose photosynthesis is responsible for providing food for the living organisms in the oceans and also for making the oxygen in our atmosphere that we breathe every second. Isn’t it exciting that these tiny organisms can make such a difference to marine life? In the future, we hope to gain more understanding of the functions of phycobiliproteins and the roles that they may play for the benefit of mankind.
Photosynthesis: ↑ A process by which plants produce food for themselves and other organisms using sunlight and carbon dioxide gas.
Chlorophyll: ↑ A chemical molecule present in plants that absorbs the sunlight for photosynthesis.
Phycobiliproteins: ↑ Colored pigments found in cyanobacteria and certain other organisms, which help in photosynthesis by absorbing certain colors of light which chlorophyll cannot absorb.
Fluorescence: ↑ The property of certain compounds to absorb one color of light and to give off another color. Phycobiliproteins use this property to change the color of light they absorb so that the light can be used for photosynthesis.
Conflict of Interest Statement
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
This manuscript has been assigned registration number CSIR-CSMCRI – 114/2016. TG gratefully acknowledges AcSIR for Ph.D. enrollment and CSIR (CSC 0105) for financial support.
 ↑ Sidler, W. A. 1994. Phycobilisome and phycobiliprotein structure. In: Bryant, D. A., editor. The Molecular Biology of Cyanobacteria. Dordrecht: Springer. p. 139–216.
 ↑ Ghosh, T., Paliwal, C., Maurya, R., and Mishra, S. 2015. Microalgal rainbow colours for nutraceutical and pharmaceutical applications. In: Bahadur, B., Venkat Rajam, M., Sahijram, L., and Krishnamurthy, K. V., editors. Plant Biology and Biotechnology: Volume I: Plant Diversity, Organization, Function and Improvement. New Delhi: Springer. p. 777–91.
 ↑ Satyanarayana, L., Suresh, C. G., Patel, A., Mishra, S., and Ghosh, P. K. 2005. X ray crystallographic studies on C-phycocyanins from cyanobacteria from different habitats: marine and freshwater. Acta Crystallogr. Sect. F 61(9):844–7. doi:10.1107/S1744309105025649