ALGAE...IN SPACE!

by SH

A few billion years after the Earth first formed, algae started producing oxygen through a process known as photosynthesis. It would take a couple more billion years for enough oxygen to accumulate and contribute to the formation of the ozone layer. This layer of the atmosphere is what protects us from the sun’s lethal ionizing radiation. But how were the algae able to survive exposure to that deadly radiation before the atmosphere protected the Earth? Scientists may have partially answered that question by analyzing the results of experiments that involved sending algae into orbit. Amazingly, the researchers found that mutations in a gene that codes for proteins involved in photosynthesis allow the algae to survive, even with no atmosphere!

Figure 1: Structure of a chloroplast. Chlorophyll, the

pigment involved in photosynthesis, is contained in

the membrane of the thylakoids.

Before we get into how the algae grew in space, let’s do a quick review. Photosynthesis is a process by which plants are able to use electromagnetic energy from sunlight, carbon dioxide, and water to make glucose for chemical energy, with oxygen being released as a by-product (1). Plants and algae that use this process contain green organelles known as chloroplasts, which are shown in figure 1. The pigment chlorophyll, which is located in the membrane of the flat, sac-like structures called thylakoids in the chloroplast, absorbs blue and red light wavelengths of light (1). Green light is not absorbed but reflected, giving plants and algae their green color (1).

The chlorophyll pigments in the thylakoids are part of a photosystem, which captures and processes light photons (1). When photons of the blue and red wavelength come in contact with the chlorophyll, the energy created excites an electron, which transfers it to a high-energy state (1). This then energizes the chlorophyll and starts the process of photosynthesis in photosystem II (PSII), the first protein complex of the system (1). After a few more steps, a sugar is produced that the algae are able use for food.

Figure 2: Chlamydomonas reinhardtii. This is an

electron scanning microscope (ESM) image

of the algae.

There are a few photosynthetic life forms that are considered model organisms for scientists to study. Model organisms are easily grown and maintained in a laboratory setting and are able to be experimented on. Chlamydomonas reinhardtii, as seen in the ECM image in figure 2, is a unicellular, photosynthetic alga that has been studied for many years. The photosystems in these algae are similar to those of vascular plants, and it has a cell wall, like plant cells, as seen in figure 3. The DNA contained in C. reinhardtii can be genetically modified, which makes it a good model for studying how the cell works. It is also easily grown and maintained on Tris-Acetate-Phosphate (TAP) medium in the lab (2). All of these things make C. reinhardtii an ideal organism to experiment on.

Figure 3: Chlamydomonas reinhardtii. A

Transmission Electron Microscope (TEM) image

is colorized to show the different
parts of C. reinhardtii.

The PSII of C. reinhardtii has a reaction core that consists of two proteins, D1 and D2 (3). These proteins bind to chlorophyll to assist in the process of energy transfer in the photosystem (3). When exposed to harsh environments, it has been found that the D1 protein is damaged, which then causes damage to the PSII (3). An example of a harsh environment would be the exposure to ionizing radiation, like the type that the algae of early Earth were exposed to. Without the protection of the atmosphere, the membranes of cell organelles are damaged. As photosynthesis occurs in the thylakoid membrane, the radiation present in space affects how well the algae is able to perform photosynthesis (3).

Since the genome of C. reinhardtii was sequenced nine years ago, scientists are now able to make mutants to see how the absence of different proteins affects the algae. In 2007, a team of scientists from Germany and Italy collaborated on an experiment to see how PSII would be affected if C. reinhardtii colonies were exposed to the ionizing radiation of space without the protection of our atmosphere (3). The team made a C. reinhardtii strain that had a mutation in the psbA gene that encodes the D1 protein in the reaction center of the photosystem and exposed some of the cells to ultraviolet light (3). Keeping some of the cells on Earth for controls, colonies of C. reinhardtii were sent into space on a Foton pacecraft, as shown in figure 4, aboard a Soyuz capsule (3). The colonies were protected from the cold and vacuum of space, but were exposed to the ionizing radiation and weightless conditions (3). After fifteen days in space, the spacecraft was returned to Earth and the C. reinhardtii colonies were collected (3).

Figure 4: Foton spacecraft. C. reinhardtii colonies are housed in the red circular area on the magenta sphere.

The researchers found that even though the C. reinhardtii colonies were exposed to ultraviolet light that was five times higher than that on Earth and ionizing radiation that was not diffused by an atmosphere, many cells survived and were still able to perform photosynthesis (3). The cells that were exposed to ultraviolet light on Earth before the trip to space survived in better condition than cells that were not exposed (3). It appears that PSII is involved in capturing the energy from ionizing radiation and using it for photosynthesis, though more studies are needed to see if this hypothesis is true (3).

In 2013, a group of scientists from Germany, Italy, and the United States worked together on another experiment involving the PSII D1 protein in C. reinhardtii in the environment of space (4). In this experiment, Chlorophyll a, which produces fluorescence when energy is dispersed in the photosynthesis process, was monitored in real time while the algae were in space. This gave the scientists a better idea of how the algae reacted while in the harsh environment of space, rather than waiting to see if any algae survived after the spacecraft landed (4). The same mutation was made in the psbA gene that encodes the D1 protein in the reaction center of the photosystem as in the experiment described previously (4). Colonies of mutated and wild type C. reinhardtii were sent into space for fifteen days on a Foton spacecraft aboard a Soyuz capsule (4). After the spacecraft landed, the algae colonies were transferred to fresh TAP medium so that they could be analyzed (4).

The real time monitoring of C. reinhardtii colonies in space during the second experiment showed that the wild type strain’s photosynthetic efficiency dropped daily, though they were still able to perform the process (4). The mutated strain, however, was able to maintain a high level of PSII activity, showing a slight increase in fluorescence emission towards the final days of the spaceflight (4). When the colonies were returned to Earth, both the wild type and the mutated strains were able to grow in the lab, though the mutated strain grew better than the wild type strain (4). This showed that mutation to the psbA gene made the D1 protein less susceptible to damage from ionizing radiation (4).

Since science has not yet found a way to make a time machine, we are not able to definitively say what happened in Earth’s history. However, scientists are able to perform experiments that give us some idea of what the Earth was like billions of years ago. Both of these experiments have shown us that even without a protective atmosphere, Earth’s early algae colonies could survive. Thanks to the single cell algae and their by-product of oxygen, other life forms on this planet have since been able to grow and thrive.

References

1.    Freeman, S., Allison, L., Black, M., Podgorski, G., Quillin, K., Monroe, J., Taylor, E. (2014) Biological Sciences, custom edition for University of Minnesota. Pearson Learning Solutions, Boston, MA.

2.    Harris, E.H. (1989) The Chlamydomonas sourcebook: a comprehensive guide to biology and laboratory use. Academic Press, INC, San Diego, California.

3.    Bertalan, I., Esposito, D., Torzillo, G., Faraloni, C., Johanningmeier, U., Giardi, M.T. (2007) Photosystem II stress tolerance in the unicellular green alga Chlamydomonas reinhardtii under space conditions, Microgravity Science and Technology 19(5/6):122-127

4.    Giardi, M.T., Rea, G., Lambreva, M.D., Antonacci, A., Pastorelli, S., Bertalan, I., Johanningmeier, U., Mattoo, A.K. (2013) Mutations of photosystem II D1 protein that empower efficient phenotypes of Chlamydomonas reinhardtii under extreme environment in space, PLoS ONE 8(5):1-10, e64352