Depending on the ambient light intensity, C. reinhardtii swims either towards or away from the light. Thereby, it is capable of adjusting its photosynthesis and optimizing its energy budget. In addition, its endogenous clock controls the behavior of the tiny cells which are only 10 micrometer in size, i.e. about 8 times thinner than a piece of paper. The algae perceive the light information mainly with a so-called eyespot that is situated at the cell equator and serves as sensory organelle to analyze the environment. Researchers have found that C. reinhardtii has several photoreceptors that perceive both the intensity and the quality of the incoming light. Some of them are directly located in the eyespot area. ”Chlamydomonas senses different wavelengths including UV and the visible spectrum (blue, green, yellow and red light) with six photoreceptors that have been described so far“, explained Maria Mittag of the Friedrich Schiller University Jena, who coordinates a team of eight scientists (financed by the German Research Foundation) who collaborate on ”Light-driven reactions in unicellular model algae“. She added: “The eyespot consists of about 200 proteins including light receptors such as the channelrhodopsins that are situated in the outermost membrane layer of the eyespot area, as well as signal transducing and structural proteins”.
Swims 12 times faster than humans
Light cannot penetrate through the layers of the eyespot; it is reflected. The alga senses the direction of the incoming light including its intensity by performing helical movements. The information about light and darkness is then transduced to the motile cilia. The alga moves forward with its two cilia by breast-style swimming. The structure of the cilia is similarly to that of the human sperm. The swimming speed of C. reinhardtii is about 120 micrometer per second. Thus, related to its cell size of about 10 micrometers, the alga swims 12 times faster than the current breast-stroke swimming record holder over 50 m, Cameron van der Burgh.
With an endogenous rhythm even in space
To find out if an endogenous clock drives the daily rhythm of the species, similar to that in humans, scientists have transferred C. reinhardtii into constant darkness. The rhythm continued even under these conditions with a period of about 24-hours: the algae know exactly when day or night should be. Chronobiologists refer to this phenomenon as ‘circadian rhythm’ and differentiate between ‘subjective day’ and ‘subjective night’. Light pulses given during ‘subjective days’ result in a movement of C. reinhardtii towards the light source while light pulses provided during a ‘subjective night’ do not. Such a daily rhythm was even observed in outer space where it continued for several days. Thus, the tiny algae know the time of day not only on Earth but also in space. Moreover, they use sunlight to re-adjust their endogenous clock based on the surrounding light-dark cycle - like humans when subjected to jetlag.
Promoted to model organism
Using different mutants, the function of organelles as well as the behavior of C. reinhardtii can be studied; this has allowed the species to become a widely-used model organism. The vegetative cells of C. reinhardtii are haploid and thus have only one copy of the genetic code originating from either of the parent cells. It is therefore relatively easy to produce mutants which have a specific phenotype that can be studied further, such as mutants lacking an eyespot or flagella. Another interesting topic to investigate using C. reinhardtii is the process of photosynthesis: C. reinhardtii can use acetate as a carbon source which means that mutants with a malfunctioning photosynthetic apparatus are still able to survive and can be used to explore this important but complex process. The fully sequenced genome of C. reinhardtii represents a further advantage for the scientific community. The alga has more than 15,000 genes that have not only homology with known plant genes but also genes from animals.
Pumping water without muscles
The liquid content of algal cells is more concentrated than that of the surrounding freshwater due to numerous dissolved substances in the cytoplasm. Thus, C. reinhardtii continuously absorbs water from its environment like a sponge. The green alga therefore has to get rid of the excess water to avoid bursting, a problem the species shares with other freshwater organisms. “Chlamydomonas actively pumps excessive water out of the cell using a contractile vacuole” described Burkhard Becker from the University of Cologne whose team is studying the pumping mechanism. Although the process by which the water enters the cell is well established, how excess water is pumped out again is still an open field, he added. “All we know is that the proteins actin and myosin, which are used in muscles, are not involved in the algal system; therefore another mechanism must apply.”
Scientific field with prestigious awards
A novel scientific area has emerged based on the analysis of light perception in C. reinhardtii. Peter Hegemann and Georg Nagel were the first to discover that the photoreceptors of the alga, the so-called channelrhodopsins, are light-controlled channels which regulate the transfer of single ions through the cell membrane. These channelrhodopsins now have applications as ‘light switches’ in the field of neurobiology when studying medically relevant pathways in real-time using light pulses as triggers. For these discoveries both scientists, who are members of the Phycology Section of the German Botanical Society, received the 700,000 Swiss Franc Louis-Jeantet prize for medicine (2013) and, in addition (Peter Hegemann, 2012), the Gottfried-Wilhelm-Leibniz prize (worth EUR 2.5 million) from the German Research Foundation.
An alga as substitute for sperm
Other medical scientists apply C. reinhardtii as a model when studying the movement of cilia. Its two cilia are in structure and function quite similar to the human sperm and other motile cilia of mammalian cells. In humans, cilia fulfill many different tasks such as the transport of eggs in Fallopian tubes or the removal of dirt particles from the airways. Defects in cilia can result in ciliopathies such as left-right asymmetry where the heart or other organs are located on the wrong side of the body, or polycystic kidney disease. The proteins involved in motility are much easier to study in the algal model system than in humans. Again, numerous available mutants e.g. where ciliary proteins are defect can help to understand these processes. Moreover, the algal cells can be cultivated in large quantities to obtain enough biological material within a relatively short timeframe to isolate cilia and study their composition. Scientists have found more than 500 different proteins that are now subject to further analysis.