Properties of Chlamydomonas reinhardtii
The unicellular green algae Chlamydomonas reinhardtii is known as a model organism, the first strains were isolated about 60 years ago (1). The algae diverged from land plants over one billion years ago and comes from a linage of Viridiplantae. C. reinhardtii is built up by a basal body with two connected anterior flagella used for environmental sensing, mating and swimming. It is approximately 10 μm in size and has a chloroplast containing its photosynthetic apparatus as well as metabolic pathways (2). The algae occurs in two genetically mating types, + and – alleles of a mating type complex. Mixing the two types will rapidly form a diploid cell that turns into a heavy walled zygospore.
Why this model organism?
C. reinhardtii is used as a model organism for several reasons. Firstly, it is very easy to culture in many different environments as long as there is access to fresh water or soil and carbon dioxide. It is also possible to grow the algae in dark if a carbon source is provided. Secondly, this microalgae has a very short generation time and is easy to manipulate genetically since its entire genome has been sequenced.
According to previous research, C. reinhardtii poses no human health risk at all (3). As a precaution however one should treat it as a biosafety class 1 when it has been genetically modified (4).
There has been a study, with an aim to see if it worked to modify the algae CRISPR-Cas9 where they have successfully modified C. reinhardtii using this system. The study was however unsuccessful to retrieve any transformants with Cas9. Since the promoter of Cas9 was constitutively activated, the research group concluded that the cells did not grow fast enough due to Cas9 being toxic to the algae. They speculated that this was probably due to the Cas9 interfering with the survival of the C. reinhardtii, although they did not speculate as to how Cas9 interfered with cell growth. (5). In that study they also suggested as a future change, that one should use a regulatory promoter instead, this has been taken to consideration in this project.
1. Pröschold et al., Portrait of a Species Chlamydomonas reinhardtii, 2005, Genetics
2. Sabeeha et al., The Chlamydomonas Genome Reveals the Evolution of Key animal and Plant Functions, 2007, Science
3. iGEM Concordia, Our Organisms and Safety (Obtained as the webpage https://2014.igem.org/Team:Concordia/Safety 2016-10-17)
4. Chlamydomonas reinhardtii (ATCC® 30483™) (obtained as a pdf 2016-10-03)
5. Jiang et al., Successful Transient Expression of Cas9 and Single Guide RNA Genes in Chlamydomonas reinhardtii, EC Journals volume 13, November 2014