Chlamydomonas reinhardtii
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| Chlamydomonas reinhardtii P.A.Dang. |
Chlamydomonas reinhardtii is a motile single celled green alga about 10 micrometres in diameter that swims with two flagella. See Chlamydomonas.
These algae are commonly found in soil and fresh water. They have a cell wall made of hydroxyproline-rich glycoproteins, a large cup-shaped chloroplast, a large pyrenoid, and an "eyespot" that senses light. Normal Chlamydomonas can grow on a simple medium of inorganic salts in the light, using photosynthesis to provide energy. They can also grow in total darkness if acetate is provided as a carbon source for catabolism.
The C. reinhardtii wild type laboratory strain c137 (mt+) originates from an isolate made near Amherst, Massachusetts, in 1945.
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[edit] C. reinhardtii as a model organism
Chlamydomonas is used as a model organism for research on fundamental questions in cell and molecular biology such as:
- How do cells move?
- How do cells respond to light?
- How do cells recognize one another?
- How do cells regulate their proteome to control flagellar length?
- How do cells respond to changes in mineral nutrition? (nitrogen, sulfur etc.)
There are many known mutants of C. reinhardtii. These mutants are useful tools for studying a variety of biological processes, including flagellar motility, photosynthesis or protein synthesis. Since Chlamydomonas species are normally haploid, the effects of mutations are seen immediately without further crosses.
In 2007, the complete nuclear genome sequence of C. reinhardtii was published.[1]
[edit] Reproduction
Vegetative cells of the reinhardtii species are haploid with 17 small chromosomes. Under nitrogen starvation, haploid gametes develop. There are two mating types, identical in appearance and known as mt(+) and mt(-), which can fuse to form a diploid zygote. The zygote is not flagellated, and it serves as a dormant form of the species in the soil. In the light the zygote undergoes meiosis and releases four flagellated haploid cells that resume the vegetative life cycle.
Curious fact: Under ideal growth conditions, cells may undergo two or three rounds of mitosis before the daughter cells are released from the old cell wall into the medium. Thus, a single growth step may result in 4 or 8 daughter cells per mother cell.
The cell cycle of this unicellular green algae can be synchronized by alternating periods of light and dark. The growth phase is dependent on light, whereas, after a point designated as the transition or commitment point, processes are light-independent. [2]
[edit] Genetics
The attractiveness of the alga as a model organism has recently increased with the release of several genomic resources to the public domain. The current draft (Chlre3) of the Chlamydomonas nuclear genome sequence prepared by Joint Genome Institute of the U.S. Dept of Energy comprises 1557 scaffolds totaling 120 Mb. Roughly half of the genome is contained in 24 scaffolds all at least 1.6 Mb in length. The sequences of all three C. reinhardtii genomes are available.
The ~15.8 Kb mitochondrial genome (database accession: NC_ 001638) is available online at the NCBI database. [1] The complete >200 Kb chloroplast genome is available online. [2] The current assembly of the nuclear genome is available online at. [3]
In addition to genomic sequence data there is a large supply of expression sequence data available as cDNA libraries and expressed sequence tags (ESTs). Seven cDNA libraries are available online [4]. A BAC library can be purchased from the Clemson University Genomics Institute [5]. There are also two databases of >50 000 [6] and >160 000 [7] ESTs available online.
[edit] C. reinhardtii DNA transformation techniques
Gene transformation occurs mainly by homologous recombination in the chloroplast and heterologous recombination in the nucleus. The C. reinhardtii chloroplast genome can be transformed using microprojectile particle bombardment and the nuclear genome has been transformed with both glass bead agitation and electroporation. The biolistic procedure appears to be the most efficient way of introducing DNA into the chloroplast genome. This is probably because the chloroplast occupies over half of the volume of the cell providing the microprojectile with a large target. Electroporation has been shown to be the most efficient way of introducing DNA into the nuclear genome with maximum transformation frequencies two orders of magnitude higher than obtained using glass bead method.
[edit] C. reinhardtii as a clean source of hydrogen production
In 1939 the German researcher Hans Gaffron (1902-1979), who was at that time attached to the University of Chicago, discovered the hydrogen metabolism of unicellular green algae. Chlamydomonas reinhardtii and some other green algae can, under specified circumstances, stop producing oxygen and convert instead to the production of hydrogen. This reaction by hydrogenase, an enzyme only active in the absence of oxygen, is short-lived. Over the next thirty years Gaffron and his team worked out the basic mechanics of this photosynthetic hydrogen production by algae. [3]
Such a production of hydrogen would only need water, sunlight and green algae. There is no need for electricity, nor is there any release of greenhouse gases. In other words, this is truly clean energy.
This hydrogen production cannot take place in the presence of oxygen. Under anaerobiotic circumstances however, the hydrogenase enzyme can produce briefly, during only a few minutes, light-mediated photosynthetic hydrogen. This reaction acts as a safety valve, dissipating the surplus of electrons created during a critical phase in the production of sugars in the chloroplast. This critical phase emerges because the reactions, transferring the electrons, happen immediately under the effect of light, while the synthesis of sugars gets going at a slower rate. Through this pumping of electrons into the production of hydrogen, the hydrogenase avoids choking the system. This reaction is gradually stopped after a few minutes by the oxygen emerging through the reactions producing the sugars.
To increase the production of hydrogen, two tracks are being followed by the researchers.
- The first track is decoupling hydrogenase from photosynthesis. This way, oxygen accumulation can no longer inhibit the production of hydrogen. And, if one goes one step further by changing the structure of the enzyme hydrogenase, it becomes possible to render hydrogenase insensitive to oxygen. This makes a continuous production of hydrogen possible. The flux of electrons needed for this production comes, in this case, no longer from the production of sugars, but is drawn from the breakdown of its own stock of starch. [4]
- A second track is to interrupt temporarily, through genetic manipulation of hydrogenase, the photosynthesis process. This inhibits oxygen reaching a level where it is able to stop the production of hydrogen. [5]
Both tracks are very promising and the practical implications to the energy supply of the world could be enormous. But industrial production of hydrogen by green algae is not yet for the very near future. At this moment the yield of the first prototypes fluctuates between 0.5 and 3 %, while 10 % could be the maximum yield of this biological process.
[edit] Notes
- ^ Merchant et al. (2007). "The Chlamydomonas genome reveals the evolution of key animal and plant functions". Science 318: 245-250. doi:. PMID 17932292.
- ^ Oldenhof, H, Zachleder, V. and van den Ende, H. 2006. Blue- and red-light regulation of the cell cycle in Chlamydomonas reinhardtii (Chlorophyta). Eur. J. Phycol. 41: 313 - 320
- ^ Anastasios Melis, Thomas Happe (2004). "Trails of green alga hydrogen research - from Hans gaffron to new frontiers". Photosynthesis Research 80.
- ^ Laurent Cournac, Florence Musa, Laetitia Bernarda, Geneviève Guedeneya, Paulette Vignaisb and Gilles Peltie (2002). "Limiting steps of hydrogen production in Chlamydomonas reinhardtii and Synechocystis PCC 6803 as analysed by light-induced gas exchange transients". International Journal of Hydrogen Energy 27 (11/12). doi:.
- ^ Anastasios Melis. Hydrogen and hydrocarbon biofuels production via microalgal photosynthesis. Retrieved on 2008-04-07.
[edit] External links
- Chlamydomonas genome sequenced published in Science, October 12, 2007
- Chlamydomonas reinhardtii resources at the Joint Genome Institute
- Chlamydomonas frequently asked questions
- The Chlamydomonas Center provides access to Chlamydomonas genomic, genetic and bibliographic information, the Chlamydomonas culture collection, and other resources for the Chlamydomonas community.
- The Chlamydomonas Teaching Center offers information on how to use Chlamydomonas in teaching and student research projects
- Chlamydomonas reinhardtii. AlgaeBase. National University of Ireland, Galway.
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