Hi everybody,
As a part of my thesis I calculated the growth rate of Chlorella and Chlamydomonas strains from wastewaters. Under similar conditions, Chlamydomonas had a higher growth rate than Chlorella. Before that, I had assumed that Chlorella would had to have the highest growth rate. How can I explain this difference? Physiological and cell cycle differences between them?
Many thanks
It is widely thought that Chlorella has higher growth rate than Chlamydomonas. This is acutally not true for some strains of Chlamydomonas that easily exceed the growth rate of Chlorella. The difference is that Chlorella can reach much higher cell densities than Chlamy and thus has in a long term bigger productivity per time. This perhaps is the reason for the common misunderstanding. As for the explanation, clearly the differences in growth rate can be explained by differences in metabolism and physiology in specific conditions and how the particular strains are able to cope with them.
Your results suggest that the culture conditions were not optimized for the taxa, or as Katrina mentioned, strain specific differences in growth rate. By the term wastewaters, I am guessing sewage. So optimized N:P ratios, trace metal abundance could all be partial explanations.
I believe both would have rather similar maximum intrinsic growth rates. However, the environmental conditions would determine the specific growth rate in your experiments. Many strains of Chlamydomonas are effective mixotrophs, using organic compounds as additional source of C, N and P, and it is possible that these types of compounds were present in your wastewater promoting the growth of Chlamydomonas.
Chlamydomonas is a motile species and therefore will have a smaller entrained boundary layer than Chlorella. This has a number of advantages - improved gas exchange between boundary layer and water column, reducing photoinhibition by lowering the boundary layer O2 concentration; improving nutrient uptake efficiency by reducing passive diffusion distances. Mobility also permits migration to areas within the water column where resources (CO2, nutrients) may be higher. I don't think Chlamydomonas is any more efficient, I just think it relies on basic physics to manipulate its' environment to its' advantage. If you can measure the velocity you can work out the boundary layer thickness and come to a conclusion about the actual advantages conferred by motility. Measurements show a speed of about 65 um/s in the light (http://www.sciencedirect.com/science/article/pii/S0176161700800450). You should also measure photosynthetic rates of each species in your conditions, not just biomass...
Many thanks for your answers! As Katerina Bisova said, my Chlorella strain reached a much higer cell density than Chlamydomonas. Both have similar conditions (light, temperature, culture volumen and aireation). I use effluents of secondary treatment as a culture media and they have som organic compounds. Also, I read that Chlorella can also growth in a mixotrophic conditions as Chlamydomonas. Jonathan Newman has an interesting point of view.
Although mobile species use more energy for locomotion rather than growth, efficient contact with nutrient rich boundary layer and avoidance of self-shading may enhance photosynthetic activity compared to stagnant species resulting in higher growth rate of Chlamydomonas. Distribution of chloroplasts density and spacing may also be a reason.
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