Results from an in-situ experiment on a sandflat reveal an increase in chlorophyll-a concentration relative to reduced light levels. The experimental period was less than 6 hours. Can benthic microalgae really respond to reductions in light this quickly? We have considered vertical migration. However, sample cores were taken at a depth of 2 cm from the sediment surface, integrating the layer over which the majority of migration occurs. What I need to find out is whether chlorophyll-a could have been synthesised in this time period as a response to reduced light conditions.
A few papers discuss photoacclimation. This paper is a start. The references within will help you too, particularly Anning et al. 2000.
I measured POC and PON under different light and nutrient conditions and within 2 hours you could see dramatic changes in nutrient concentration and stoichiometry in the species of algae I was working with.
I have not direct answer but you should also considered that algae have light independent chlorophyll biosynthesis, i.e. they can synthesize chlorophyll in the dark, so the regulation of chlorophyll biosynthesis is not so simple like in angiosperms e.g. Arabidopsis, where light is the dominant factor by two ways: 1) light reduces protochlorophyllie to chlorophyllide 2) light stimulates expression of GluTR, the key enzyme in aminolevulinic acid biosynthesis. The algae have light independent enzyme, which catalyzes the reduction of protochlorophyllide to chlorophyllide also in the dark. Mutants in this enzyme have yellow in the dark phenotype.
Your topic reviels several questions about the chlorophyll synthesis in living cells in relation to the light conditions.
* To synthesise Chlorophyll, light is needet for the phototransformation of proto- chlorophyllide into chlorophyllyde. Some algae and some conifers are able to synthesise chlorophyll in the dark (by using internal NADPH for the reduction steps).
As the light dependency of the phototransformation is a quantic reaction, it is very fast (below ms), so that the chlorophyll synthesis is (in presence of light) limited by the speed of dark reactions. Therefore both factors play a limiting role: Light intensity and time. As a quantum reaction, more light will hit more protochlorophyllide molecules. However more light will increase completely different photoinhibition reactions. Therefore the light dependency curve will show an optimum. As you are working with Algae, the growth rate of your Algae culture will becoming the determining factor, so that all "baby Algae" will be green as well.
In summary: The light reaction in chlorophyll synthesis is very fast and depends for a cell on the light intensity. However the REGENERATION of the phototransformed Protochlorophyllide pool takes about 10 to 15 MINUTES. That means after a strong and short light flash the yield of Chlorophyll synthesis is very slow. Therefore it is well possible that low light conditions can become very efficient to keep an appreciable amount of chlorophyll in each cell of a growing cell culture. It's a chain of KEY-reactions to optimise growth under given light reactions. Each organism under different conditions will react differently. However each case can be carefully investigated.
(For basic literature see the old papers by SIRONVAL and BROWERS about chlorophyll synthesis. Many papers in Photosynthetica (and others) about 35 years ago). Good luck for your interesting case study.
I published on this, way back in 1980, in a now-defunct journal, Marine Biology Letters ("Time course of light intensity adaptation in a marine diatom", Mar. Biol. Lett. 1, 175−183). I used a continuous culture of a diatom, and wanted to discriminate between cell growth and physiological acclimation. If memory serves, it took a day or two.
With respect to the above, in that experiment published in 1980, it took 10 h for the diatom to acclimate to the new, reduced, light intensity. So, 6 h is probably reasonable.
Dear Daniel,
I agree with Reto on the capacity of algae to synthesize chlorophyll in the dark (dark-dependent protochlorophyllide oxidoreductase) and in the light (light-dependent protochlorophyllide oxidoreductase). Usually, the genome contains several genes encoding for the light-dependent protochlorophyllide oxidoreductase.For instance, the genome of the green microalga Chlamydomonas reinhardtii contains two genes. The enzymes have been demonstrated to be responsible for chlorophyll synthesis at different light-intensities.
If you are interested, forward to me your e-mail address and I will send you reprints on this topic.
Best wishes,
Benoît Schoefs ([email protected])
Thank you all for taking time to respond and for your insightful comments and suggestions. I am convinced that chlorophyll-a synthesis could occur over the time period of my experiment. Reto and Benoit, am I correct in stating that increases in chlorophyll-a relative to lower light conditions may be due to a shorter time lag in the reaction chain of light-independent of chlorophll-a biosynthesis (greater lag phase in light dependent biosynthesis)?
Dear Daniel,
I think I understand what you mean. However,, I would not formulate exactly as such becasue there are two thinks.
1) when both light-dependent and light-independent prototochlorophyllide oxidoreductase are present, the light-independent one could functions continuously. At least it was what I measured in pine needles. On the top of this synthesis, the light-dependent system is producing chlorophyll at a rate that depends to several factors including irradiance and NADP availability. So when a decrease of the light intensity would slow down this lsat process keeping the first one unchanged in principle.
2) usually, when the light needed for photosynthesis is decreased, one could observe an increase in the size of the light-harvesting antenna. This increase would require chlorophyll that have to be produced. To me there are two producing ways: (a) involvement of protochlorophyllide reductase andn (b) transformation of Chl b to Chl a, this last possibility being probably valid only in green algae as diatoms and red algae have no Chl b.
Best wishes,
benoît
I'll just add one more piece of evidence that algae can increase their chlorophyll content on timescales shorter than a a day, this paper compares different model formulations for photoacclimation to data from laboratory experiments. http://www.aslo.org/lo/toc/vol_46/issue_7/1794.pdf
I think that algae considered have light independent chlorophyll biosynthesis, i.e. they can synthesize chlorophyll in the dark, so the regulation of chlorophyll biosynthesis is not so simple like in angiosperms e.g. Arabidopsis, where light is the dominant factor by two ways: 1) light reduces protochlorophyllie to chlorophyllide 2) light stimulates expression of GluTR, the key enzyme in aminolevulinic acid biosynthesis. The algae have light independent enzyme, which catalyzes the reduction of protochlorophyllide to chlorophyllide also in the dark. Mutants in this enzyme have yellow in the dark phenotype.
Very interesting discussion. I have a related question; in the cases where the cyanobacterial mats dry out. The chlorophyll wont appear at the surface because of migration downward. But after exposure to longer time the chl couldnt be detected even by HPLC. Now the question, if we assume that chl is degraded, how long does need to resynthesize the chl again?
Chlorophyll synthesis can be quite fast. For example, in Ostreococcus, the smallest picoeukaryotic phytoplankton, expression of genes for chlorophyll synthesis is regulated through the day-night cycle. That is they synthesize chlorophyll at specific times of the day-night cycle, in order to optimize their use of limited resources. https://www.biomedcentral.com/content/pdf/1471-2164-11-192.pdf
hi
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