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#29
09-22-2013, 10:13 PM
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1) The dark reactions are a misleading term that really is not used much anymore, although many scientists still say it out of habit. It leads people like you to believe these reactions can only occur in the dark, when that is not true. These reactions occur only while the light-dependent reactions are producing ATP, since ATP is required to fix carbon. They are more accurately called the "Calvin-Benson Cycle" or "Light independent reactions". There are examples of carbon fixation only occurring at night, but they are in multi-cellular plants often called "CAM Plants" (this is a mechanism is prevent water loss in hot terrestrial environments). 2) Inside a dense algae culture, there is a "photic zone" (light) and a "non-photic zone" (no light). Algae cultures are constantly being circulated, usually using an air pump. This transports microalgae cells between these two zones. Since the light-dependent reactions occur on a nearly instantaneous timescale, the rate limiting part of photosynthesis are the light-independent reactions/ Calvin cycle. All photosystems in a microalgae cell can become saturated instantaneously at high enough light intensities, meaning there are not enough electron carriers to process any more incident photons. These electron carriers are re-generated during the Calvin cycle. This is the physiological reason why the Calvin cycle limits photosynthetic productivity. 3) Therefore, if the goal is to maximize growth rates, ideally the algae culture will be circulated such that each cell spends milliseconds in the non-photic parts of the culture and nanoseconds in the photic zone. This ensures a constant supply of electron acceptors to process incoming photons. So, finally, to answer your question: the dark time occurs when the algae cells are in the non-photic parts of the culture. If you want to maximize productivity, this dark time should not be hours, it should be milliseconds. This is achieved by transporting the cells in and out of the photic zone really quickly via circulating the culture with air bubbles, NOT by turning the lights off for several hours. 4) If you do turn the lights off for several hours, you are only regenerating the electron carriers ONCE! Once the lights are off, those electron carriers are no longer needed for photosynthesis, and will not be useful until the lights are turned back on again. Once the lights are off, the algae cell will no longer be producing sugars from photosynthesis. Instead, it will consume them via cellular respiration in order to keep the cell alive. This is obviously not a productive time for the algae culture. It does not keep the culture "healthy". Quote:
Excessive light intensity can cause damage to the cell. This is due to a couple of reasons. 1) the photosystems cannot process more than a certain number of photons due to a limited supply of electron carriers. 2) Excess energy can form reactive oxygen species, which cause all sorts of damage. It should be stated that cells are capable of adapting to a variety of intensities in time. Extending the photoperiod under excessive light intensity will cause more harm than good. I am not saying you should illuminate the cultures with excessive light intensity. That doesn't make sense. I am saying to maximize productivity, cultures should be illuminated at a non-damaging light intensity 24/7. I have not read this book you are referring to. For my everyday reference, I use "Handbook of microalgae culture" as well as "Algal Culturing Techniques". I would want to read the full passage you are referring to before commenting. You have severely misunderstood another reference you have cited, so I am honestly highly sceptical of your interpretation of this book as well. The concept of 24/7 light for algae cultures is not a debated topic in the field of microalgae biotechnology, so I honestly have no idea why the authors would say that, and I assume there is more to the story. The fact that they refer to "color temperature" shows me this is not a highly scientific publication to begin with. In this science, wavelength and spectrum are referred to, not "color temperature". Quote:
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"Dark phase respiration" - Are these your words or the authors? It seems to be some combination of "Dark reactions" and "Cellular respiration" which are completely different processes. The dark reactions occur during daylight! They cannot occur at night. Cellular respiration occurs all the time. Plants consume oxygen all the time, but fortunately they produce more than they use while illuminated. At night, they only consume it. It sounds like you are referring to cellular respiration in your description, so remember this occurs all the time (not just at night). Quote:
I too have cultured many different species of micro algae, but that doesn't teach you about their physiology. It teaches you how to inoculate cultures, aerate them, light them, and keep them going, and if you are observant, a bit about their life cycle. It does not teach you that, for example, they have a limited supply of electron carriers, nor when the Calvin cycle reactions occur. There are a few practical reasons to light algae at 16/8, as Myka has said. It consumes less electricity and it prevents high pH's if you do not aerate with CO2 to keep it down. But there are no physiological reasons for it. 
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