Quote:
Originally Posted by mameroo2000
As a plant to grow properly you need to provided the right habitat, which depends of the species they need usually a light intensity of 2500-5000 lux (250-500 footcandles) Conversion of foot candles to lux or lumens is: one ft. c = 10.8 lumens or lux, for nannochloropsis oculta (the mos common phyto to culture) you need 2,500-8,000 lux...You would get that intensity in a led warm white light on a 16 hours on that will allow the micro algae to breething co2 and you need 8 hours in full darkness for breething oxygene...like this system you will have a balance with your PH level inside the reactor chamber. Light shading by algae cells would become limiting as density increase, at high densities incoming light is shade from all but the cells currently at the outer surface of the reactor, that is mean is time to harvest the culture.
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Microalgae do not require a prolonged period of darkness to "breath oxygen". Microalgae require ATP to fix CO2. They produce ATP only when illuminated, and therefore will not fix CO2 in darkness. As soon as CO2 fixation ceases, growth will also cease. I'm not saying you can't grow microalgae if you give them a dark period, I'm saying growth is maximized if they are illuminated 24/7.
Here are a few references that explain the reasoning, which is firmly based on the well established physiology of plant cells and photosynthesis.
"..long dark periods (i.e., of the order of magnitude of several hours) generally result in biomass loss, as well as decline in growth rates, because microalgae switch to respiration processes; several authors have indeed suggested that a suitable dark period should be of the order of miliseconds (Kommareddy and Anderson 2004)—which would be more appropriately seen as an intermittent cycle"
From - Carvalho et al. 2011. Light requirements in microalgae photobioreactors: an overview of biophotonic aspects. Applied Microbiology and Biotechnology, 89, 1275 - 1288.
"This internal shading (clearly visible in that light does not pass through the culture’s optical path, being essentially fully absorbed in the outer surfaces), results in cells receiving light intermit- tently, a phenomenon augmented by the fact that light energy attenuates exponentially in passing through the culture column. The higher the cell density, the shorter the depth light penetrates into the culture. Two light zones are thereby established in the culture: the outer illuminated volume, in which light is sufficient to support photosynthesis (i.e. the photic zone); and the dark volume, in which net photosynthetic productivity cannot take place, since light intensity is below the compensation point (Fig. 8.1). The higher the population density (and the longer the optical path), the more complex it becomes to address the basic requirements for efficient utilization of strong light, i.e. an even distribution of the available light to all cells in the culture, at an optimal dose per cell (to be elucidated somewhat later).
Clearly then, when mutual shading prevails, cells are not exposed to continuous illumination but rather to cycles of light and darkness (L–D cycle), which may take scores of milliseconds to a few seconds to complete, depending on the optical path and the extent of turbulence in the culture. The endless combinations of light intermittency expressed in L–D cycles to which the individual cells are exposed at a given instant, relate to two basic para- meters: first, the ratio between the light and the dark period in the cycle and second, the frequency of the cycle. As shall be elucidated, the higher the frequency of the L–D cycle, the more efficient strong light may be used for photosynthesis."
From Richmond (2005) - page 127/128. "Handbook of Microalgae Culture".