Does anyone know how the position of the light source in a PBR will affect the production of hydrocarbons OR growth rates? I ask this important question because of one reason: the density of oil is 10% less than the density of water and oil-laden algae will rise to the top (I posit that this is perhaps the reason they form hydrocarbons to begin with). Algae are phototropic (right? why else would many have flagella?) and tend to avoid areas of darkness. Thus, if the light is placed on the bottom, there will be no incentive for the algae to produce hydrocarbons as doing so would take them further away from the light source.

 

No one seems to care about exactly where the light is placed. I have seen many companies using vertical bioreactions through which light enters from the side in order to conserve space. Maybe they should start giving second thought to this approach? 

TrevorPrinceton:

    if the light is placed on the bottom  ...   further away from the light source. 

In my part of the world, the sun always shines from the top and the ground is the bottom. So Im curious why/how you would have light coming up from the bottom?

TrevorPrinceton:

I have seen many companies using vertical bioreactions through which light enters from the side in order to conserve space. Maybe they should start giving second thought to this approach? 

I think many PBR's would rather be at a vertical angle to the sun, just like other solar uptake devices like PV and Solar water heaters.

 

 I will not have the light coming up from the bottom. I was just using the bottom as an example.

    If your theory were correct, there would then be two possibilities.  The first is that this mechanism is "blind", in that it produces hydrocarbons regardless of context.  The second is that this mechanism is "smart", in that it produces hydrocarbons dependent on the orientation of the light source.  If I had to choose one, I would probably choose the first simply because the algae would generally not need, therefore not develop a "smart" system, since the sun is always "up".  However, I can think of few examples where the smart system might be beneficial.

    That said, if we assume the first, light placement shouldn't matter, all else being equal.  However, over time in an artificial environment, it is feasible that the algae may develop a "smart" system, where it was not before, as it is forced to adapt to this new envirnment.

      It would be a good experiment to perform with light source at the top as the control and the light source at the bottom as the experimental.  Measure lipids, etc.

Silly Froggy, couldn't you work out he was talking about artificial light

Lab scale PBR's will mostly employ artificial light. If you have an air supply pumping air through an aquarium air stone and you get good circulation then the best position is side on. This way all algae will circulate past the light.