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CO2 mist effect - relief from photorespiration?

Discussion in 'Advanced Strategies and Fertilization' started by wapfish, Nov 4, 2005.

  1. wapfish

    wapfish Prolific Poster

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    Hi Tom, this is mainly a question for you, but I'd appreciate feedback from anyone else also.

    With regard to the CO2 mist effect, where plants grow better using a venturi reactor to create microbubbles, I wanted to pursue the idea that this phenomena might actually have its roots (no pun intended :) ) in the fact that high light/CO2 tanks can in fact become limited for growth by photorespiration.

    Maybe this idea is way off base, but isn't it possible that what's really happening is that the venturi is mechanically forcing O2 out of solution, perhaps to a much greater extent than CO2 (ie CO2 microbubbles redissolve rapidly whereas the O2 microbubbles do not), and in this way the dissolved O2 concentration in the tank gets lowered much more than the dissolved CO2? If photorespiration were initially limiting for plant growth, preferentially reducing the O2 concentration would be expected to enhance CO2 utilization and thereby increase growth.

    This would mean that the microbubbles are mostly oxygen of course, or at least that the bubbles that persist are mostly oxygen.

    I don't know if it's possible to force a gas to leave solution by forcing a (saturated/supersaturated) solution through a small orifice, but it seems like this would happen. Another more important point is whether the dissolved O2 concentration in a high light/high CO2 tank could rise to anywhere near what might be considered necessary for photorespiratory effects in aquatic plants.

    Anyway, the main question, aside from speculations about mechanisms, is whether you feel photorespiratory effects might somehow be playing a role in the mist effect. In researching this business a bit more last night I came to realize that you've got a bit of history behind you in terms of photorespiration, so I'd appreciate any thoughts you have on this one way or the other.

    Thanks.
     
  2. Tom Barr

    Tom Barr Founder
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    Re: CO2 mist effect - relief from photorespiration?

    Oh no, it's not off base in the least and PR(hence forth photorespiration = PR) has not been studied much in submersed systems.
    These are very good questions.

    High DO levels are toxic and plants can lose nearly 40% of their fixed carbon at higher levels of PR. A number of things increase PR though besides O2(Salt stress, higher temps, water vapor loss from CO2 demand-not an issue with us, excess light).

    Few studies have been done in terms of aquatic plants and PR. Hydrilla, coontail, milfoil and some other weeds have most of the research. There is an excellent paper I have on PR in aquatics laying around here some wheres.

    If you are interested, I'll dig it out.
    I'll have an issue on O2 cycling for the BarrReport in the near future.
    Ca++ is next up.

    I'm 100% positive I will make changes to the CO2 mist idea but I wanted to get folks thinking and trying it first.

    A number of things can counter act the PR, adding higher CO2 levels is one.
    The other is how much O2 will cause a negative response.

    Or perhaps a better question/s:
    At what range does the benefits outweigh the gain with O2 levels? Can we add too much O2 with plants alone?

    I had suggested high O2 and PR would be the cause for algae decline in a well planted tank suggesting algae had less mechanisms to deal with the higher O2 levels than the plants.

    This idea of mine proved false.
    Algae species changed was all, but the total algae biomass remained the same.

    Damn!

    I was hoping so much that was true it at the time.

    How is it that the O2 is being forced out of solution with the internal reactor? There are no bioballs and the flow is always out, not in, the reactor is not pressurized with any backpressure etc. The orifice alone I can run while adding CO2 with another reactor to see if any O2 or gas accumulates.

    I think if you try this:

    Running a control reactor w/venturi without CO2
    Then a reactor with CO2 being added in the same tank, you will have your answer and rule out the orifice and degassing.

    But perhaps some "seed" gas is needed? N2 would be good.

    This is why this entire thing is interesting, we have conflicting observations from all 3 methods of dissolving the gas.

    A comparative approach is a lot of work and takes longer, but it's what I'm trying to do.

    I'll try and hunt down some of the papers for you, you can try PR for wetland/aquatic/macrophytes in a search engine and also off of FL's IFAS's aquatic plant database.

    Regards,
    Tom Barr
     
  3. wapfish

    wapfish Prolific Poster

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    Re: CO2 mist effect - relief from photorespiration?

    Hi Tom. Thanks for the info. I'm relatively new to aquariums, but do have a basic grounding in biology. I couldn't help but wonder about PR with all the enthusiasm given to "pearling" etc by planted tank enthusiasts.

    Didn't know any of this. Excess light? That's also not likely to be relevant for us is it? Or is it?

    I'd certainly like to take a look if you can find it. Just the reference would probably be enough.

    I understand that this is true at the enzyme level in the sense that CO2 and O2 are competitive inhibitors for RUBISCO, but in a plant in an aquarium, if photosynthesis were running strongly and O2 levels had already risen high enough to cause PR, it would seem that adding more CO2 would only give a temporary fix as oxygen accumulated and rose to even higher levels in the tank.

    Absolutely.

    Cool hypothesis and interesting result, even if it wasn't what you were looking for.
    Exactly. The idea of a "seed" might be key. If DO is in fact supersaturated (a real possibility, given that algal blooms in lakes, etc are known to give rise to O2 supersaturation) then couldn't a "seed" microbubble theoretically cause O2 to come out of solution in the same way a seed crystal causes a supersaturated solid to precipitate? As the venturi setup was originally described, the seed microbubble would be from the CO2 being injected, which drags out supersaturated O2 that has accumulated in the tank from all the photosynthesis. An N2 microbubble should do the same thing. Just have to make sure the CO2 levels are kept the same when comparing one case with another.

    The bottom line for PR effects to be involved, though, is that DO levels have to drop after turning on the venturi reactor. If they don't, PR effects are pretty much ruled out aren't they? Can't be reducing PR if DO isn't lower.

    Righto. I'll hunt around a bit as you suggest. Thanks.
     
  4. PK1

    PK1 Guru Class Expert

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    Re: CO2 mist effect - relief from photorespiration?

    This may be relevant to the discussion:

    A few months ago I installed a powerhead in my 90 gal. to increase flow in one side of the tank, and turned it down to about 1/2 capacity by using its built-in flow controller.

    I always got small bubbles coming out of the powerhead 4-6 hours after the lights were on, but not from the discharge of my eheim filter. When I looked at the powerhead design, I realized that the flow controller was reducing the orifice size at the "intake" of the pump and not at the discharge. This meant that there was a pressure drop between the intake orifice and the impeller, which probably caused the O2 to come out of the solution towards the end of the day when concentration of O2 was higher in the solution.

    As for why this was not the case with my eheim, I theorized that there was no pressure drop between the impeller and the intake since the canister was installed under the aquarium and the syphon action alone would probably have a higher flow than the pump, so no pressure drop existed (the pump didn't have to suck the water in the canister, it was freely flowing in by itself). One way to confirm this would be to raise the canister and put it at a higher level than the water to see if the same thing would happen - or simply close the inlet valve enough to create pressure drop to see if the same thing would happen. It would take a while to see the bubbles since they would get stock in the canister but eventually they would come out.

    At the time I removed the powerhead since I didn't like the bubbles all over my tank. I wonder if I completely removed the restriction at the inlet of the pump and put it back in the same thing would happen.

    If the bubbles I was seeing were actually O2 coming out of the solution, and this would help reduce photorespiration, then maybe I should put the powerhead back in!

    This a very interesting discussion and I look forward to more back-and-forth on this topic.
     
  5. wapfish

    wapfish Prolific Poster

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    Re: CO2 mist effect - relief from photorespiration?

    It's interesting that a simple powerhead with a pressure differential is able to "generate" bubbles from an oxygen-laden solution. Although Tom's venturi reactor is not specifically pressurized, a venturi by definition involves a pressure differential, so I do believe there is the potential for the same sort of thing to be happening there. It would certainly be interesting in your situation to put the powerhead back with and without the inlet restriction and observe bubble formation and plant growth in the two situations. Potentially, you might find pretty much what Tom did, achieving it in a slightly different way. Still, DO measurements would be necessary to implicate or to rule out PR involvement.
     
  6. Tom Barr

    Tom Barr Founder
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    Re: CO2 mist effect - relief from photorespiration?

    The orifice will cause cavitation, but whether this is O2, or CO2, CO2 is at a much higher level in terms of supper stauration, 10x vs O2's at about 100%, maybe 1.5 if someone has their tank chugging along.

    Generally it takes a fair amount of pressure changes to cause this though.

    Regards,
    Tom Barr
     
  7. Tom Barr

    Tom Barr Founder
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    Re: CO2 mist effect - relief from photorespiration?

    I'm not so sure it is O2. CO2 is also at a much much higher super saturated level, 6-10x more than O2.

    It's weird though, why would it only take a little super saturation for this to occur?

    Also, look at the mist size PM late in the day compared with AM.
    It tends to be finer and persist longer.

    George Bowes was a past prof of mine, he's the guy that discovered Oxygenase activity in Rubisco.
    I have a few paper's of his, he worked with Hydrilla and an single celled algae.

    I'll post them here for folks.

    Regards,
    Tom Barr
     
  8. wapfish

    wapfish Prolific Poster

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    Re: CO2 mist effect - relief from photorespiration?

    Not sure I follow this. By definition, a supersaturated solution has a concentration of solute that is higher than its solubility.

    O2 is relatively insoluble in water, 8 ppm being a reasonable value at moderate temperature and atmospheric pressure. Supersaturated values would therefore be anything in excess of 8 ppm. As I said earlier, O2 supersaturation is common nature, with lakes and ponds frequently achieving O2 supersaturation when algae bloom and photosynthesize. It seems likely that O2 levels in planted tanks might also reach supersaturated levels, though I've not yet come across a specific reference where this has been demonstrated. Knocking a supersaturated solute out of solution is relatively easy either mechanically (shaking, scratching) or chemically (seed crystal), which is why it wouldn't take much to cause O2 to come out of solution if it were indeed supersaturated.

    CO2, on the other hand, is nowheres near supersaturated in aquarium tanks, even with injection. Although the actual concentration of dissolved CO2 is routinely much higher than O2 (6-10X more), the solubility limit for CO2 in water is way up at around 1700 ppm or so at moderate temperatures (eg http://jcbmac.chem.brown.edu/myl/hen/carbondioxideHenry.html ). Since CO2 is not supersaturated, it will be much harder to get out of solution (if it can in fact be done at all without going to extremes). Dispersing injected CO2 as microbubbles, however, could (I think!) cause supersaturated O2 to "precipitate" out of solution in much the same way that a "scratch" can act as a seed to precipitate a supersaturated solid out of solution. It provides a nucleation center for the O2 to latch onto.

    Supersaturated crystals tend to be tiny if they're knocked out of solution rapidly, so one hypothesis might be: early bubbles = primarily dispersed injected CO2, while later bubbles = primarily O2 lost from a supersaturated solution...?
     
  9. PK1

    PK1 Guru Class Expert

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    Re: CO2 mist effect - relief from photorespiration?

    I tend to agree with wapfish, a less soluble gas would be easier to precipitate out of the solution (or in this case bubble to the surface).

    Another thing to add is that at the time I was running CO2 24/7, and had no bubbles coming out of the powerhead in the morning, but mainly in the evening. This strengthen the case for the bubbles being O2. Of course I am conjecturing & cannot put science behind this as eloquently as Wapfish did without more research and experimentation.

    I will give this another try when I have some time... If I notice anything more or anything different I'll add to the thread.
     
  10. defdac

    defdac Lifetime Members
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    Test for negative effects of photorespiration

    If it is photorespiration limiting growth it would be easy to see with a time-lapse movie shooting a fastgrower with one frame every minute?

    It should grow much faster early in the morning when O2-levels are low than later in the day with supersaturated O2-levels?
     
  11. wapfish

    wapfish Prolific Poster

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    Re: CO2 mist effect - relief from photorespiration?

    Hi Tom, I was thinking more about this and now realize what you meant about CO2 being supersaturated. Per Henry's law, we do have more CO2 dissolved in our tanks (~30 ppm) than should be there based on the partial pressure in the atmosphere, which is why agitation, etc causes it to come out. The CO2 is in fact supersaturated with respect to the air-water interphase. On the other hand, CO2 gas bubbles within the tank, since they are 100% CO2, would not be supersaturated with respect to the surrounding liquid. For a 100% CO2 atmosphere (a bubble), maximum solubility is per the chart I referenced earlier ~ 1700 ppm.

    Anyway, this stuff is pretty technical and could be entirely irrelevant with regard to explaining the enhanced plant growth. If the dissolved O2 goes up when using the venturi reactor, forget PR! I'm anxious to see how it all sorts out in your experiments.
     
  12. Tom Barr

    Tom Barr Founder
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    Re: CO2 mist effect - relief from photorespiration?

    It may take awhile.
    And there are many competing notions.

    What I do know is that the method is cheap, works like nothing I've ever seen except small isolated regions next to the diffusers that produced mist.

    I suggest folks try it, we have plenty of time to work out why. I thought I might be able to address it in a month, but nope.

    Still, many are considering the CO2 a great deal now and looking at it much more critically after back burning it for so long.

    Regards,
    Tom Barr
     
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