“Are you referencing the article that states that photosynthesis in aquatic plants is saturated at 30 ppm?”

yes, I am. That link is a very nice, but non-scientific, white paper that includes the statement “approximately 30 mg/L free CO2 is required to saturate photosynthesis in submerged aquatic plants” with no supporting evidence.

“Sure it could. That could be the upper limit under conditions of near absolute stillness. Increasing circulation would only decrease the CO2 needed to saturate photosynthesis”

Under conditions of absolute stillness, the amount of CO2 needed to saturate photosynthesis would be dependant on the amount of light. Also quite likely dependant on the species of plant.

-jd

It sounds to me like you two are agreeing on the principle, and disagreeing on how to say it. Semantics are a frequent obstacle. Perhaps the reccuing use of the word "saturation" is the cause. After all, to saturate something is to fill it to capacity; so it's somewhat confusing to talk about saturation "levels". There is only one level of saturation - saturation ;).

To me it sounds like you are debating (but mostly agreeing) the minimum concentration of CO2 needed to NOT be a limiting factor during photosynthesis (with respect to light intensity, flowrate, etc).

I have no problem with the concept of saturation. What I have difficulty with is the measurement technique. We’ve established that the local environment at the surface of the plant is different than that of the surrounding water because of the difficulty of transporting CO2 across the boundary layer. We also know that the amount of that difference is dependant on several variables (consumption rate, current, turbulence, bubbles, etc.). How can it be simple to define saturation of the local environment (surface of the plant) by using a measurement made on a different environment (the surrounding water) ?

I will admit this is getting boring. Do any Botonists have comments on CO2 saturation ?

-jd

Just a currious question. We are discussion the boundry layer of the plant. Is there also a "boundry layer" surrounding the surface of a bubble? Will we also get localized co2 concentrations in that potential boundry layer?

Yes, but it is not accurate to compare the boundry layer of macrophyte tissue to that created by the surface tension of water. They are not comparable, and the mechanisms creating the questions are very different.

I'm speculating why you ask, and if I'm right, I'll head off your question...

Even if a bubble of CO2 were holding completely still, with no water movement, it would still need to create a "boundary layer" of 1500ppm (saturation concentration) to stop CO2 from dissolving any further. A bubble of CO2 can not "hold" any boundary layer of concentration if it is moving, or if any current is moving past it. In fact, the law of diffusion wouldn't allow this even if the CO2 bubble was completely still (although it would take longer to dissolve).

A couple key points I have to keep reminding myself:

O2 "saturation" in water = 12 ppm O2 (but this is actually dependant on water temperature and salinity)

CO2 "saturation" in water = 1500 ppm CO2

Photosynthesis "saturation" in plants = 30 ppm CO2

I think "maximum" would be a better word for "saturation" in that last one.

Assuming I am reading things right, the references I could find in technical literature after a brief search indicate that photosynthesis saturation with CO2 is possibly much higher than 30ppm. –caveat, these papers are for saltwater plants/algae since the CO2/HCO3- relationship makes things much more complicated in our lightly buffered freshwater systems.

“High Apparent Affinity of Photosynthesis for CO2. The results presented on Figure 4, a and b show that photosynthesis of C.crispus is saturated with a little more than 330 ul/l in the gas phase, which corresponds to 150 ul/l (i.e. 5 uM) of free dissolved CO2 concentration in seawater,”


that is 150ppm

Oxygen Uptake and Photosynthesis of the Red Macroalga, Chondrus crispus, in Seawater
EFFECTS OF LIGHT AND CO2 CONCENTRATION
Plant Physiol. (1984) 75, 919-923


Also:
“there is no apparent inhibition of photosynthesis at high CO2 concentrations”

Inorganic Carbon Source for Photosynthesis in the Seagrass Thalassia hemprichii (Ehrenb.) Aschers
Plant Physiol. (1984) 76, 776-781

That one had measurements up to 100ppm

Both these are in very high flow at saturation light levels. (there is that saturation word again).

-jd

Saturation is not a maximum. Saturation is the point at which additions cannot be used. If a typist can type 30 wpm, and they are taking dictation, they will be saturated at 30 wpm. The speaker can talk faster, but it's not going to make them able to type faster. Same with CO2 and plants.

To say that there is no one number for saturation is to needlessly confuse the issue. You could say the same thing about my hypothetical typist. It would be like saying that there's no one number that could represent one's typing speed because it's so dependent on things like the having no broken fingers, the speaker speaking loud enough to be audible, whether the speaker is speaking the same language as the typist, whether there are any distractions in the office at the time, etc. You could say the same of anything. You could say, "how long is an inch really?" It's influenced by the temperature of the object being measured, the speed at which that object is moving (thanks Einstein), and the strength of the gravitational field in which the measurement is taken. Of course everything is influenced by variables, and those variables are the reason controlled studies are important, not a reason to discount controlled studies.

As far as photosynthesis in aquatic plants, saltwater is a very different beast. I don't know of any submerged saltwater plant. I believe there are only algae that live permanently submerged in saltwater.

I agree that the article at aquabotanic is a tenuous one as far as scientific credibility goes, but that's all there is about submerged plants as far as I can find (except for some pay journal articles).

You've been on the mark re: many aspects of CO2, jbaker. Be careful not to start discreditting yourself.

http://www.seagrasswatch.org.au/seagrass.html

That's why I said, "I believe" and not "There are not." I was under the impression that sea grasses could only be submerged for short periods.

I think Ted was just teasing you jb. You guys need to use smilie's every now and again. :proud:

My take-away from this thread is that just getting the CO2 dissolved into the water may not be enough; you need to help get the CO2 from the water to the plant.

To test this, I added a powerhead next to my internal CO2 reactor with it’s flow directed at a clump of pigmy chain swords that had not been growing well. I was amazed to see that not only did the chain swords quickly turn bright green and start sending out runners, but stands of Ludwigia, Rotala, and Java Fern on the opposite end of the tank (where the current went after deflecting off the front glass) also had a dramatic increase in growth rate. Pearling thru out the tank is increased. This increased growth is so dramatic that I am now having to start increasing NO2 and PO4 dosing to keep up ! After seeing this, I am now working on adding an XP3 with an external reactor to get some stuff out of the tank and provide a large increase in flow.

-jd

jd,

So, to clarify, you did not use the "microbubble" diffuser method, but rather, simply increased circulation (of CO2 enriched water) in your tank?

Just a little something I posted a while back in this thread. :cool:

No CO2 microbubbles were introduced to the tank. just a powerhead located next to the output of my internal reactor blasting at the pigmy chain swords.

However, when the swords pearl the bubbles get blown off before they can inflate to the normal size, so there is quite a few micro bubbles flying around the tank in the afternoon.

but again, my intention was to see the effect of increased flow -and in my case it had a very profound impact on growth.

-jd