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[APD] Aquatic photosynthesis RE: was night growth

>I agree that net carbon gain could be one way to define growth, but I don't think it is really the most useful >definition for this situation. I think many would consider plant growth to be an increase in size and mass of a >plant.  Size increases come from cell division and especially cell elongation.  The carbon required in making new >cells comes from stored carbon, not CO2 that drifts into the the cells from the air. 

All the carbon, stored or otherwise, is from CO2.
The storage is still from the CO2 at some point.
CO2 is only taken in during the day.

The biomass=> the dry weight etc does not increase at night(some uptake of other nutrients etc may occur and cause a slight increase). Shape might change, but that is NOT growth. 

What you see as far as elongation/usage of stored reserves does mean that's growth, which is what you are arguing for. In terms of simplcity, a change in size might be easy, but it's still not growth(an increase in total biomass). 

Folks are likely more concerned with daily or weekly growth rather than diurnal growth. Maybe I'm wrong, but few seem to really care that much about their night "growth". Tom asked, I gave the definition that I know from two different fields that are specific for FW Limnology and Botany.

Whether these are useful?
That's another arguement.

I think the light/dark measurements are useful using O2 and CO2. 
Saying yea, the plants grew x amount of inches at night is not useful and suggest to folks that plants "grow" at night. I guess I should leave my lights off more then huh?
How far do we take this night growth before you hit the compensation point where respiration = production?

Maximizing production is the goal in most cases or control of production at least. 
Like us, plants need their rest but they are not growing during that time, they are using reserves. 
That's not growth. 

> And cell elongation is essentially pumping water into the cell vacuoles and stretching them out.  The water >uptake needed for this increases mass and the stretching of the cell and thereby increases the size of the stem 
>(or leaf or other >structure).  I seriously doubt that someone reporting that they have a lot of plant growth >would say that they've >seen a lot of carbon getting stored, they would say the stems are getting longer and the >leaves are getting >bigger.  

Botantist typically do dry weight analysis, then measure the mass differences to get around the water issue.
I can water a plant and increase it's size dramatically, doesn't mean it "grew" at all. It's just a sponge. 

If I drank a gallon of water, did I grow?
If I took a leak, did I shrink? 

>While the exact timing of carbon uptake may be useful, or at least worth debating amongst physiologists, I think >for the average aquatic gardener (and even a plant systematist like myself!) a physical increase in size (due to >cell elongation and division) is the ideal definition.

It is ideal:-)?
If you want to say so I guess...

> If nothing else, keep in mind what Tom Wood's original question was.  I think your definition of growth applied > to his observation would lead him to think he was imagining things when the plants were actually increasing in 
> size.

No, it does not. It distingush between storage of reserves that where produced during the day time. 
No carbon reserves are produced at night, therefore no growth (net biomass gain) occurs at night. 
Usage of storage products does not imply growth.
That's my point.
He is not imagining anything, just seeing a clearer picture of what is happening and a better understanding. 

>I'm curious about your statement on O2: "Some also define growth or primary production as O2 evolved, >especially in aquatic photosynthesis."
>Could you clarify that a bit further?  I'm not disagreeing with it, I just don't understand what you are referring to.

In AP, O2 evolution from primary producers(plants, algae, BGA/bacteria) give off known amounts of O2 relative to carbon fixed.

See any Limnology text.

What are the rates of gross production, net production and respiration for a planted tank?
This can be measured rather simply for a planted tank by aquarist. Note: this is for a non CO2 tank but you can use just the O2 to get a close approximation, then turn off the CO2 for a day to get the CO2 produced by the plants. You will need to seal the tank off somehow to keep the O2/CO2 from coming in from the above air.
A plastic wrap etc might be used.

#1To  convert from mass of oxygen released to mass of carbon fixed, the values of oxygen production and carbon consumption must be multiplied by the ratio of moles of carbon to moles of oxygen (12 mg C/32 mg O2=0.375).

#2 The oxygen measurements must also be corrected for: respiratory quotient = molecules of CO2 liberated during respiration = 1 molecules of oxygen consumed. 

Note: this is for a non CO2 planted tank since we add CO2 and this would skew the results, BUT we can still estimate based on O2 production alone and then go back and get the CO2 w/o the CO2 on but the respiration rate will be less than in a CO2 enriched tank.  The example method below is for a non CO2 planted tank. 
If you remove the CO2 enrichement from a planted tank and try this, the O2 produced will be limited due to CO2 limitations at high light.
Photosynthetic quotient = molecules of O2 liberated during photosynthesis 1.2 molecules of CO2 assimilated
The first step is to figure out the average values (mg/l oxygen) for the initial, light and dark Tank.

For respiration, this is relatively easy.
Respiration = Initial - Dark  

The first conversion is to convert from grams of oxygen used to grams of carbon (the conversion is the ratio of the molecular weights; NOTE: you can only convert from oxygen to carbon for the moles of oxygen either released or used -- where photosynthesis or respiration has occurred.  Then you need to find an hourly respiration rate (easy, since the incubation time is say for 12 hours, just divide by 12).  Then, because we assume that respiration rates are the same throughout the day(they are not but to make things simpler), just multiply by 24 hours to get the daily respiration rate.  Finally, to convert from a per liter rate to tank volume, you first convert from liters to gallons or simply leave your tank volume in liters.
So the units are mg C liter/day

Gross primary productivity is also relatively easy.  Gross primary production is equal to the light minus the dark Tank. Convert from grams of oxygen to grams of carbon as before, but then you need to divide by the photosynthetic quotient to account for the production of compounds other than carbohydrates by the plants.  Because primary productivity is only occurring during the daylight hours, you have measured the total day's primary productivity with your incubation over the daylight hours.  Thus, there is no other conversion to per day.  You need to convert to per gallons from liters(or leave in liters and use that for your tank volumes) as before.  

Net primary production is where this gets a little complicated.  Net primary production = light-initial.  Then you can convert to carbon using the molar ration and photosynthetic quotient as before.  This is the EXCESS carbon (over respiration costs) being produced.  However, you only measured this during the 12 hours of daylight, for the daily rate, you will also need to subtract off the extra 12 hours of respiratory cost that will occur at night, without any more photosynthesis to offset them.  The extra respiratory costs can be determined by calculating an hourly rate from your daily respiration rate above, and multiplying by 12.


Generally, O2 is the best measurement of growth and production in an aquatic environment for the gardener.
Hence most look at pearling for gauging their tank's health and additions of PO4/higher CO2 levels really brings out the production in plants rather than me suggesting them to measure grams of Carbon fixed:)

Looking at night "growth", elongation or storage etc is not going to really do anything worthwhile for the aquatic plant tank grower or is it? 

Looking at production in terms of plant growth and seeing if you have a better growth rate based on your O2 levels seems far more useful than measuring night elongation/storage usage.

Tom Barr


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