# Phun Physiology Phact Phour



## Phil Edwards (Jan 22, 2004)

Why do plants grow, if not exactly thrive, in low light tanks with no special treatment? With a high load of well fed fish they get all of their nutrient needs met, but what about carbon? Enter the Compensation Point.

Plants have a number of compensation points that are related to CO2 produced and CO2 used. So, what is a compensation point? It's the point where cellular respiration emits the same amount of CO2 that photosynthesis takes up. Basically, it's where the plant is generating its own CO2. 

Plants, like animals, are constantly respiring. That is oxidizing sugars in order to provide the energy necessary to maintain their existing tissue and put on new growth. The general formula for this is C6-H12-O6 (Glucose) -> 6 CO2 + 6 H20. On the flip side, the general formula for Photosynthesis is 6 CO2 + 6 H2O -> C6-H12-O6. I'm sure Tom's going to ask some silly question about enzymatic details, and there are a lot, but for purposes of this discussion we'll stick with the above reaction equations. 

The easiest to talk about is CO2 compensation point. It's basically the equilibrium between respiration and photosynthesis. X of this in = X of this out. However, the CO2 comp. point is affected greatly by two things, light and temperature. 

Light compensation point. This is the point along an illumination curve where CO2 in = CO2 out. For some plants this point is higher and for others it's lower. As a general rule, the higher on the light axis of the curve one goes the greater the imbalance is in favor of photosynthesis. This is why at moderate to higher light we have to have some external (to the plant) source of carbon, whether it be soil decomposition, Excel, or CO2 gas. 

Temperature compensation point. Temp. comp. point is the opposing force to the light comp. point. As temperature increases so does the metabolic rate of a plant. This would cause an imbalance in favor of respiration, and respiration emits CO2, so this is a good thing, right? To a degree. 

As T increases the plant's need for photosynthates also increases. If Ps (photosynthesis) doesn't increase as well the plant is going to start using up carbohydrate and lipid reserves that under the circumstances it might not have much of. Most of the energy produced by Ps goes into maintenance of current tissues and not into the building of new growth. If a plant is already not producing much because of low light and limited carbon it won't have much in the way of stores. Increased respiration without a summary increase in Ps will eventually cause the plant to start cannibalizing itself for the necessary elements. 

Another important factor to consider in the increase of temperature is RuBISCO activity. RuBISCO is the enzyme that binds CO2 and O2 to photosynthetic carbon intermediates (RuBP). At higher temperatures RuBISCO has a tendancy to bind O2 to RuBP instead of CO2 and this is bad news for the plant. This mistake is going to cost the plant energy to recover some, but not all, of that metabolically "lost" carbon. 

Ok, so what does this all mean? At lower temperatures and lower light plants are able to generate the majority of the carbon they need to survive/maintain current tissue. If we're really lucky there might be enough to produce a small amount of new growth. A high fish load to process food and produce some metabolic CO2 will be what tips the chemical balance in the plants' favor and will allow them to put on new growth, albeit slowly. 

That, my friends is why low light tanks with no extra treatments can sustain plant growth. Why are certain plants are better able to survive in these conditions? As my favorite tv cook is fond of saying....that's another show.


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## Sir_BlackhOle (Jan 25, 2004)

Very interesting post!


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## Laith (Sep 4, 2004)

I agree, very interesting!

So at lower temps (22-24C?) and lower light (1-1.5wpg?) and an adequate bioload, we'll have a balanced system where the plants are healthy and also manage to produce new growth (I assume with very little algae...), though slower than a in a high light situation?

Tell me again why we're pouring light, CO2, ferts, time and money into our tanks?  ](*,)


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## Phil Edwards (Jan 22, 2004)

Laith said:


> I agree, very interesting!
> 
> So at lower temps (22-24C?) and lower light (1-1.5wpg?) and an adequate bioload, we'll have a balanced system where the plants are healthy and also manage to produce new growth (I assume with very little algae...), though slower than a in a high light situation?
> 
> Tell me again why we're pouring light, CO2, ferts, time and money into our tanks?  ](*,)


You *should* have a balanced system it's not guaranteed, but is more likely than not. Algae, well, that's another matter entirely and it depends on the ability of the plant mass to do whatever it is they do that discourages algal growth.

We pour light, CO2, ferts, time, and money into our tanks because we like the horticultural aspect of forcing growth and the results of that work. Plants may be able to survive and grow to a point under the lower light conditions, but it's proven that they do better when their nutritional needs are met and the above stuff does that.


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## Bert H (Mar 2, 2004)

Nice article Phil.


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## plantbrain (Jan 23, 2004)

So Phil , who discovered the O2 oxygenase activity of RUBSICO(hint: he's an old prof of mind at UF) and what is this "lost" referred to in plant biology?

What are the other 2 by product losses from that "process"?
Also, what if the organisim(like many algae) do not possess perxisomes?
How do they cope?

Regards, 

Tom Barr


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