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Aquatic photosynthesis and respiration
By Matheus Carvalho.
Respiration is a fundamental physiological process present in all aerobic living beings. It is also important for the global carbon cycle, but very few measurements are available for respiration rates of aquatic communities in the light. Without this measurement, it is also difficult to know the real magnitude of gross primary production.
People measure photosynthesis and respiration by doing incubations in closed vessels. In a closed vessel containing natural water in the light, carbon transits between the inorganic pool (water) and the organic pool (organisms) by means of photosynthesis and respiration, as shown below:

Then, people measure the net exchange in the dark (another incubation), to obtain respiration, and then, by difference, calculate photosynthesis (or gross primary production) in the light. However, nobody really knows whether respiration in the dark is the same as that in the light, and thus this procedure can be erroneous.
Another way to measure photosynthesis is to add 14C to water and measure how much accumulates in the cells (represneted below by blue cells appearing):

The problem with the 14C method is that no one knows whether sometimes 14C instead of 12C goes back to water during respiration. I other words, it is not possible to know whether net or gross production is being measured.
I am working on a solution for this problem and will try to publish a paper about that soon.

Other interests: Fractionation / discrimination of carbon stable isotopes in photosynthesis and respiration by algae

In photosynthesis, 12C is absorbed faster than 13C. This difference in absorption speed is called isotopic fractionation.
My study has shown that when photosynthesis is stronger, the difference in the velocity of absorption of different C isotopes (isotopic fractionation) diminishes, as shown below:

Notice that in slower photosynthesis the proportion 12C/13C entering the cell is 5/1, while in the faster photosynthesis it is 3/1. In other words, proportionally more 13C enters the cell when photosynthesis is faster.
I have also tested the influence of temperature on fractionation in photosynthesis, and found that for a same photosynthetic rate fractionation increased with temperature:

Look that the proportion of 13C absorbed is higher at the lower temperature. The photosynthetic rates are the same for both temperatures (look carefully, counting the atoms being absorbed, and you will see!).

Overall, my results can be explained based on a demand-supply control of fractionation. When there is more carbon available (carbon supply), it becomes easier for 12C to react faster, and thus fractionation values go up (towards 3%). The opposite is true when carbon is not very available. Hence, when photosynthesis is strong, carbon is less available, and fractionation values go down, as in the results obtained in my research. When temperature increases, diffusion rates of molecules also increase, making carbon more available, which increases fractionation, also as in the results of my research.

More recently, I investigated about the fractionation that happens in respiration by seaweeds. I found that the overall fractionation was in average small (0.3%), and variation could be somewhat explained by the consumption of different organic substrates (lipids or carbohydrates).

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