Hi

I have to decide on an experiment in Biology for school. It is an extremely important project, so I want to make sure that I am on the right track. One problem is, that I don't have so much time, so I have to do this experiment as soon as possible...

The basic idea is to put the Elodea and the photosynthetic bacteria together in a container and observe the oxygen production of Elodea. Thus, it is about the competition for CO2.

Firstly I wanted to ask if this is actually possible, in terms of, if something will happen - is it doable? If the bacteria will take enough CO2 to actually have an influence on the Elodea? Of course it depends on the amount of bacteria, but just generally speaking.

Secondly, I know there are many issues about how to control this experiment. I would measure the O2 production of Elodea by using a common method - placing the Elodea inside a graduated cylinder under water; like it is shown here (I would use a gas syringe).
The problem is, how can I make certain that the bacteria isn't getting inside the graduated cylinder and is therefore adding O2 to the O2 produced by the Elodea - this would kind of screw up the measurement. I would take an measurement of O2 produced only by Elodea (without bacteria) to be able to compare. I don't know how I could measure the amount of O2 produced only by the bacteria... maybe using a pH indicator to at least proof there is photosynthesis going on.

Anyway, it's probably also about the type of bacteria I use. I've read about the cyanobacteria (Anabaena sp.) and was thinking, since it is called "blue-green algae" if this type is maybe floating on the surface of the water? I mean, if it is, I could be quite certain that it isn't interfering with my measurement.

If the idea of using bacteria is not good or doable, I could maybe use something different, like algae or another water plant. Maybe someone has a better idea in regard to this?

My next problem is, that I would need the ideal concentration of CO2 for this experiment. If I have too much there won't be competition going on. So another question: How/where can I find proper information about the CO2 concentration needed? Any recommendations?

Ok, I hope I could explain the basic concept and all my problems...
Of course I'll also research on my own - its just, as mentioned, I don't have a lot of time and I am trying to get this done soon.

Thanks for any tips, answers, explanations, etc.!

Since it is for school, make it as simple as possible. Forget cyanobacteria. Take flowering Elodea (Magloniophyta) and some green algae, for example Chara (Charophyta). Weigh them and take in first container 2x grams of Elodea, in second 2x grams of Chara and in third x grams of Elodea and x grams of Chara, but put elodea under one and Chara under the other graduated cylinder. It is important that you always use same volume of water in which you submerge your plants... I'll make a sketch and post it here. Since you have both plants in one container, they'll be competing for the same amount of CO2. And production of O2 should be half of production prom the first or second container... In theory. M.

Sketch. M.

Hi

Thanks for your help!

I already dropped the idea to do it with bacteria, it involves too many difficulties.
From the pictures (wikipedia) it seems like Elodea and Charophyta have a similar structure(?).
Anyway, do you think I should add baking powder to increase the CO2 concentration, or is the CO2 in the tap water enough?

don't forget that if you don't seal the vessels then they will have the co2 replenished from the atmosphere. you may not be able to see a reduction of co2. you may be able to see if there is any difference in the rate of oxygen generation but that may be due to factors other than competition for co2.

Well, Elodea are vascular ("higher", flowering) plants, and Charophyta are algae. So there is some difference between them. Since cyanobacteria are relatives of chloroplasts (endosymbiotic theory), you are actually assessing photosynthesis capability of two evolutionary descendants of cyanobacteria .
Backing soda... I wouldn't. I suppose it would change pH of the water and in that way change normal conditions for plants. You can try it, but I wouldn't.
About CO2 replenishment from the atmosphere, that is true, you really should somehow try to seal your containers hermetically. I thought that in open vessels you could have more "natural" conditions, like the one in pond... In that way you will get the maximum CO2 in water, but still, you might get differences in O2 production (or not). You can do it both ways.
Also, observe temperature of the water, illumination, pH... You might get better O2 production in Elodea in higher temperatures or vice-versa. I really don't know, that's why you should do the experiment . M.