It is a known fact that magnesium is a very important part of photosynthesis and chlorophyll. I have looked everywhere to try to find out this question but I have never really gotten a solid answer.
What is magnesium's role in the chlorophyll molecule and what if it was replaced with another alkaline earth metal, like beryllium or calcium?
You couldn't just replace the Magnesium directly without making other changes.
Taking beryllium as the first example.
In Plant-like organisms there are at least two different types of Chlorophyll, they work in pairs synergistically, one as an oxidizing agent and one as a reducing agent in the ATP and NADPH cycle (synthesis of glucose), (more on Photosynthesis). You would need to consider binding energies between the Mg+2 ion (Cation) and any corresponding Anion. Binding energies for beryllium are much higher, meaning you would need higher-energy photons to be absorbed by your new version of chlorophyll (let's call it NVC).
The thing about the chlorophyll molecules is that they have tails acting as antennas, tuned to a specific wavelength of light, corresponding to it's absorption spectrum. In the case of your NVC, you'd need to tune it to a higher frequency - ie. truncate the tail of the molecule, and in a wavelength-specific way.
The electromagnetic spectrum is shown in this table, higher energies (shorter wavelengths) nearer the top.
I'm not enough of a physical chemist to tell you how you can do this, but I can say that beryllium would likely lead to your plants needing ultra-violet and beyond energy photons, perhaps even into the x-ray spectrum to work, if it would at all.
Edit: calcium's binding energy turns out to be less than magnesium, perhaps a longer-tailed NVC is in order and absorption of infra-red light would be the method of photosynthesizing.
I initially thought this is not about world building, but then you asked about switching magnesium for something else.
Most chlorophylls have mangesium in their chlorin core. Chlorophylls absorb energy from light and then transfer that energy to other chlorophylls by ressonance. This is a very delicate and fine tuned (no pun intended) mechanism. Changing any part of the whole molecule changes its color - seriously, check the difference between chlorophylls A and B - and thus:
In theory you can make any change to the molecule as long as it does not become poisonous and still absorbs light in some spectrum. For example, pheophytin is a blueish chlorophyll.
In practice, though, nature selects for what is most efficient. Red and purple-leaved plants do exist, but looks like the green variety of chlorophyll is unmatched in photosynthetic efficiency. A different planet with a different parent star might make it so that some other color is best. But under a sky that is blue most of the day and red by the end of it? Go for something that absorbs those colors, causing the leaf to be green. In other words, magnesium-based chlorophylls.
Also notice that the atom you wish to use must be abundant. You would be hardpressed evolving niobium-based chlorophylls regardless of their color, for example.
