Algae role in an ecosystem?

Phytoplankton (specifically algae and photosynthetic bacteria) are responsible for the production of about 50% of the Earth's O2. That is a fairly large role in a natural ecosystem.

I don't know exactly how much O2 is produced by macrophytes (higher plants) in a typical aquatic ecosystem, but O2 is required for the respiration of plants, fish, and other organisms. Algae take care of that need pretty well.

I would venture a guess that any "healthy" aquatic environment would include algae. I also don't believe its presence would necessarily signify an "unhealthy" aquatic environment although I imagine an excess thereof applies implications of what one might call an imbalance.

Diatoms alone are responsible for 80% of the O2, not 50% or less.
Algae is the base of the food chain for most of the world, it regulates the the cloud cover and hence the amount of radiation the surface recieves from the sun. DSMO is a gas that diatoms give off. This forms vapor droplets in the air. When there is little light coming in, there is little formation of water vapor, when there is a lot, the diatoms grow more and produce more. This cycle has been known for some time now.

If you want the "role", you need to be more specific in your question, in a lake? In a stream? In your tank? etc..........
Generally it's the main starting point between inorganic nutrients and living systems.

Unless you are a plant/algae(or a few chemotrophic deep sea or iron reducing bacteria), you need to eat something that is/was living.

Regards,
Tom Barr