For bacteria: I believe their role is to provide nitrogen in the sand by eating algae poop. Nitrogen in the sand is required for plants in that sand to grow.
You can test this a bit:
1. Create a new sphere and fill it with water. Add a bit of sand at the bottom
2. Add algae and fastforward.
3. The algae will multiply, causing the o2 to shoot up.
4. The algae will die off because of the high o2. Their dead bodies (purple dots) will litter the sea floor.
5. Put in some bacteria.
6. You can watch the bacteria eat the purple dots. Note that the sand changes color slightly as they do - I think that's the sand becoming nitrogen-rich.
Yes the sand seems to darken a bit under where bacteria were eating, that might be the nitrogen, although it doesn't seem to spread over the rest of the sand, not sure if the plants are getting it.
Also you need to keep adding bacteria because the keep dying off. They don't have the mobility to find all the dead algae, so they are gone pretty quickly.
You can test this a bit:
1. Create a new sphere and fill it with water. Add a bit of sand at the bottom
2. Add algae and fastforward.
3. The algae will multiply, causing the o2 to shoot up.
4. The algae will die off because of the high o2. Their dead bodies (purple dots) will litter the sea floor.
5. Put in some bacteria.
6. You can watch the bacteria eat the purple dots. Note that the sand changes color slightly as they do - I think that's the sand becoming nitrogen-rich.