How does algae build its calcium carbonate shells? Does it just use calcium carbonate that's already dissolved in ocean water? Or does it create calcium carbonate? I've seen hints that they remove calcium from the water to create the shells, but no details.
It is my understanding that sea creatures (flora and fauna) take calcium from the surrounding waters and combine it with Carbon from dissolved CO2 to build their skeletons.
At least that was what they taught me while I was studying for my Marine/Environmental Chemistry degree.
I haven't done any research since graduating though.
One factor which can dramatically effect the rate of growth within sea creatures is the availability of Calcium in the water. Why do you think corals grow in warm seas? Warm water makes the CaCO3 much more bio-available. In cold waters they have to expend more energy to extract it and to stop it re-dissolving into the surrounding waters.