The world is a steamy tropical ball reminiscient of Earth in the time of the dinosaurs. There is no permanent ice cover anywhere on Earth. An arctic continent, entirely within the polar circle, is surrounded on its seaward edge by giant mushroom forests.
These forests exhibit year-round oceanic temperatures between 10 and 20 C. There is moderate rain and heavy fog all year. The fungus that make up these forests are symbionts. Their mushroom-like fruiting structures have evolved into complex shapes filled with photosynthesizing bacteria or algae to generate energy. In this way, the biome operates much like a coral reef with the fungus taking the place of corals. During the endless days of arctic summer, the phototrophic bacteria (or zooxanthellae) generate energy; in the long months of sunless winter, the fungus absorb nutrients and decompose matter in the soil.
I want my mushrooms to be able to build skeletons for themselves to take all the wierd shapes and colors seen in a reef. Corals can build calcium carbonate shells because the raw materials are floating all around them. Calcium ions are present at 411 ppm in seawater, while bicarbonate are 145 ppm. But calcium is not that common in a wet forest that never freezes, like these mushroom forests. For example, studies of calcium in soil and biomass in various rainforests show some forests have as few as 300 kg of calcium per hectare; while even the most plentiful rainforests only about about 4000 kg. That may seem like alot, but 4 tons of limestone equivalent is not nearly enough to decorate a whole hectare with giant mushroom-trees and intricate mushroom-brain-corals.
What can symbiotic fungi use to build hard, rock-like skeletons in the way that coral reefs do?