Seaweeds usually live in shallow water, because they need sunlight for energy. Those in shallower waters may be battered by the waves, and the survivors tend to have tough 'stems' and strong holdfasts fixed on rocks, such as those of oarweed. They are too small to be seen singly, but colonies sometimes form a brownish 'tidemark'. Over millions of years, their silica-based skeletons have formed deposits on the sea floor up to 300m thick. This is dug up as fuller's earth, once used to clean wool and now to decolourize oil and gas.
Lichens are a remarkable partnership between algae and fungi.
The algae are tiny green balls, which make the food to feed the fungi. The fungi make a protective layer above and beneath, and hold water. Some lichens provide food for reindeer in the Arctic. The orange lichen here is Xanthoria parietina, which grows on roofs in the country. Lichens are very sensitive to air-pollution and will not grow where the air is dirty.