Algae are rich in minerals, vitamins and trace elements. Fossil algae too are valuable. The ornamented silica shells of diatoms form huge fossil 'death assemblage' deposits of diatomaceous earth. In 532 AD, light-weight diatomaceous earth bricks allowed the builders of Constantinople's Hagia Sophia cathedral to construct its 32.6 m diameter dome. They occur in most habitats, especially in those unsuitable for the growth of higher plants.
Lichens are amongst the most indispensible of all bio- indicators.
Growing in sites of often limited nutrient availability, they take up mineral ions with great efficiency. Their sensitivity to sulphur dioxide indicates both local levels of pollution and acid rain. Their tolerance and accumulation of metals, including radioactive isotopes, similarly provides evidence of a range of industrial pollutants.