We consider Pisciarelli (Campi Flegrei, Italy) as a model site for investigating the factors which influence the distribution of microalgae in hydrothermal environments. This hydrothermal system is liquid-dominant with very low pH values both in water and in the soil; spring water has a temperature of up to 92 °C. At Pisciarelli, chemico- physical conditions are extremely variable in different sites, seasons and years. Five ecological stations were identified on the basis of temperature, pH and fluid vents conditions. Different algal communities have been observed according to their adaptability to these controlling factors. Cyanidiales were found to be largely diffused in the entire area, proving very successful at adapting to environmental changes. Chlamydomonas sp. and Pinnularia sp. were scattered in at least three of the five ecological stations. Some green acid-tolerant algae, such as Pseudococcomyxa simplex and Chlorella protothecoides var. acidicola, were strongly influenced by temperature and pH fluctuations and mainly limited to one station. Dunaliella acidophila is confined to a narrow area, being able to live exclusively in restricted pH and temperature ranges. In some sampling sites the peculiar chemico-physical conditions together with silica oversaturation cause the formation of a thick silica crust around cell walls. Generally, algae could act as a nucleation site for neoforming minerals, offering in some cases an ideal substrate for the formation of a thick silica crust on their surface.
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