Photosynthesis in the caves of Frasassi (Italy).

Photosynthetic organisms inhabiting the caves of Frasassi (Italy) were identified and aspects of their strategy of acclimation to the cavern environment investigated. The habitats of the organisms were characterized with special reference to light, the inorganic carbon system and the availability of inorganic N, P and S. The availability of nutrients was low. The availability of inorganic C was mostly affected by pH, but it was generally high enough to support photosynthesis at the photon flux density measured. The blue-green alga Phormidium sp. was by far the most abundant photosynthetic organism in the caves. The adaptation of Phormidium to growth at low light and abundant inorganic carbon was mediated by a down-regulation of photosynthesis and the CO2-concentrating mechanism. The down-regulation of these processes is probably an adaptive adjustment that paces the energy output to the limited light input. Heterotrophy may also be important for the survival of Phormidium.