Abstract:Monte Conca Cave is a karst system placed in Messinian evaporites, consisting of an active cave and a resurgence located on the massif of Monte Conca, Campofranco within the "Riserva Naturale Integrale di Monte Conca". A sulfidic spring is located in the terminal gallery of the cave. To characterize the physical and chemical parameters of the Monte Conca Cave and of the sulfidic spring, air temperature, relative humidity, water pH, and concentrations of dissolved sulfides, nitrates and sulfates were monitored. The high sulfide consumption rate in the sulfidic spring, evaluated by a kinetic study, suggests that biotic consumption is dominant. Moreover, snottites and filamentous floating mats, rich in sulfur and nitrate suggest a microbial activity related to the sulfur cycle. Iron content was also evaluated in water and snottites, given its involvement in microbial activity. The microbial mats could be the source of an autotrophic system in close correlation with the biological cycle of many species of living organisms found near the spring. Some of them show typical troglobitic characteristics, while the presence and abundance of others depends on the water amount. The greater abundance of taxa found close to the sulfidic spring suggests a complex food web associated with it. The monitoring lasted a year and half has highlighted the difference between chemical-physical parameters of the cave and the sulfidic spring, emphasizing its typical microenvironment. Messina M., Grech T., Fiorenza F., Marletta A., Valenti P. and Petralia S., 2015. Sulfidic spring in the gypsum karst system of Monte Conca (Italy): chemistry and microbiological evidences.