Geosphere-biosphere interaction in extreme environments: microbial life's capability of shaping VOCs emissions from the soil in the Solfatara Crater (Campi Flegrei, southern Italy)

Volcanic and hydrothermal areas represent extreme environments, being affected by high-temperature and low-pH fluid emissions. Microbial life develops in such a harsh and spatially rapidly changing environment, affecting and being affected by the local geochemical conditions. Whilst efforts have been made to investigate the interaction between biotic and abiotic factors in deep-sea hydrothermal vents, with relevant implications for the origin of life on Early Earth, the occurrence and extent of interplay between microbial communities and hydrothermal fluids in continental solfataric fields are still poorly understood. Solfatara Crater (Campi Flegrei, Italy) represents a perfect natural laboratory to study the relationship between extremophilic microbial communities and CO2-rich fluids uprising from deep hydrothermal reservoirs. Here, microbial activity was suggested to exert a trivial control on hydrothermal emissions from diffuse degassing. In particular, evidences of shallow production and consumption of volatile organic compounds (VOCs), involving both hydrocarbons and heteroatomic (S-, O- and Cl-bearing) species, were recognized (Tassi et al., 2015), pointing to a crucial role of biochemical processes in shaping the composition of VOCs diffusively emitted from the soil of the crater. In this study, geochemical and microbiological data of soil and gas samples from two different depths (10 and 30 cm) along 5 vertical profiles and 2 sediment samples from two acidic mud pools (Fangaia Piccola and Fangaia Grande) are presented. Microbial diversity was analyzed by 16S rRNA gene sequencing and in situ hybridization in the framework of Census Of Deep Life Sequencing Opportunities 2016 - Deep Carbon Observatory. Despite the extreme temperatures (up to 70 °C) and pH (from 1.3 to 3.6) values, the total prokaryotic abundances ranged from 7.23 × 106 to 439 × 106 cell/g WW. However, remarkable phylogenetic variations in both Archaea and Bacteria communities were observed at changing environmental conditions (pH, T, soil gas fluxes and interstitial soil gas contents) among the selected sites. These analytical results provide a detailed picture of biodiversity in a hydrothermal environment and give insights into the complex inter-dependence between the chemical and isotopic features of the fluid constituents involved and the microbially-driven processes. These findings represent an important step forward towards a better comprehension of terrestrial solfataric fields, opening the way to follow-up investigations on the estimation of diffuse VOCs emissions from volcanic/hydrothermal areas and the development of novel bioremediation and biomedical applications. Tassi F., Venturi S., Cabassi J., Capecchiacci F., Nisi B. & Vaselli O. 2015. Volatile Organic Compounds (VOCs) in soil gases from Solfatara crater (Campi Flegrei, southern Italy): Geogenic source(s) vs. biogeochemical processes. Appl. Geochem., 56, 37-49.