Changes in prokaryotic community composition accompanying a pronounced temperature shift of a shallow marine thermal brine pool (Panarea Island, Italy)

Hot Lake is a recently described thermal brine pool off Panarea Island (Eolian Islands, Italy) where emitted fluids are highly saline and rich in CO 2 and H 2 S. The prokaryotic community composition in surface sediment samples was analyzed by high-throughput Illumina sequencing targeting the V3 region of the 16S rRNA at two time points that differed mainly with respect to temperature conditions, high-temperature (94 °C, HT09) and low-temperature (28.5 °C, LT10). Bacterial richness and diversity were greater than those of Archaea under both temperature conditions. In contrast to Bacteria , diversity and evenness of Archaea greatly increased at LT10. While the phylogenetic analysis indicated the presence of members mostly affiliated with the same taxonomic groups, their relative abundances differed from HT09 to LT10, resulting in different bacterial and archaeal assemblages. Both HT09 and LT10 were dominated by members of the Epsilonproteobacteria . Within this subphylum, bacteria of the genus Sulfurimonas were most frequently detected at HT09, while Arcobacter prevailed at LT10. The abundance of other dominant taxonomic groups (≥1 % of Illumina reads) also correlated with temperature conditions. Members assigned to hyperthermophilic Euryarchaeota ( Thermococci ) or to thermophilic ( Caldiserica ) and thermoresistant ( Firmicutes ) bacterial taxa were dominant at HT09, while those related to non-thermophilic Bacteroidetes , Fusobacteria and Actinobacteria were dominant at LT10. Several, probably photosynthetic, members of the Alphaproteobacteria , Chlorobi, Cyanobacteria and Chloroflexi were recovered under both temperature conditions. The co-occurrence of photosynthetic and chemolithotrophic microorganisms represents a unique feature of shallow vents such as Hot Lake.