Microbial diversity in a shallow-hydrothermal brine pool as revealed by Illumina sequencing

Hot Lake is a recently described thermal brine pool off Panarea Island (Italy) where emitted fluids are highly saline and rich in CO2 and H2S. The prokaryotic community structure and composition from sediment samples differing in temperature (94°C, HT and 29°C, LT) were compared by a massive parallel sequencing (Illumina) technique. This technique enabled us to reveal also microorganisms occurring at very low abundance. Bacteria strongly dominated over Archaea in both conditions. Despite the sequence analysis indicated the presence of members of the same taxa, their relative abundances differed at the two temperature conditions, determining different bacterial and archaeal communities. Proteobacteria (mainly consisting of Epsilonproteobacteria) dominated in both samples, followed by Bacteroidetes. Dominant phylotypes at HT were Caldiserica, Firmicutes and Aquificae, while at LT were Gammaproteobacteria, Fusobacteria, Actinobacteria and Spirochaetes. Dominant epsilonproteobacterial genera, all involved in the oxidative phase of the sulfur cycle, were Sulfurimonas, Sulfurovum and Arcobacter at HT, while Arcobacter, Sulfurimonas, Sulfurovum and Sulfurospirillum at LT. On the other hand, sequences referred to sulfate-reducing Deltaproteobacteria were dominant at HT. Other than the chemosynthetic activity, sustained by inorganic compounds released from vent emissions, the availability of sunlight represented an additional energy source for less abundant, diverse photosynthetic bacteria mainly represented by Chlorobi at HT, and by Alphaproteobacteria, Cyanobacteria and Chloroflexi at LT. Euryarchaeota dominated the archaeal community, followed by Crenarchaeota and Korarchaeota. Hyperthermophiles (Thermococci and Thermoprotei), considered as predominant in deep-sea high temperature sites, and halophiles (Halobacteria) were more abundant at HT, while methanogens (Methanococci) were prevalent at LT. Although Archaea represent a minor component of microbial community, they could play a relevant role in the carbon cycle at the high-temperature condition. In this high-CO2 and salinity, and low-pH shallow vent habitat, a microbial succession occurred in response to the shift from high- to low-temperature conditions.