Bacterial communities in the 'petola' microbial mat from the Sečovlje salterns (Slovenia)

The Sečovlje saltern is one of the few remaining solar salterns for traditional, seasonal salt production. The bottom of the crystallizer ponds is covered with a microbial mat, known as the 'petola', that has continuously been cultivated from medieval times. Outside the salt production season, the petola is fertilized with anoxic marine mud and covered with saline water; during the season, it is covered by brine. Here, we have applied culture-independent techniques and microelectrode-based activity measurements to study the bacterial communities in three different layers of the petola during the peak of the harvesting season. For reference, we used nonactive petola that had been abandoned for several years. The upper 2 mm of the petola were dominated by the cyanobacterial species Coleofasciculus chthonoplastes and the Phormidium/Lyngbya group, and Gammaproteobacteria (Acinetobacter sp.), while the third anoxic layer was dominated by as yet uncultured phyla. The nonactive petola showed a higher biodiversity. Oxygen and sulfide concentrations differed between the mats studied, in terms of the depth of oxygen penetration and diel changes. This study provides the first molecular insight into the microbiology of the petola, and it represents an important contribution towards understanding the geomicrobiological cycles of the traditional Sečovlje saltern.