Dr. Francisco Rodríguez-Valera

Haloquadratum, Salinibacter and Spiribacter, three halophiles, three metagenomic stories

The major revolutions that took place at the end of the past century and the begining of the new one nurtured by the molecular approach (rRNA genes) and the high throughput sequencing (cheap and easy genomes and metagenomes) have had a profound influence in our understanding of hypersaline environments. In a way they have been like a scale model of the revolution that is taking place in Microbiology at large. The existence of square cells that turned out to be the dominant archaeon in saturated brines was brought up by 16S rRNA sequencing of solar saltern DNA that latter on allowed the isolation of Haloquadratum walsbyi in pure culture. This peculiar microbe dominates neutral saturated bines worldwide and in spite of its sequencing as genomes and metagenomes still carries a large load of unexplained mysteries starting from its own massive dominance of habitats in which many other microbes can grow succesfully. Its strategies to succeed might shed light on other similarly successful microbes such as Pelagibacter in the surface ocean. A by-product of the efforts to isolate Haloquadratum was the unexpected discovery and subsequent isolation and genome sequencing of Salinibacter an interesting example of convergent evolution in which a bacterium, likely by the acquisition of several archaeal properties (like having carotenoids in the membrane or several rhodopsins, some typically archaeal such as halorhodopsin) has also succeeded in colonizing saturated brines and being relatively successful, although its actual specific habitat remains unclear. Latter on, through extensive metagenomic studies carried out is differently concentrated brines of solar salterns we still discovered another yet hidden microbe: Spiribacter a truely moderate halophile thriving at intermediate salinities and sharing features with Salinibacter such as the salt-in strategy and the presence of xanthorhodopsin. Still, a last challenge remains, the nanohaloarchaea, found in saturated brines of different pH.