Retinal-binding proteins mirror prokaryotic dynamics in multipond solar salterns

Microbial opsin (i.e. retinal-binding protein) dynamics has been studied along a salinity gradient in Santa Pola solar salterns (Alicante, Spain) by using culture-independent approaches and statistical analyses. Five ponds of salinities ranging from 18% to above 40% were sampled nine times along a year. Forty-three opsin-like sequences were retrieved by denaturing gradient gel electrophoresis and clustered into 18 different phylogroups, indicating that their diversity was higher than expected according to previous data. Moreover, the statistical correlation between environmental factors controlling microbial community structure and dynamics of environmental rhodopsin proteins indicated almost identical temporal fluctuations between the opsin-related sequences and their corresponding putative 'producers' in nature. Although most sequences were related to others previously detected in hypersaline environments, some pond-specific opsins putatively belonged to previously uncharacterized hosts. Furthermore, we propose that subtle changes in the bacteriorhodopsin 'retinal proton binding pocket', which is key in the photocycle function, could be the molecular basis behind a fine 'photocycle-tuning' mechanism to avoid inter/intraspecies light-competition in hypersaline environments.