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Environmental setting of Socompa Stromatolites. Panels A–C: Sampling site at the shore of the alkaline Socompa Lake containing stromatolites of domical and columnar morphology. Panel D: Transverse cut of the sample with clearly visible laminations. Panel E: Magnification of stratification revealing layers 1 to 6, from top to bottom. They consist of a pinkish-white layer on the surface (1, orange dot), a dark-green layer (2, green dot), a light-brownish layer (3, red dot), a dark-brownish layer (4, purple dot), a light-brownish layer (5, yellow dot), and a dark-brown layer (6, black dot) in successive strata below. m.a.s.l., meters above sea level
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Modern non-lithifying stromatolites on the shore of the volcanic lake Socompa (SST) in the Puna are affected by several extreme conditions. The present study assesses for the first time light utilization and functional metabolic stratification of SST on a millimeter scale through shotgun metagenomics. In addition, a scanning-electron-microscopy app...
Citations
... The deepest anoxic layers are inhabited by sulfate reducer members of Deltaproteobacteria, Bacteroidetes, and Firmicutes, next to a high diversity and equitable community of rare, unclassified, and candidate phyla [18]. Shotgun metagenomics on SST revealed a broad range of genes encoding photoreceptors and pigments, and a remarkable abundance of microbial rhodopsins and photolyases whose distribution within the mat followed the UV and PAR light gradient [19,20]. Interestingly, MR was abundant even at deep anoxic layers where only limited infrared light exists. ...
A putative xanthorhodopsin-encoding gene, XR34, was found in the genome of the moderately halophilic gammaproteobacterium Salinivibrio socompensis S34, isolated from modern stromatolites found on the shore of Laguna Socompa (3570 m), Argentina Puna. XR-encoding genes were clustered together with genes encoding X-carotene, retinal (vitamin-A aldehyde), and carotenoid biosynthesis enzymes while the carotene ketolase gene critical for the salinixanthin antenna compound was absent. To identify its functional behavior, we herein overexpressed and characterized this intriguing microbial rhodopsin. Recombinant XR34 showed all the salient features of canonical microbial rhodopsin and covalently bound retinal as a functional chromophore with λmax = 561 nm (εmax ca. 60,000 M-1 cm-1). Two canonical counterions with pK values of around 6 and 3 were identified by pH titration of the recombinant protein. With a recovery time of approximately half an hour in the dark, XR34 shows light-dark adaptation shifting the absorption maximum from 551 to 561 nm. Laser-flash induced photochemistry at pH 9 (deprotonated primary counterion) identified a photocycle starting with a K-like intermediate, followed by an M-state (λmax ca. 400 nm, deprotonated Schiff base), and a final long wavelength-absorbing N- or O-like intermediate before returning to the parental 561 nm-state. Initiating the photocycle at pH 5 (protonated counterion) yields only bathochromic intermediates, due to the lacking capacity of the counterion to accept the Schiff base proton. Illumination of the membrane-embedded protein yielded a capacitive transport current. The presence of the M-intermediate under these conditions was demonstrated by a blue light-induced shunt process.
High-Altitude Andean Lakes have been described as polyextremophilic environments and plenty photoinduced processes have been documented. Lake Diamante metagenomic taxonomical analysis revealed mostly members from Archaea domain of life. In this context, we searched for Light Oxygen Voltage photoreceptors where 24 of these Archaeal sequences were part of two major groups (A and B) except for 3 domains that did not group even with the reference sequences included for the analysis. Several signatures of halo-adapted proteins were found amongst archaeal sequences, i.e. abundance of acidic residues on the surface, increased number of arginine residues at the expense of lysines and a multiplicity of salt bridges. Heterologous expression of one of these domains, ALovD-1, showed that the photophysics for the dark and the light adapted states was fairly conserved, but most importantly that is a slow cycling type, and a moderate halophilic photoreceptor since it conserves activity at 0.5 M of monovalent salt. This is the first Archaeal LOV domain characterized in the literature to our knowledge.
