Abstract
Microbial consortia inhabiting evaporitic salt nodules at the Atacama Desert are dominated by unculturable cyanobacteria from the genus Halothece. Halite nodules provide transparency to photosynthetically active radiation and diminish photochemically damaging UV light. Atacama cyanobacteria synthesize scytonemin, a heterocyclic dimer, lipid soluble, UV-filtering pigment (in vivo absorption maximum at 370 nm) that accumulates at the extracellular sheath. Our goal was to demonstrate if UV-A irradiations modulate scytonemin biosynthesis in ground halites containing uncultured Halothece sp. cyanobacteria. Pulverized halite nodules with endolithic colonization were incubated under continuous UV-A radiation (3.6 W/m2) for 96 h, at 67% relative humidity, mimicking their natural habitat. Scytonemin content and relative transcription levels of scyB gene (a key gene in the biosynthesis of scytonemin) were evaluated by spectrophotometry and quantitative RT-PCR, respectively. After 48 h under these experimental conditions, the ratio scytonemin/chlorophyll a and the transcription of scyB gene increased to a maximal 1.7-fold value. Therefore, endolithic Halothece cyanobacteria in halites are metabolically active and UV radiation is an environmental stressor with a positive influence on scyB gene transcription and scytonemin biosynthesis. Endolithobiontic cyanobacteria in Atacama show a resilient evolutive and adaptive strategy to survive in one of the most extreme environments on Earth.
Highlights
The Atacama Desert is considered one of the driest and oldest extreme environments under permanent desiccation, with one of the highest levels of solar radiation on Earth [1,2,3]
Previous reports on the Atacama lithobiontic microbial communities inhabiting halite nodules showed that rain events are practically absent in this hyperdesert; resilient microbial assemblies can overcome the permanent desiccation by obtaining liquid water from regular fog events and NaCl-dependent deliquescence when at least 75% relative humidity (RH) is reached within the halites [12,13]
Our research objective was to test the effect of UV-A irradiations on scytonemin biosynthesis in endolithic cyanobacteria colonizing halites, mimicking the regular environmental conditions
Summary
The Atacama Desert is considered one of the driest and oldest extreme environments under permanent desiccation, with one of the highest levels of solar radiation on Earth [1,2,3] This coastal nonpolar desert has unique geomorphological features that include longitudinal mountain ranges, salt-rich basins and a central hyperarid depression; all of these features provide diverse habitats with microbiota under continuous scrutiny [4,5,6,7,8]. Among the various salt pans in Atacama, Salar Grande is a non-fossil coastal basin located at the eastern slopes of the Coastal Range, at the Tarapacá Region in northern Chile (Figure 1a,b) [9,10] This basin, rich in evaporitic rocks, has an annual mean temperature of 20 ◦ C, and a relative humidity (RH) and UV irradiation close to 50% and 550 μmol s−1 m2 , respectively [4]. NaCl deliquescence starts when the relative humidity reaches 75% and atmospheric water is trapped by the salt crystals, forming saline brines within the micropores in the halites [15]
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