DAWN ILLUMINATION SERVES AS A SIGNAL PREPARING DESERT CYANOBACTERIA TOWARDS FORTHCOMING DEHYDRATION

Filamentous cyanobacteria inhabiting desert biological soil crusts, among the harshest environments on Earth, must cope with frequent hydration/dehydration cycles. Water is mostly provided in the form of early morning dew followed by. Inability to recover after fast desiccation suggested that the cells must prepare themselves toward the forthcoming dehydration, but the nature of the signal involved was unknown. Sampling the cells along the dehydration curve for physiological analyses, global and specific gene expressions showed that the abiotic conditions preceding natural dehydration are critical for recovery after rewetting. Transcript profiling on RNA isolated during simulated natural dehydration showed the impacts of the rising illumination. The latter is critical, as darkness severely inhibited both revival after rewetting and the changing expression of many genes. L. ohadii possesses 4 genes encoding typical phytochrome, the red/far red sensitive photo-sensor involved in many functions in various organisms from bacteria to plants. Addition of far-red light to simulated natural conditions during dehydration severely inhibited recovery after rewetting, even more than darkness. It also abolished the marked change in expression of many genes otherwise strongly responding. An exception was the orange carotene binding protein (OCP) that plays an essential role in dissipation of excess light energy in cyanobacteria. OCP was severely inhibited by darkness but not significantly by FR. Blocking the blue light spectra inhibited the revival after re-wetting, although less so than darkness, but completely blocked the expression of OCP. Our data provide the mechanism whereby BSC-inhabiting cyanobacteria prepare themselves towards the inevitable forthcoming dehydration with rising dawn illumination, sensed by photo-sensors, as the signal. The evolutionary origins of the function of phytochromes and cryptochromes are not really known. We raise and analyze the possibility that ancient stromatolites-inhabiting cyanobacteria adopted these genes from anoxygenic photosynthetic bacteria as predictive sensors for the approaching desiccation and high illumination.