לוח שנה משולב

The sunlit surface layer of the world’s oceans is a critical biome for the functioning of the Earth system. It constitutes the habitat to a tremendous diversity of viral, bacterial and unicellular eukaryotic groups (autotrophic and heterotrophic) whose intricate populations structure and trophic interactions drive nearly half of the global primary productivity and is central in regulation of elemental cycles and climate homeostasis. In a decade where genomic tolls are enabling a robust assessment of the marine microbial biodiversity our understanding of the mechanisms underlying trophic interactions and the complexity of organisms’ life cycles is still fragmentary. Here, I present two studies that I have been developing over the last years. In a first study, we showed using both field observation and laboratorial experiments that copepods, abundant migrating crustaceans that graze on phytoplankton, as well as other zooplankton can accumulate and act as viral-vectors mediating the transmission of viruses infecting Emiliania huxleyi, a bloom-forming marine microalgae that plays an important role in the carbon cycle. We propose that such zooplankton-driven mechanism can boost host-virus contact rates and potentially accelerate the demise of large-scale phytoplankton blooms in the oceans. In a second study, we explore the mechanisms by which viral infections impact the life cycle of E. huxleyi. In earlier studies we demonstrated that the haploid phase of E. huxleyi is unrecognizable and therefore resistant to viruses that specifically kill the diploid phase, and that exposure of diploid cell to virus induces transition to a phenotypically-like haploid phase. We proposed that such escape strategy via life phase switch, the ‘‘Cheshire Cat’’ escape strategy, enables diploid blooming cells to evade viral attack. Recent morphological and genetic characterization of cells exposed to viruses starts now to shed light on the mechanisms underlying life phase transition dynamics, opening new perspectives of future research.

The solar wind is a stream of hot (T~106K) magnetized plasma emerging from the Sun and expand into the interplanetary space at high speed of several hundred km/s. However, the exact plasma acceleration and heating mechanisms are not fully understood. The solar wind is classified in two types according to their coasting velocity: slow and fast. The slow solar wind reaches ~400 km/s is highly variable and dense compared to the fast wind streams, and is associated with coronal streamers. The fast wind is associated with coronal holes - regions of low-density plasma, and it is the dominant form of the solar wind during periods of solar minimum activity reaching asymptotic speed ~1000 km/s. The solar wind was observed by space probes from 0.29 AU and beyond, and was studied using remote-sensing spectroscopic observations at its sources in the corona. The magnetic and velocity fluctuations in the solar wind exhibit turbulent power spectrum that agrees with Kolmogorov turbulence scaling, and steeper spectrum in the kinetic resonant dissipation range. The fast solar wind ion temperature is often anisotropic with Tperp>Tparallel. Observations show that the slow and fast wind differ in heavy ion composition, and that heavy ion temperatures and speed, are often hotter than protons and electrons. The acceleration and heating of the solar wind was modeled in the past with single-fluid MHD equations. However, multi-fluid and kinetic modeling are required to account for the plasma properties, and study the heating processes of the solar wind heavy ions. I will present an overview of solar wind plasma observations, and show the results of solar wind plasma models using MHD, multi-fluid, and kinetic hybrid approaches that include heavy ions such as O5+, Mg9+, and He++. I will show results of synthetic UV observations that use the results of the models facilitating the interpretation of spectroscopic data. I will discuss the impact of the modeling on our current understanding of the solar wind plasma acceleration and heating and its sources in the solar corona.

Cas9 is an RNA-guided DNA endonuclease found in clustered regularly interspaced short palindromic repeats (CRISPR) bacterial immune systems. Recently, Cas9 and CRISPR-derived guide RNAs have been repurposed for powerful genome engineering applications in plants and animals, yet the underlying molecular cues that enable programmable DNA recognition have been poorly understood. Using a combination of structural, biochemical, and single-molecule approaches, we determined the mechanisms of DNA target search and recognition. We show that Cas9 consists of two structural lobes that undergo RNA-induced conformational changes to create a central DNA-binding channel. DNA binding requires the initial recognition of a short trinucleotide motif known as the PAM, and PAM interactions trigger Cas9 catalytic activity. Building on these insights, we developed new applications including programmable RNA manipulation with Cas9, and a split-Cas9 enzyme that will enable enhanced spatiotemporal control of genome engineering events in cells. Finally, ongoing work with evolutionarily related CRISPR-Cas immune systems has revealed how distinct RNA-guided protein complexes have converged on common mechanisms of target recognition.