All Activities

Atlantic algae studies

Several members of the lab of Prof. Assaf Vardi in the Department of Plant and Environmental Sciences were on board the 2019 Tara Atlantic expedition to the coast of Brittany, France, taking samples of Emiliania huxleyi, a unicellular alga that forms massive annual oceanic blooms. The biotic interactions that regulate the fate of these blooms play a profound role in determining carbon and nutrient cycling in the ocean and feedback to the atmosphere.

Annual E. huxleyi spring blooms are frequently terminated following infection by a specific large dsDNA virus (EhV) that belongs to the coccolithovirus group. Viral infection leads to a rapid remodeling of the E. huxleyi genetic transcriptome that results in rewiring of many cellular metabolic pathways.

Prof. Vardi and his team aimed to take a holistic approach to untangle the complexity of alga-virus-bacteria interactions during an E. huxleyi bloom in the North Atlantic Ocean. During the international Tara expedition, they were able to study many different parameters of their samples, including cell, virus and bacterial abundance, single-cell sorting, single-cell RNA-seq, 16S and 18S sequencing for microbiome diversity, electron microscopy, metabolomics, and lipidomics. In addition, air samples were collected daily for microbiome, chemical, and physical composition of aerosols. All these samples were shipped back to the Weizmann Institute and are currently being processed.

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Tara Pacific expedition

Prof. Ilan Koren of the Department of Earth and Planetary Sciences is working to understand the composition and size of marine aerosols (extremely small particulates of a couple microns or less) and investigate how they depend on environmental variables, in order to reveal the microphysical properties of clouds and better calculate their influence on the radiation budget used in climate models. These improved models can help them understand the effect of aerosols on marine ecosystems and climate.

Members of the Koren group were sent to measure marine aerosol properties during the Tara Pacific Expedition, a 2.5 year research project that began on May 2016 in Lorient, France and returned on October 2018. They installed a custom-made filter collector for biological, chemical and morphological analyses of the marine aerosols. One of their aims is to determine the composition and size of individual particles with diameters as small as 0.8 microns (μm). Since returning with their samples, the Koren team has been performing single-particle analysis using a scanning electron microscope (SEM) with an energy dispersive X-ray analyzer to quantify the size, shape, quantity, and elemental composition of all aerosols. They are using the Quantax 800 automatic software from Bruker which was recently acquired by the SEM unit of the Weizmann Institute.

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Mediterranean cyclones

Dr. Shira Raveh-Rubin and her team are working to understand the airstreams that act as air mass conveyor belts, which bring cold, dry artic air southward and can trigger intense Mediterranean storm systems. They tested the sensitivity of dry intrusion airstreams with their Lagrangian computer definition by systematically computing airmass trajectories for a test period from November 2009 – April 2010. They examined a number of dry inclusion trajectories in the Mediterranean domain for a range of combinations of changes in the minimum air pressure difference measured over different time scales.

They found that the air parcels descend through a range of pressure differentials from 250 to 500 pascals of pressure (hPa), and the time window for the descent varied between 24 to 96 hours. Their results, combined with individual case studies of known impactful cyclones in the Mediterranean region, indicate that intrusions that show a pressure difference of at least 350 hPa in 48 hours lead to significant storm activity.

Based on their chosen criterion, the team is generating a new dataset for the years 1979-2018. They will use the same criterion for a new reanalysis of the ERA5 dataset that was used for the study of the Mistral wind in the Gulf of Lion, a phenomenon directly related to such cold and dry intrusions. The new datasets will serve as a basis for subsequent studies aiming to understand high-impact weather and the coupling of the atmospheric circulation to the water cycle in this region. 

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Cryo-electron microscopy of phytoplankton

Dr. Assaf Gal from the Department of Plant and Environmental Sciences is trying to understand the intracellular mechanisms of minerals formed by unicellular marine algae. With his team, he is focusing on coccolithophores, an abundant group of phytoplankton, which produce coccoliths, mineralized calcite arrays. The formation of the coccolith crystals is exquisitely controlled by the cell. Dr. Gal and his group are striving to understand the fundamentals of the process in order to understand problems related to cell physiology, oceanography, and materials applications.

In a collaboration with Dr. Julia Mahamid at the European Molecular Biology Laboratory, Dr. Gal and Dr. Zohar Eyal, a postdoc working in his lab, have conducted very promising experiments on campus and in Heidelberg, using cryo-electron tomography—an imaging technique used to produce high-resolution three-dimensional views of samples.

Dr. Eyal has established the experimental pipeline in which the calcifying cells are vitrified on a transmission electron microscope grid, and then thinned to 200 nanometers (nm) by a focused ion beam instrument. The final step in this process is acquiring the electron microscope images at different tilt angles. All aspects of this process are now properly functioning in the newly purchased cryo-electron microscopes on campus. Dr. Gal and his team are collecting data from their samples.

[Image on the left: (A, B) Sections in P. carterae whole cells show both the homogenous lumen of the vacuole that is punctuated with calcium and phosphorus rich (Ca-P) bodies, and the contrasting dense internal organization of the ion-rich compartments (orange arrowheads) that contain particles, spheres, membranes, and more. (Credit: Journal of Structural Biology, 2020)]

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Deep sea information

Energy-efficient mobile autonomous systems, such as gliders and drifters, are helping scientists measure the currents and water mass properties in the eastern areas of the Levantine Basin (Eastern Mediterranean Sea) and study the complex circulation features governing the dynamics near the Israeli coast and in the open sea. 

Prof. Aldo Shemesh from the Department of Earth and Planetary Sciences and his team examined data from two distinct seasons in order to investigate seasonal variability. Two glider missions were carried out in two different seasons: one in summer 2018 and one in winter 2019. In both experiments, Prof. Shemesh and his group collaborated with an Italian effort to study the region between Israel and Cyprus. In this region, mesoscale and sub-mesoscale eddies are predominant, and strongly interact with a persistent coastal current. They are currently working on analysing the collected data and preparing their results for scientific publication.

Algae-bacteria interactions

Dr. Einat Segev of the Department of Plant and Environmental Sciences studies the complex interactions between bacteria and phytoplankton. Using a laboratory model system, she was able to demonstrate that bacteria exhibit a dynamic range of interactions, from mutualism to pathogenicity, with coccolithophores.

Her lab’s current projects aim to reveal which bacteria associate with coccolithophores and how they influence natural coccolithophore communities – specifically, Emiliania huxleyi – in the Gulf of Aqaba, off the coast of Eilat. They discovered that different communities of bacteria characterize different phases of E. huxleyi growth.

They found that these bacterial communities correlate highly to the concentration of E. huxleyi and available nutrients in the environment. Low concentration of the alga in the beginning of growth resulted mainly in bacterial communities that are adapted to environments with low nutrient concentration.

However, when the alga reaches high concentrations, the scientists observed a shift in the bacterial community. This shift was characterized by a dominance of bacterial communities that are well adapted to environments with high concentration of nutrients. The results indicate that E. huxleyi contributes to the nutrient availability in the environment, and as a consequence to the changes in the bacterial communities.

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