Director of Aquatic Ecology and Biological Oceanography Laboratory.Read More
Director of Aquatic Ecology and Biological Oceanography Laboratory.Read More
Jointly supervised with Prof. Yishai Weinstein- Faculty of Geography and Environment, Bar Ilan University.Read More
Jointly supervised with Prof. Danny Tchernov and Prof. Michael D. Krom, Department of Marine Biology, Haifa UniversityRead More
Jointly supervised with Dr. David Ezra, Department of Plant Pathology & Weed Research, ARO The volcani center.Read More
TONGA is a multidisciplinary project dedicated to the study of the control of ocean productivity and carbon export driven by micronutrients from hydrothermal origin. It is based both on a 37-day oceanographic cruise in the Western Tropical South Pacific and modelling work and involves hydrothermal geochemists, physical oceanographers, trace element chemists (ocean and atmosphere), biogeochemists, biologists and modelers.
Name: Tslil Bar, Ph.D student
Institute: Department of Marine Biology, Faculty of Natural Sciences, University of Haifa.
Title of PhD project: Reconciling nutrient supply, phytoplankton C:N:P ratios and bacterial community diversity in the eastern Mediterranean Sea
Alfred Redfield suggested that the elemental composition of phytoplankton reflects the dissolved nutrient ratios of the deep ocean (C:N:P 106:16:1). He also hypothesized that phytoplankton elemental stoichiometry controls ocean chemistry through the remineralization of exported material. Yet, many studies since have demonstrated that phytoplankton cellular C:N:P is flexible and changes both between taxa and as a response to nutrient supply and availability (physiological plasticity). My aim is to evaluate Redfield’s hypothesis regarding the predominant phytoplankton groups (Prochlorococcus, Synechococcus, and picoeukaryotes) and assess their contribution to the carbon flux in the eastern Mediterranean Sea (EMS) with its high N:P supply ratio and low nutrient availability.
My main goals in this research are to determine seasonal changes in cell stoichiometry (C:N:P) of the predominant phytoplankton groups in the coastal and open sea in the EMS, and the impact of increased N:P supply ratios on these groups and the nitrogen sources that they are utilizing (using δ15N isotopic signatures).
The elemental composition of marine plankton and particulate organic matter has biogeochemical implications for C cycling and export, for, understanding nutrient limitation, and for ecosystem and food web models.
This study will provide the first taxa specific C:N:P and N isotopic signatures of phytoplankton in the EMS and will help to understand more thoroughly the interaction between phytoplankton ecophysiology and sinking materials.
Trichodesmium spp. are diazotrophic cyanobacteria that exist as single filaments (trichomes) and as macroscopic colonies of varying shapes, that may form enormous surface accumulations (‘blooms’) visible to the naked eye. Trichodesmium’s unique ability to fix atmospheric Nitrogen along with its massive blooms, makes this fully photoautotrophic genus a vital player in the biogeochemical cycling of basic elements in contemporary oceans.
Although research has been done on this globally important diazotroph, little is known about its cell division physiology. During my work, I will try to deepen the knowledge on the physiology of cell division in single trichomes and within colonies.
The ocean is a significant sink of anthropogenic CO2, in large part because organic matter is exported to oceanic depths driving the biological sequestration of carbon in the ocean’s interior. Organic matter export depends on the supply of external nutrients to the euphotic zone (by processes such as deep mixing and biological fixation of atmospheric dinitrogen (N2)) and the subsequent production of organic matter by photosynthesis (defined as “new” production).
The Gulf of Aqaba offers a unique opportunity to observe, at high temporal resolution, both mechanisms supplying “new” nutrients. The oligotrophic Gulf is surrounded by land on three sides and characterized by a thermohaline circulation pattern caused by high evaporation rates, and exhibits strong seasonal variability mainly due to deep winter mixing and strong summer stratification.
My main goal is to understand the relative contribution of N2 fixation and deep winter mixing to “new” production and subsequent “export” in the northern Gulf of Aqaba and to simulate and predict the response of the system under changing environmental scenarios.
Ronen Alkalay- Jointly supervised with Prof. Yishai Weinstein- Faculty of Geography and Environment, Bar Ilan University. Prof. Ilana Berman-Frank- Leon H. Charney School of Marine Sciences, University of Haifa, and Dr. Timor Katz- Israel Oceanographic and Limnological Research.
Title of PhD project: Particulate carbon export in the Levantine Basin (East Mediterranean Sea)
About 10-30% of the net primary production, produced in the oceans photic zone, is exported downwards by the biological pump. This pump is an important buffering agent of atmospheric CO2, and it forms significant foundation for most of the heterotrophic life in the deep-sea. It is commonly estimated that 80% of the export settles as particulate matter mainly via passive sinking of aggregates and fecal pellets aided by mineral ballast. Nevertheless, in oligotrophic areas, such as the Levantine Basin, observations and modelling suggest that of the exported carbon, usually only ca. 10% survives the travel through the oceanic Twilight Zone (TLZ, i.e. 200 to ~1,000 m depth) and reaches the deep, Dark Zone.
In this study, the first of its kind in this area, we have deployed and operated the “Deep-lev moored observatory”, which is located 50k of shore, at water depth of 1500m.
In this observatory we are using a variety of 16 sensors to collect physical and biological parameters,
The main components of the system are three sediment traps to study the flux of particulate matter.
The flux of particulate matter consists mainly on biogenic and lithogeny substances.
In the research I am involved with we are using two complementary methods –
Due to the unique characteristic of the Levantine basin it may serve as a model for carbon fluxes in a warming, oligotrophic regions in the ocean.
Alkalay, R., Pasternak, G., Zask, A., 2007. Clean-coast index-A new approach for beach cleanliness assessment. Ocean Coast. Manag. 50.
Alkalay, R., Zlatkin, O., Katz, T., Herut, B., Halicz, L., Berman-Frank, I., Weinstein, Y., 2020. Carbon export and drivers in the southeastern Levantine Basin. Deep. Res. Part II Top. Stud. Oceanogr. 171, 104713.
Avnaim-Katav, S., Herut, B., Rahav, E., Katz, T., Weinstein, Y., Alkalay, R., Berman-Frank, I., Zlatkin, O., Almogi-Labin, A., 2020. Sediment trap and deep sea coretop sediments as tracers of recent changes in planktonic foraminifera assemblages in the southeastern ultra-oligotrophic Levantine Basin. Deep. Res. Part II Top. Stud. Oceanogr. 171, 104669.
Katz, T., Weinstein, Y., Alkalay, R., Biton, E., Toledo, Y., Lazar, A., Zlatkin, O., Soffer, R., Rahav, E., Sisma-Ventura, G., Bar, T., Ozer, T., Gildor, H., Almogi-Labin, A., Kanari, M., Berman-Frank, I., Herut, B., 2020. The first deep-sea mooring station in the eastern Levantine basin (DeepLev), outline and insights into regional sedimentological processes. Deep. Res. Part II Top. Stud. Oceanogr. 171, 104663.
STERN, N., ALKALAY, R., LAZAR, A., KATZ, T., WEINSTEIN, Y., BERMAN-FRANK, I., & HERUT, B. (2020). Unexpected massive enmeshments of the Sharpchin barracudina Paralepis coregonoides Risso, 1820 in mesopelagic sediment traps in the Levantine Basin, SE Mediterranean Sea. Mediterranean Marine Science, 21(1), 47-51.
PhD Title: Trichomes to blooms in the marine diazotrophic cyanobacterium – Trichodesmium
Trichodesmium is a filamentous, non-heterocystous cyanobacteria whose filaments (trichomes) are composed of 10-100s of cells with similar morphologies. Trichomes can be found as single filaments, spherical (“puffs”) or fusiform (“tufts”) colonies . The colonies provide unique habitats for other organisms (metazoans, bacteria, and viruses) and serve as hot-spots for microbial mediated nutrient transformations within the oligotrophic oceans. While this phenomenon is a well-known trait of Trichodesmium the mechanisms for the formation of these structures are not understood.
In my research, I am interested in determining what causes the single filaments to create colonies; to reveal the cues and mechanisms involved in creating colonies from single trichomes. My work will combine live-imaging microscopy as well as molecular and physiological techniques.
The Mediterranean Sea has nurtured and sustained its surrounding populations for thousands of years. Yet, food and energy requirements of increasing populations have contributed to a greater human (anthropogenic) footprint on the Mediterranean. The recent discoveries of large natural gas reservoirs (1.8% of world reserves) and possibly oil in the Levant Basin of the eastern Mediterranean Sea (EMS) have stirred excitement over the new prospects yet raise stakeholders concerns about the environmental risks involved in the large-scale exploitation of these offshore resources.
Marine hydrocarbon pollution affects pelagic, benthic, and coastal populations as well as commercially grown organisms. Primary producers and bacterioplankton form the basis of marine food webs and are essential for biogeochemical cycling of nutrients such as carbon (C), nitrogen (N), and phosphorus (P). Many studies have examined the response of individual species of algae or bacteria with several studies examining the integrated responses of natural communities of marine pelagic and coastal microbial populations including primary producers to either acute or chronic hydrocarbon pollution. Yet, the large variability of both pollutants and the natural environments/ecosystems make community responses hard to predict requiring experimentation according to specific locality and conditions.
This project will be funded starting October 2014, candidates for PhD are encouraged to apply.
Biofilm prevention in the desalination industry
Limitations of global freshwater supplies have stimulated the application of desalination technology with desalinized water coming online worldwide at a rate of 40 to 50 million m3 d−1. Currently, about 50% of global desalination is based on filtration through reverse osmosis (RO) membranes requiring effective pretreatment procedures upstream to reduce fouling, maintain performance and extend membrane lifetime and to ensure the manufacturers requirements for membrane recovery yield. Transparent exopolymer particles (TEPs) are sticky, organic microgels, ranging in size from ∼0.4 to >200 μm, present in large numbers in all aquatic environments. Recently, TEPs have been implicated as an important factor in the development of aquatic biofilm and are part of the extracellular polymeric substances (EPSs) that form the matrix of microbial biofilms.
In our research we examined the direct involvement of these microgel particles in biofilm development. We showed that protobiofilm and TEPs in the feedwater contributed to fast development of biofilm and not EPSs generated by adhering, single bacteria, or bacterial aggregates. In addition, experiments comparing the initial stages of biofilm formation in filtered or in untreated seawater clearly illustrate the importance of protobiofilm and TEPs in accelerating aquatic biofilm formation.
Desalination impacts on microbial coastal populations
The increasing importance and expansion of seawater desalination technologies and large scale coastal plants – enhances both the “visibility” and essentiality of ensuring environmental sustainability of the impacted coastlines. Our study (2015-2019) added important knowledge to the scant information that has been published either locally or globally to examine the impacts of desalination discharges on the coastal microbial communities comprising the foundation of the aquatic food webs. Our overarching goal in this project was to characterize and predict the responses of microbial and phytoplankton communities to their exposure to both enhanced salinities and chemical discharges resulting from desalination plants.
The comprehensive results of this study including the in-situ sampling, experimental manipulations, hydrodynamic simulations of plume and intake effects, and modeling brine (and temperature impacts) on the aquatic food-web all demonstrate the following. Impacts are influenced by both seasonality and site-specificity with salinity and temperature both driving biological changes. Therefore, site-specific monitoring and assessment of changes of the microbial populations is essential. Moreover, Coastal environments, exposed to long term discharge of brine, may exhibit cumulative chronic effects and affect the ecosystems more dramatically. The assessment of ecological impacts, from the rapidly expanding desalination industry, on coastal marine environments and their biota should be included as a routine monitoring tool and not be based solely on the results of short term studies such as this one.
Photosynthesis and fixation of atmospheric dinitrogen which are two fundamental processes that are performed by organisms at the basis of aquatic food webs – the phytoplankton and bacterioplankton. Dinitrogen fixation is a biological transformation carried out only by a subgroup of prokaryotic organisms (diazotrophs) that can utilize atmospheric nitrogen (N2), unavailable for most organisms, and convert it into a form of nitrogen that is used for growth. This process is extremely important in many nitrogen-poor surface waters of the oceans, and injects a new source of bioavailable nitrogen to areas where nitrogen limits growth and primary production by the ocean’s tiny plants – the phytoplankton. In our research we explore how diazotrophs influence bio-geochemical cycling of carbon and nitrogen in the face of climatic changes. These changes include global warming and ocean acidification due to increased dissolution of atmospheric CO2 in the oceans.
Collaboration with Dr. Yeala Shaked (IUI- Hebrew University) on iron uptake of Red Sea populations of Trichodesmium (a globally important N2 fixer in the tropical and subtropical oceans with blooms extending over thousands of kilometers) has illustrated that the colonies actively take up and shuttle Fe along the filaments to the center of the colony where it is dissolved and assimilated into the cells. We have also demonstrated that the future projections of high CO2 in the oceans can enhance nitrogen fixation and growth of this marine cyanobacterium and indicate that Trichodesmium will thrive in the future warmer and more acidified oceans.
Further research on other diazotrophs in the Gulf of Eilat shows that a large diversity including bacterioplankton. These are not restricted like the phytoplankton to the upper sunlit areas of the surface oceans, and can fix nitrogen in deep dark layers. We measure N2 fixation rates from oceanic zones that have traditionally been ignored as sources of biological N2 fixation; the dark, fully oxygenated, nitrate (NO3–)-rich, waters of the oligotrophic Gulf of Aqaba and the eastern Mediterranean. Our results suggest that while N2 fixation may be limited in the surface waters of the oligotrophic Mediterranean and Red Seas, N2 fixation from the deeper and dark ocean layers may contribute significantly to new N inputs, yet these inputs are currently not included in regional or global N budgets.
Tal Ben-Ezra- Jointly supervised with Prof. Danny Tchernov and Prof. Michael D. Krom, Department of Marine Biology, Haifa University
Quantifying and Understanding the seasonality of nutrient limitation in the EMS
The Eastern Mediterranean Sea (EMS) is a unique body of water that was recently suggested to have a seasonal switch in the nutrient limiting primary productivity (PP). High nitrate+nitrite (N+N) in winter, while soluble reactive phosphate (SRP) below detection limits (BDL), and with the progression of summer, N+N decrease in the photic zone. Thus at the end of summer, the system was characterized as co-limited. Both the temporal and spatial resolution of this switch is in need of further investigation. Furthermore, the secondary limiting nutrient i.e. the access concentrations of the nutrient that is not currently limiting and is available in the photic zone is also an important parameter in order to evaluate the system’s plasticity and productive potential. In this study, firstly I intend to perform monthly nutrient measurements on EMS off-shore water. Secondly, I intend to perform seasonal bioassays based on Alkaline Phosphatase Activity (APA) and nutrient enrichment titrations as a tool to investigate how the microbial community responds to nutrient enrichments, how far is the system from one type of limitation or another, how it changes with season and how sharp the transition between the two limitations is according to the changing nutrient regimes during different seasons. This method allows us to ask questions about the limitation dynamic but from the organisms’ perspective i.e. what nutrients does the microbial community “see”? This fundamental understanding of the system is crucial in order to formulate science-based management decisions and sustainable usage of this valuable resource.