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.

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.

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.

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 –

1. Measurements of captured material by time series sediment traps.
2.  Radioisotope technique, based on a radioactive disequilibrium between 238U and 234Th.  Disequilibrium arises when 234Th adheres to particulate matter and sinks.

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.

Publications

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.

Ynon Deutsch- Jointly supervised with Dr. David Ezra‎,
Department of Plant Pathology & Weed Research, ARO The volcani center.

Registered as a student in the Department of Marine Biology, Leon H. Charney School of Marine Sciences, University of Haifa.

Title of PhD project: Endophytes from algae for use in biological control of pathogens and pests in aquaculture.

Project description:

Endophytes are micro-organisms that live inside the plant (e.g. algae) tissue without causing external symptoms. The plant provides a living niche and food for the endophytes while the endophytes provide protection against pathogens, pests and induce tolerance to abiotic and biotic stresses. Some endophytes produce and secrete biologically active compounds that prevent bacteria, fungi and pests from harming the host plant.
Diseases are a major and influential harmful factor in the aquaculture industry. Chemical pesticides and antibiotics are widely used in order to cope with pathogens and pests. Currently, the ability to deal with disease outbreaks of different types in aquaculture is very limited.
Our aim is to exploit this capability of the endophytes, by isolating and characterizing endophytes with significant biological activity, from fresh and sea water macro-algae, and use them and their secondary metabolites to control pathogens and pests in aquaculture.

We have isolated endophytes from several Mediterranean seaweeds and fresh water algae, and found many to have biological activity against known and common aquaculture pathogens.

We have introduced some of those active; GFP labeled, endophytes back into their origin host (Fig 1). Our aim is to track and study their localization, interactions with other microorganisms in the algae (microbiome) and to examine their ability to survive and produce the biologically active compounds in the algae.

In addition, we are examining the possible use of the endophytes’ active compounds (by using analytical chemistry methods) for control of aquaculture diseases.

Figure 1. Active, GFP labeled endophyte introduced into an alga ‎‎(Ulva sp.). Bright field image shows alga’s cells (A). Green ‎fluorescence filter shows the endophyte in between alga’s cells (B). ‎