Research
Our lab studies marine chemical ecology - the way aquatic organisms communicate through chemistry, the chemicals that mediate these interactions, and the way these interactions and chemicals affect entire ecosystems. This is a fascinating field of research that combines ecology, limnology/oceanography, biochemistry, molecular and cellular biology, physics and computational biology. In addition, studying these interactions often brings with it the discovery of novel chemical compounds which have biotechnological, pharmacological or medical uses - antibiotics, for example, are often synthesized by microorganisms in order to fight other microbes. Some of the questions we are interested in are: Do marine bacteria communicate using chemical signals? What are the roles of toxins in jellyfish, sea anemones, coral and hydra? How do marine invertebrates protect themselves against pathogens? Can understanding such interactions help us model marine communities and predict how they will change in a changing world?
Here are some specific projects we are working on:
How do bacteria interact in the dilute open ocean?
Every drop of seawater contains around one million microorganisms (bacteria, small algae and other organisms such as ciliates and diatoms), which form the base of the marine food chain. Interactions between marine microbes such as symbiosis and competition determine the structure of the microbial community, and also directly affect global processes such as cloud formation, ocean acidification and greenhouse gas dynamics.
In our lab we study interactions between Prochlorococcus, a tiny single-celled microbe, which is the most abundant photosynthetic organism on Earth, and co-occurring bacteria, which make their living by consuming and respiring organic molecules. We study how growing Prochlorococcus and heterotrophic bacteria together ("co-culture") affects the physiology of each organism, as well as the regulation of the various genes in their genomes. We use these data to construct mathematical models, which describe and interpret how the lives of these organisms are intertwined.
Understanding and modeling the simple laboratory co-culture will give us insight and tools for interpreting the relationships between large and complex communities of these organisms in the ocean, and their impact on the global carbon cycle.
The image below shows the effect of hundreds of different heterotrophic bacteria on the growth of Prochlorococcus in culture. From Sher et al, ISMEJ 2010
When and where to bacteria and algae produce toxins and signalling molecules?
Microorganisms produce an astounding variety of signalling molecules, antibiotics and toxins. In our lab, we aim to determine whether there are specific ecological niches which favour the production of such molecules. There are two major motivations for this study: first, harmful blooms caused by toxic algae (especially cyanobacteria) are becoming more common due to climate change and habitat pollution. Understanding the dynamics of such blooms, and being able to predict where such blooms occur, may help alleviate their effect on the economy (e.g. fisheries) and on human and ecosystem health. Second, it has been suggested that toxins and similar “secondary metabolites” are produced when microorganisms need to compete or fight with other microbes. Determining under what conditions such “microbial chemical warfare” occurs more commonly may help us in the search for new antimicrobial compounds to combat the emergence of drug-resistant pathogenic bacteria. In this study, we are making use of the unique geography of Israel, which provides a wealth of different freshwater habitats - from alpine to deep desert – in which to search for toxic algae and new antimicrobial compounds, all within easy reach of our lab. We search for such molecules primarily in metagenomes from these environments.
The image below shows the dynamics of bacterioplankton over three years in an aquaculture reservoir, showing annual blooms of potentially-toxic cyanobacteria. From the PhD thesis fo Sofi Marmen
What are the ecological roles of toxins in cnidarians?
Cnidarians such as hydra, sea anemones, corals and jellyfish are simple, mostly sessile animals that depend on bioactive chemicals for survival. Cnidarians utilize sophisticated stinging cells (nematocytes) to inject paralyzing venom into their prey, predators or competitors. In addition to the nematocyte venom, we and others have recently shown that cnidarians produce toxins in other tissues, and that these toxins are used for many biological roles: protection from pathogenic bacteria, digestion of prey and, potentially, regulation of development.
We hypothesize that, in cnidarians, bioactive compounds secreted both as localized point sources (nematocyte discharges) and across extensive body surfaces combine to create complex "chemical landscapes". These landscapes may affect the surrounding community on scales from microns to, in the case of coral reefs, hundreds of kilometers. As a first step towards describing such landscapes in detail and understanding their ecological importance, we are characterizing the chemical armament of corals and jellyfish. We are also studying the distribution patterns of jellyfish in the Eastern Mediterranean, and testing the hypothesis that the decline of major jellyfish swarms is due to potential microbial pathogens.
The image below shows the localization of a pore-forming toxin, Hydralysin, in the body of the Green Hydra, Chlorohydra viridissima. The toxin is in green. Cell nuclei are in blue, symbiotic algae are in red. From Sher et al, FASEBJ 2008
