The marine filamentous N₂ fixing (diazotroph) cyanobacteria Trichodesmium spp. form extensive blooms contributing 25 to 50% of the estimated rates of N₂ fixation in the oligotrophic subtropical and tropical oceans. Trichodesmium’s dominant role in carbon and nitrogen cycling has prompted investigations examining the effects of rising sea surface temperatures and elevated atmospheric pCO₂ (leading to ocean acidification) on the growth and abundance of this organism.
We examined the effect of elevated pCO₂ and light on the physiology and gene expression of key genes in Trichodesmium. Genes that participate in nitrogen metabolism, Ci fixation, and photosynthesis were most affected by changes in pCO₂, temperature and the time within the diurnal period. High pCO₂ shifted transcript patterns of all genes, resulting in a more synchronized diel expression. Concurrently, we found no significant changes in the protein pools or in total cellular allocations of carbon and nitrogen (i.e. C : N ratio remained stable). Moreover, increased temperatures and high pCO₂ resulted in higher C : P ratios. The plasticity in phosphorous stoichiometry combined with higher enzymatic efficiencies lead to higher growth rates. We demonstrate that shifted cellular resource and energy allocation among those components will enable Trichodesmium grown at elevated temperatures and pCO₂ to extend its niche in the future ocean, through both tolerance of a broader temperature range and higher P plasticity.

Currently we are investigating two other physiological phenomena in Trichodesmium: the mechanisms of colony and bloom formation, and the molecular mechanisms of toxin production.

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.

We have specifically examined changes in natural populations of diazotrophs from the Red- and the Mediterranean Seas under the combined effects of elevated CO₂ and higher temperatures as well as variations in essential nutrients such as phosphate and iron.
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 (Photo 1). 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 (Photo 2).

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.

Biological fixation of atmospheric dinitrogen can contribute significant amounts of biologically available dinitrogen to the nitrogen-limited surface waters of the oceans and induce subsequent biological production

Read More

Trichodesmium trichome showing DNA stained in blue

Programmed cell death (PCD) is an irreversible, genetically controlled form of cell suicide that is essential to promote and maintain genetic stability and is critical for the regulation of cellular and tissue homeostasis in metazoans. PCD has been observed in a variety of unicellular organisms including prokaryotic bloom-forming cyanobacteria, chlorophytes, dinoflagellates, diatoms, and coccolithophores. Trichodesmium also displays autocatalytic PCD in response to stressors such as oxidation, high irradiance, and Fe-depletion.
In the oceans Trichodesmium forms extensive blooms in nutrient-poor tropical and subtropical regions. These massive blooms generally collapse several days after forming, but the cellular mechanism responsible along with the magnitude of associated C and N export, are as yet unknown. Our work from laboratory simulations demonstrate that extremely rapid development and abrupt, PCD-induced demise (within 2-3 d) of Trichodesmium blooms lead to greatly elevated excretions of transparent exopolymers and a massive downward pulse of particulate organic matter. Our results mechanistically link autocatalytic PCD and bloom collapse to quantitative C and N export fluxes and suggest that PCD may impact biological pump efficiency in the oceans.