- WELCOME 2022 -

Zuckerberg Institute for water research (ZIWR) is inviting you to our special virtual seminar series on BIOFILMS, BIOFOULING, AND BIOAGGREGATES. The topics of the talks will focus on various bacterial interactions in natural environments and artificial systems. The seminar will include ONE talk per WEEK between 2021 to 2022 and given by leading scholars from the US, Europe, and Israel. The seminar series is free of charge with no prerequisites. Zoom link will be distributed without any pre-registration.

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05-01-2022; 13:15 ISRt

Prof. Helle Ploug

Department of Marine Sciences, University of Gothenburg, Gothenburg, Sweden

N2-fixation within the O2 and pH microenvironments of colony-forming, brackish and marine Cyanobacteria

​Zoom link: https://us02web.zoom.us/j/88340808048?pwd=MVJ3V1RIRkJVSEpqMnpJN1EvREM3Zz09

Abstract

Several genera of filamentous, N₂ fixing cyanobacteria, e.g., Aphanizomenon, Nodularia, and Trichodesmuim, occur as individual trichomes and form mm large colonies in lakes, brackish waters or in the ocean. These cyanobacteria can cover their N-demand relative to C-fixation completely by N₂ fixation, and they often release recently fixed N₂ as ammonium to the ambient water. Thus, colony formation is often thought to facilitate N₂-fixation in these cyanobacteria. However, recent research involving O₂ and pH microsensors, stable isotopic tracers combined with secondary ion mass spectrometry for cell-specific C and N₂ fixation measurements, and diffusion models have demonstrated a high variation of cell-specific N₂ fixation rates in colony-forming cyanobacteria, and that the chemical microenvironments within colonies do not always favour N₂-fixation. Colonies collected in the field are often highly supersaturated with O₂ during light and only rarely anoxic in their centre during darkness. In connection with O₂ evolution and consumption during photosynthesis and respiration, the pH within colonies is highly dynamic and can vary from 7.4 to 9.0 depending on light conditions in brackish waters. In oceanic waters with higher buffering capacity, pH can vary between 7.92 and 8.17 within Trichodesmium colonies. At the same time, ammonium release from excess N₂ fixation causes steep concentration gradients of ammonium in the nM to mM range within colonies. The ammonium is partly assimilated by associated microbiota, rather than by the cyanobacteria, and partly released to the ambient water. A diffusion model demonstrates that the capacity to do N₂-fixation is a prerequisite to meet N-demands during the formation of mm large colonies because these otherwise would be severely diffusion limited by ammonium supply from ambient waters with ammonium concentrations in the nM to mM range.