- 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.
