As the most ancient phytoplankton group cyanobacteria were responsible for the original oxidization of the Earth’s atmosphere. Due to their ability to fix atmospheric N₂ diazotrophic cyanobacteria can thrive in oligotrophic tropical and subtropical provinces, exerting a large influence on global N and C cycles. Future expansion of these oligotrophic regions to higher latitudes due to surface ocean warming and increased stratification is expected to increase the magnitude of marine N₂ fixation (Boyd and Doney 2002). Recent studies further suggest that elevated CO₂ concentration stimulates photosynthesis as well as N₂-fixation in the bloom-forming Trichodesmium (Levitan et al. 2007, Barcelos e Ramos et al. 2007, Hutchins et al. 2007). The underlying processes for the strong CO₂ sensitivity in this important diazotroph are currently unknown.

Experiments aim to quantify and understand the strong CO₂-dependence in this group. To this end, Trichodesmium and other cyanobacteria are cultured under different future scenarios. Carbon uptake/fixation and nitrogen fixation are key processes, which compete for energy and reducing power. Since cyanobacteria possess a RubisCO that has one of the lowest CO₂ affinities, increasing CO₂ levels could favour this group either by directly increasing the carboxylation efficiency of RubisCO or indirectly by reducing the energy costs of their carbon concentrating mechanisms (CCMs). The consequent reallocation of resources may enable enhanced N₂ fixation. First results on CCM regulation in Trichodesmium (Kranz et al. 2008) indeed show changes in CCM efficiency and point to shifted resource allocation between key processes under different CO₂ levels.

To further improve our understanding, experiments also focus on the diurnal cycle of photosynthetic O₂ evolution and N₂ fixation in this species. Both processes appear mutually exclusive since the nitrogenase enzyme is irreversibly inhibited by O₂. The separation of both processes in time requires close regulation and possibly involves light-dependent respiration (like Mehler reaction). To assess all of these aspects of photosynthesis and N₂ fixation a suite of techniques measuring the cellular fluxes for oxygen, carbon, nitrogen as well as photosynthetic electrons is used. We further attempt to develop an MIMS-based method to directly measure N₂-fixation. The laboratory research will be complemented by field studies.

Collaborations: Ilana Berman-Frank and Orly Levitan (Bar Ilan University, Ramat-Gan, Israel); Peter Ralph (UTS, Syndey, Australia); Ondrej Prasil (University of South Bohemia, Czech Republic).

Related publications: Kranz et al. 2011, Kranz et al. 2010b, Kranz et al. 2010a, Levitan et al. 2010, Kranz et al. 2009, Rost et al. 2008.

Dr. Sven Kranz