Nicola Wannicke, Achim Herrmann, Michelle M. Gehringer

• Heterocystous Cyanobacteria of the genus Nodularia form major blooms in brackish waters, while terrestrial Nostoc species occur worldwide, often associated in biological soil crusts. Both genera, by virtue of their ability to fix N2 and conduct oxygenic photosynthesis, contribute significantly to global primary productivity. Select Nostoc and Nodularia species produce the hepatotoxin nodularin and whether its production will change under climate change conditions needs to be assessed. In light of this, the effects of elevated atmospheric CO2 availability on growth, carbon and N2 fixation as well as nodularin production were investigated in toxin and non-toxin producing species of both genera. Results highlighted the following: Biomass and volume specific biological nitrogen fixation (BNF) rates were respectively almost six and 17 fold higher in the aquatic Nodularia species compared to the terrestrial Nostoc species tested, under elevated CO2 conditions. There was a direct correlation between elevated CO2 and decreased dry weight specific cellular nodularin content in a diazotrophically grown terrestrial Nostoc species, and the aquatic Nodularia species, regardless of nitrogen availability. Elevated atmospheric CO2 levels were correlated to a reduction in biomass specific BNF rates in non-toxic Nodularia species. Nodularin producers exhibited stronger stimulation of net photosynthesis rates (NP) and growth (more positive Cohen’s d) and less stimulation of dark respiration and BNF per volume compared to non-nodularin producers under elevated CO2 levels. This study is the first to provide information on NP and nodularin production under elevated atmospheric CO2 levels for Nodularia and Nostoc species under nitrogen replete and diazotrophic conditions.