Response of <i>Nodularia spumigena</i> to <i>p</i>CO<sub>2</sub> – Part I: Growth, production and nitrogen cycling

Heterocystous cyanobacteria of the genus Nodularia form extensive blooms in the Baltic Sea contributing substantially to the total annual primary production. Moreover, they dispense a large fraction of new nitrogen to the ecosystem, when inorganic nitrogen concentration in summer is low. Thus, it is of great ecological importance to know how Nodularia will react to future environmental changes, in particular to increasing carbon dioxide (CO₂) concentrations and what consequences there might arise for cycling of organic matter in the ocean. Here, we determined carbon (C) and dinitrogen (N₂) fixation rates, growth, elemental stoichiometry of particulate organic matter and nitrogen turnover during batch growth of the heterocystous cyanobacterium Nodularia spumigena under glacial (180 ppm), present (380 ppm), and future (780 ppm) CO₂ concentrations. Our results demonstrate an overall stimulating effect of rising pCO₂ on C and N₂ fixation, as well as on cell growth. An increase in pCO₂ resulted in an elevation in growth rate, C and N₂ fixation by 23%, 36% and 25%, respectively (180 ppm vs. 380 ppm) and by 27%, 2% and 4%, respectively (380 ppm vs. 780 ppm). Additionally, elevation in the carbon and nitrogen to phosphorus quota of the particulate biomass formed (POC:POP and PON:POP) was observed at high pCO₂. Our findings suggest that rising pCO₂ stimulates the growth of heterocystous diazotrophic cyanobacteria, in a similar way as reported for non-heterocystous diazotrophs. Implications for biogeochemical cycling and food web dynamics, as well as ecological and socio-economical aspects in the Baltic Sea are discussed