Response of Nodularia spumigena to pCO₂ – Part 2: Exudation and extracellular enzyme activities

Abstract. The filamentous and diazotrophic cyanobacterium Nodularia spumigena plays a major role in the productivity of the Baltic Sea as it forms extensive blooms regularly. Under phosphorus limiting conditions Nodularia spumigena have a high enzyme affinity for dissolved organic phosphorus (DOP) by production and release of alkaline phosphatase. Additionally, they are able to degrade proteinaceous compounds by expressing the extracellular enzyme leucine aminopeptidase. As atmospheric CO₂ concentrations are increasing, we expect marine phytoplankton to experience changes in several environmental parameters, including pH, temperature, and nutrient availability. The aim of this study was to investigate the combined effect of CO₂-induced changes in seawater carbonate chemistry and of phosphate deficiency on the exudation of organic matter, and its subsequent recycling by extracellular enzymes in a Nodularia spumigena culture. Batch cultures of Nodularia spumigena were grown for 15 days under aeration with low (180 μatm), medium (380 μatm), and high (780 μatm) CO₂ concentrations. Obtained pCO₂ levels in the treatments were on median 315, 353, and 548 μatm CO₂, respectively. Extracellular enzyme activities as well as changes in organic and inorganic compound concentrations were monitored. CO₂ treatment–related effects were identified for cyanobacterial growth, which in turn influenced the concentration of mucinous substances and the recycling of organic matter by extracellular enzymes. Biomass production was increased by 56.5% and 90.7% in the medium and high pCO₂ treatment, respectively, compared to the low pCO₂ treatment. In total, significantly more mucinous substances accumulated in the high pCO₂ treatment, reaching 363 μg Xeq L⁻¹ compared to 269 μg Xeq L⁻¹ in the low pCO₂ treatment. However, cell-specific rates did not change. After phosphate depletion, the acquisition of P from DOP by alkaline phosphatase was significantly enhanced. Alkaline phosphatase activities were increased by factor 1.64 and 2.25, respectively, in the medium and high compared to the low pCO₂ treatment. We hypothesise from our results that Nodularia spumigena can grow faster under elevated pCO₂ by enhancing the recycling of organic matter to acquire nutrients.