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

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 mu atm), medium (380 mu atm), and high (780 mu atm) CO2 concentrations. Obtained pCO(2) levels in the treatments were on median 315, 353, and 548 mu atm 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 pCO(2) treatment, respectively, compared to the low pCO(2) treatment. In total, significantly more mucinous substances accumulated in the high pCO(2) treatment, reaching 363 mu g Xeq L-1 compared to 269 mu g Xeq L-1 in the low pCO(2) 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 pCO(2) treatment. We hypothesise from our results that Nodularia spumigena can grow faster under elevated pCO(2) by enhancing the recycling of organic matter to acquire nutrients.