Response of Nodularia spumigena to pCO₂ – Part 3: Turnover of phosphorus compounds

Abstract. Diazotrophic cyanobacteria form extensive summer blooms in the Baltic Sea driving the surrounding surface waters into phosphate limitation. One of the main bloom-forming species is the heterocystous cyanobacterium Nodularia spumigena. N. spumigena exhibits accelerated uptake of phosphate through the release of the extracellular enzyme alkaline phosphatase whose activity also serves as an indicator of the hydrolysis of dissolved organic phosphorus (DOP). The present study investigated the utilisation of DOP and its compounds (e.g., ATP) by N. spumigena during growth under different CO₂ concentrations, in order to estimate potential consequences of ocean acidification on the cell's supply with phosphorus (P). Cell growth, the phosphorus pool, and four DOP compounds (ATP, DNA, RNA, and phospholipids) were determined in three setups with different CO₂ concentrations (average 341 μatm, 399 μatm, and 508 μatm) during a 15-day batch experiment. The results showed stimulated growth of N. spumigena and a rapid depletion of dissolved inorganic phosphorus (DIP) in all pCO₂ treatments. DOP uptake was enhanced by a factor of 1.32 at 399 μatm and of 2.25 at 508 μatm compared to the lowest CO₂ concentration. Among the measured DOP compounds, none was found to accumulate preferentially during the incubation or in response to a specific pCO₂ treatment. However, at the beginning 61.9 ± 4.3% of total DOP were not characterised but comprised the most utilised fraction. This is demonstrated by the decrement of this fraction to 27.4 ± 9.9% of total DOP during the growth phase with a preference at high pCO₂. Our results indicate a stimulated growth of diazotrophic cyanobacteria at increasing CO₂ concentrations which is accompanied by increasing utilisation of DOP as an alternative P source.