Response of <i>Nodularia spumigena</i> to <i>p</i>CO<sub>2</sub> – Part 2: Exudation and extracellular enzyme activities

Abstract

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.

CO<sub>2</sub> increases <sup>14</sup>C primary production in an Arctic plankton community

“…However, higher export fluxes and therewith a higher loss of organic matter from the water column in the high pCO 2 bags were not directly observed during this study . (ii) Recent studies suggest that bacterial processes such as organic matter solubilization and hydrolysis by extracellular enzymes are enhanced by ocean acidification (Grossart et al, 2006;Piontek et al, 2010Piontek et al, , 2013Yamada and Suzumura, 2010;Endres et al, 2013). Higher activities of hydrolytic enzymes were observed at reduced pH also during side-experiments of this study (Piontek et al, 2013), and may have resulted in faster degradation of organic matter, including autotrophic biomass.…”

Response of <i>Nodularia spumigena</i> to <i>p</i>CO<sub>2</sub> – Part 3: Turnover of phosphorus compounds

“…The activity of alkaline phosphatase was determined as described in detail by Endres et al (2013) using 4-methylumbelliferyl (MUF)-phosphate (Hoppe, 1983). This fluorescent substrate analogue was added to 180-µl subsamples and incubated in duplicates for 3.5-4.5 h in the dark at 25 • C. Seven different concentrations ranging from 0 to 150 µmol l −1 were tested.…”

“…Cell numbers on average were 4.69 ± 1.64 × 10 5 , 4.54 ± 1.59 × 10 5 , and 4.73 ± 1.28 × 10 5 cells l −1 for the low, medium, and high treatment, respectively (Wannicke et al, 2012, Table 2). In Endres et al (2013) it is discussed that if bacteria were attached to each other, to Nodularia or to gel particles we might have underestimated the bacterial abundance by flow cytometry. Furthermore, Wannicke et al (2012) mentioned the possibility of staining non-viable bacteria cells with SYBR GREEN which might have been included in the enumeration.…”

“…This will be studied as part of two other investigations based on the same experimental setup. Wannicke et al (2012) and Endres et al (2013) focused on N-cycling, and exudation and extracellular enzyme activities, respectively. Here we investigated dissolved P pools, in particular their variation during the growth of the diazotrophic cyanobacterium Nodularia spumigena under conditions of pCO 2 elevation, and their contribution to Nodularia nutrition.…”

Effects of CO<sub>2</sub> perturbation on phosphorus pool sizes and uptake in a mesocosm experiment during a low productive summer season in the northern Baltic Sea

“…In CO 2 manipulation experiments, particulate phosphorus dynamics were studied to determine effects on C : P stoichiometry of phytoplankton (Riebesell and Tortell, 2011;Sugie and Yoshimura, 2013) and PO 4 concentration dynamics to estimate its utilization . CO 2 effects on phosphorus pool sizes and PO 4 uptake have so far been studied by Tanaka et al (2008) in the Raunefjorden, Norway and by Unger et al (2013) and Endres et al (2013) in laboratory experiments with cultures of Nodularia spumigena. In order to reduce the gap of knowledge, we studied the impact of elevated CO 2 on phosphorus pool sizes, the DOP composition, and PO 4 uptake of a northern Baltic Sea plankton community.…”