Response of Nodularia spumigena to pCO2 – Part 1: Growth, production and nitrogen cycling

Heterocystous cyanobacteria of the genus Nodularia form extensive blooms in the Baltic Sea and contribute 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 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 Baltic Sea. Here, we determined carbon (C) and dinitrogen (N2) fixation rates, growth, elemental stoichiometry of particulate organic matter and nitrogen turnover in batch cultures of the heterocystous cyanobacterium Nodularia spumigena under low (median 315 μatm), mid (median 353 μatm), and high (median 548 μatm) 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 during incubation days 0 to 9 resulted in an elevation in growth rate by 84±38% (low vs. high pCO2) and 40±25% (mid vs. high pCO2), as well as in N2 fixation by 93±35% and 38±1 %, respectively. C uptake rates showed high standard deviations within treatments and in between sampling days. Nevertheless, C fixation in the high pCO2 treatment was elevated compared to the other two treatments by 97% (high vs. low) and 44% (high vs. mid) at day 0 and day 3, but this effect diminished afterwards. Additionally, elevation in carbon to nitrogen and nitrogen to phosphorus ratios 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 the non-heterocystous diazotroph Trichodesmium. Implications for biogeochemical cycling and food web dynamics, as well as ecological and socio-economical aspects in the Baltic Sea are discussed

Effect of ocean acidification on production and decomposition of exudates - first results from a joint batch experiment

The effects of increasing CO2 concentrations (180, 380, and 780 ppm, representing past, present-day, and future atmospheric pCO2, respectively) on marine production, bacterial growth and activity as well as degradation of organic matter was studied during a joint bipartite laboratory experiment (Part I: Production, Part II: Degradation) of BIOACID subprojects 1.2.1, 1.2.2, 1.23 and 1.2.4 in April/May 2010. Here we report on the effect of pCO2 on the turn-over of dissolved organic matter (DOM) and transparent exopolymer particles (TEP) as well as activity rates of extracellular enzymes. The latter play an important role in the turn-over of DOM as they process organic matter degradation as well as nutrient regeneration. Ocean acidification (OA) is expected to affect the enzymatic hydrolysis, resulting in changes of the microbial decomposition of exopolymers. During Part I, growth and total TEP production of Nodularia was significantly enhanced at 780 ppm. TEP production normalized to chlorophyll a was highest at 180 ppm, suggesting that cell growth was more stimulated by CO2 than TEP production. During Part II, degradation of TEP was low in all treatments with highest decline at 780 ppm. Throughout the Part I, aminopeptidase activity increased over time in all CO2 treatments, whereas alpha- and beta-glucosidase activity remained very low. Inorganic phosphate was rapidly depleted in all treatments. Activity rates of extracellular phosphatase were highest at 780ppm, which is confirmed by strongest decline of dissolved organic phosphorus (DOP) in these treatments. No phosphatase activity was measured after removing Nodularia cells in Part II. These results suggest that ocean acidification may increase the rates of organic phosphorus recycling and therewith indirectly support algal growth

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 has a high enzyme affinity for dissolved organic phosphorus (DOP) by production and release of alkaline phosphatase. Additionally, it is 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 aerated with three different pCO2 levels corresponding to values from glacial periods to future values projected for the year 2100. 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 was influencing exudation and 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 and simultaneously increasing exudation. During the growth phase significantly more mucinous substances accumulated in the high pCO2 treatment reaching 363 μg Gum Xanthan eq l−1 compared to 269 μg Gum Xanthan eq 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. In conclusion, our results suggest that Nodularia spumigena can grow faster under elevated pCO2 by enhancing the recycling of organic matter to acquire nutrients

New perspectives on nitrogen fixation measurements using N-15(2) gas

Recently, the method widely used to determine N-15(2) fixation rates in marine and freshwater environments was found to underestimate rates because the dissolution of the added N-15(2) gas bubble in seawater takes longer than theoretically calculated. As a solution to the potential underestimate of rate measurements, the usage of the enriched water method was proposed to provide constant N-15(2) enrichment. Still, the superiority of enriched water method over the previously used bubble injection remains inconclusive. To clarify this issue, we performed laboratory based experiments and implemented the results into an error analysis of N-15(2) fixation rates. Moreover, we conducted a literature search on the comparison of the two methods to calculate a mean effect size using a meta-analysis approach. Our results indicate that the error potentially introduced by an equilibrium phase of the N-15(2) gas is -72% at maximum for experiments with very short incubation times of 1 h. In contrast, the underestimation was negligible for incubations lasting 12-24 h (error is -0.2%). Our meta-analysis indicates that 84% of the measurements in the two groups will overlap and there is a 61% chance that a sample picked at random from the enriched water group will have a higher value than one picked at random from the bubble group. Overall, the underestimation of N-2 fixation rates when using the bubble method relative to the enriched water method is highly dependent on incubation time and other experimental conditions and cannot be generalized

Growth and production of Nodularia spumigena under elevated CO2 concentrations.

This study presents results from a joint laboratory experiment investigating the effect of elevated pCO2 and P limited conditions (P <0.5 μM) on growth and production of the Nodularia spumigena. Batch cultures were grown under three different CO2 - level (180, 380, and 780 ppm) over a 15 days growth phase. The filament number increased in all treatments but s most pronounced under the highest concentrations of CO2. Correspondingly, chlorophyll increased over the first 10 days, but decreased drastically while the phosphate pool was already exhausted at day 4 of the experiment. Growth rates depended on abundance and chlorophyll were significantly higher at 780 ppm. Moreover, C fixation rates were elevated in the highest pCO2 treatment by 14-31%, N2 fixation rates by 9-60% relative to present day CO2 conditions. A higher pCO2 level seemed to have compensated to a large extend P limited growth. Furthermore, our results suggest a CO2 limitation of nitrogen fixers at the present partial pressure of CO2, which was also found in previous experiments with the open ocean species Trichodesmium