Dissolution of anthropogenic CO2 increases the partial pressure of CO2 (pCO2) and decreases the pH of seawater. The rate of Fe uptake by the dominant N2-fixing cyanobacterium Trichodesmium declines as pH decreases in metal-buffered medium. The slower Fe-uptake rate at low pH results from changes in Fe chemistry and not from a physiological response of the organism. Contrary to previous observations in nutrient-replete media, increasing pCO2/decreasing pH causes a decrease in the rates of N2 fixation and growth in Trichodesmium under low-Fe conditions. This result was obtained even though the bioavailability of Fe was maintained at a constant level by increasing the total Fe concentration at low pH. Short-term experiments in which pCO2 and pH were varied independently showed that the decrease in N2 fixation is caused by decreasing pH rather than by increasing pCO2 and corresponds to a lower efficiency of the nitrogenase enzyme. To compensate partially for the loss of N2 fixation efficiency at low pH, Trichodesmium synthesizes additional nitrogenase. This increase comes partly at the cost of down-regulation of Fe-containing photosynthetic proteins. Our results show that although increasing pCO2 often is beneficial to photosynthetic marine organisms, the concurrent decreasing pH can affect primary producers negatively. Such negative effects can occur both through chemical mechanisms, such as the bioavailability of key nutrients like Fe, and through biological mechanisms, as shown by the decrease in N2 fixation in Fe-limited Trichodesmium.
Posts Tagged 'nitrogen fixation'
Ocean acidification slows nitrogen fixation and growth in the dominant diazotroph Trichodesmium under low-iron conditions
Published 17 October 2012 Science ClosedTags: biological response, chemistry, nitrogen fixation, prokaryotes
Effect of ocean acidification on cyanobacteria in the subtropical North Atlantic
Published 9 August 2012 Science ClosedTags: biological response, nitrogen fixation, North Atlantic, primary production, prokaryotes
Cyanobacteria make significant contributions to global carbon and nitrogen cycling, particularly in the oligotrophic subtropical and tropical gyres. The present study examined short-term (days) physiological and acclimation responses of natural cyanobacterial populations to changes in pH/pCO2 spanning the last glacial minimum, ~8.4/~150 ppm, to projected year 2100 values of ~7.8/~800 ppm. Fe- and P-replete colonies of Trichodesmium increased N2-fixation rates (nmol N colony−1 h−1) at pH 7.8 by 54% (range 6 to 156%) over ambient pH/pCO2 conditions, while N2-fixation at pH/pCO2 8.4 was 21% (range 6 to 65%) lower than at ambient pH/pCO2; a similar pattern was observed when the rates were normalized to colony C. C-fixation rates were on average 13% (range −72 to 112%) greater at low pH than at ambient pH and 37% (−53 to 23%) greater than at high pH. Whole community assemblages dominated by Prochlorococcus and Synechococcus (47 to 95% of autotrophic biomass), whether nutrient-replete or P-limited, did not show a clear response of C-fixation rates to changes in pH/pCO2. Comparison of initial and final C-fixation responses across pH/pCO2 treatments suggests rapid acclimation of cellular physiology to new pH/pCO2 conditions. Changes in cell size and pigment content for Prochlorococcus and Synechococcus were minor and did not vary in a consistent manner with changes in pH/pCO2. These results for natural populations of all 3 cyanobacteria concur with previous research and suggest that one important response to changes in ocean pH and pCO2 might be an increase in N2 and C fixation by Trichodesmium under nutrient-replete conditions. The response of single-cell cyanobacteria to changes in pH/pCO2 will likely be indirect and controlled by the response to other variables, such as nutrients.
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Response of Nodularia spumigena to pCO2 – Part 1: Growth, production and nitrogen cycling (update)
Published 7 August 2012 Science ClosedTags: biogeochemistry, biological response, growth, nitrogen fixation, primary production, prokaryotes
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.
No stimulation of nitrogen fixation by non-filamentous diazotrophs under elevated CO2 in the South Pacific
Published 9 July 2012 Science ClosedTags: biological response, field, nitrogen fixation, prokaryotes, South Pacific
Nitrogen fixation by diazotrophic cyanobacteria is a critical source of new nitrogen to the oligotrophic surface ocean. Research to date indicates that some diazotroph groups may increase nitrogen fixation under elevated pCO2. To test this in natural mixed plankton communities, four manipulation experiments were carried out during two voyages in the South Pacific (30-35oS). High CO2 treatments, produced using 750ppmv CO2 to adjust pH to 0.2 below ambient, and “Greenhouse” treatments (0.2 below ambient pH and ambient temperature +3°C), were compared with Controls in trace metal-clean deckboard incubations in triplicate. No significant change was observed in nitrogen fixation in either the high CO2 or Greenhouse treatments over five day incubations. qPCR measurements and optical microscopy determined that the diazotroph community was dominated by Group A unicellular cyanobacteria (UCYN-A), which may account for the difference in response of nitrogen fixation under elevated CO2 to that reported previously for Trichodesmium. This may reflect physiological differences, in that the greater cell surface area:volume of UCYN-A and its lack of metabolic pathways involved in carbon fixation may confer no benefit under elevated CO2. However, multiple environmental controls may also be a factor, with the low dissolved iron concentrations in oligotrophic surface waters limiting the response to elevated CO2. If nitrogen fixation by UCYN-A is not stimulated by elevated pCO2, then future increases in CO2 and warming may alter the regional distribution and dominance of the different diazotroph groups, with implications for dissolved iron availability and new nitrogen supply in oligotrophic regions.
