Interactive effects of light, nitrogen source, and carbon dioxide on energy metabolism in the diatom Thalassiosira pseudonana

Due to the ongoing effects of climate change, phytoplankton are likely to experience enhanced irradiance, more reduced nitrogen, and increased water acidity in the future ocean. Here, we used Thalassiosira pseudonana as a model organism to examine how phytoplankton adjust energy production and expenditure to cope with these multiple, interrelated environmental factors. Following acclimation to a matrix of irradiance, nitrogen source, and CO2 levels, the diatom's energy production and expenditures were quantified and incorporated into an energetic budget to predict how photosynthesis was affected by growth conditions. Increased light intensity and a shift from inline image to inline image led to increased energy generation, through higher rates of light capture at high light and greater investment in photosynthetic proteins when grown on inline image. Secondary energetic expenditures were adjusted modestly at different culture conditions, except that inline image utilization was systematically reduced by increasing pCO2. The subsequent changes in element stoichiometry, biochemical composition, and release of dissolved organic compounds may have important implications for marine biogeochemical cycles. The predicted effects of changing environmental conditions on photosynthesis, made using an energetic budget, were in good agreement with observations at low light, when energy is clearly limiting, but the energetic budget over-predicts the response to inline image at high light, which might be due to relief of energetic limitations and/or increased percentage of inactive photosystem II at high light. Taken together, our study demonstrates that energetic budgets offered significant insight into the response of phytoplankton energy metabolism to the changing environment and did a reasonable job predicting them.

Effects of increasing seawater carbon dioxide concentrations on chain formation of the diatom Asterionellopsis glacialis

Diatoms can occur as single cells or as chain-forming aggregates. These two strategies affect buoyancy, predator evasion, light absorption and nutrient uptake. Adjacent cells in chains establish connections through various processes that determine strength and flexibility of the bonds, and at distinct cellular locations defining colony structure. Chain length has been found to vary with temperature and nutrient availability as well as being positively correlated with growth rate. However, the potential effect of enhanced carbon dioxide (CO2) concentrations and consequent changes in seawater carbonate chemistry on chain formation is virtually unknown. Here we report on experiments with semi-continuous cultures of the freshly isolated diatom Asterionellopsis glacialis grown under increasing CO2 levels ranging from 320 to 3400 µatm. We show that the number of cells comprising a chain, and therefore chain length, increases with rising CO2 concentrations. We also demonstrate that while cell division rate changes with CO2 concentrations, carbon, nitrogen and phosphorus cellular quotas vary proportionally, evident by unchanged organic matter ratios. Finally, beyond the optimum CO2 concentration for growth, carbon allocation changes from cellular storage to increased exudation of dissolved organic carbon. The observed structural adjustment in colony size could enable growth at high CO2 levels, since longer, spiral-shaped chains are likely to create microclimates with higher pH during the light period. Moreover increased chain length of Asterionellopsis glacialis may influence buoyancy and, consequently, affect competitive fitness as well as sinking rates. This would potentially impact the delicate balance between the microbial loop and export of organic matter, with consequences for atmospheric carbon dioxide.

Colimitation of the unicellular photosynthetic diazotroph Crocosphaera watsonii by phosphorus, light, and carbon dioxide

We describe interactive effects of total phosphorus (total P = 0.1-4.0 µmol/L; added as H2NaPO4), irradiance (40 and 150 µmol quanta/m**2/s), and the partial pressure of carbon dioxide (P-CO2; 19 and 81 Pa, i.e., 190 and 800 ppm) on growth and CO2- and dinitrogen (N-2)-fixation rates of the unicellular N-2-fixing cyanobacterium Crocosphaera watsonii (WH0003) isolated from the Pacific Ocean near Hawaii. In semicontinuous cultures of C. watsonii, elevated P-CO2 positively affected growth and CO2- and N-2-fixation rates under high light. Under low light, elevated P-CO2 positively affected growth rates at all concentrations of P, but CO2- and N-2-fixation rates were affected by elevated P-CO2 only when P was low. In both high-light and low-light cultures, the total P requirements for growth and CO2- and N-2-fixation declined as P-CO2 increased. The minimum concentration (C-min) of total P and half-saturation constant (K-1/2) for growth and CO2- and N-2-fixation rates with respect to total P were reduced by 0.05 µmol/L as a function of elevated P-CO2. We speculate that low P requirements under high P-CO2 resulted from a lower energy demand associated with carbon-concentrating mechanisms in comparison with low-P-CO2 cultures. There was also a 0.10 µmol/L increase in C-min and K-1/2 for growth and N-2 fixation with respect to total P as a function of increasing light regardless of P-CO2 concentration. We speculate that cellular P concentrations are responsible for this shift through biodilution of cellular P and possibly cellular P uptake systems as a function of increasing light. Changing concentrations of P, CO2, and light have both positive and negative interactive effects on growth and CO2-, and N-2-fixation rates of unicellular oxygenic diazotrophs like C. watsonii.

The influence of ocean acidification on nitrogen regeneration and nitrous oxide production in the northwest European shelf sea

The assimilation and regeneration of dissolved inorganic nitrogen, and the concentration of N2O, was investigated at stations located in the NW European shelf sea during June/July 2011. These observational measurements within the photic zone demonstrated the simultaneous regeneration and assimilation of NH4+, NO2- and NO3-. NH4+ was assimilated at 1.82-49.12 nmol N/L/h and regenerated at 3.46-14.60 nmol N/L/h; NO2- was assimilated at 0-2.08 nmol N/L/h and regenerated at 0.01-1.85 nmol N/L/h; NO3-was assimilated at 0.67-18.75 nmol N/L/h and regenerated at 0.05-28.97 nmol N/L/h. Observations implied that these processes were closely coupled at the regional scale and that nitrogen recycling played an important role in sustaining phytoplankton growth during the summer. The [N2O], measured in water column profiles, was 10.13 ± 1.11 nmol/L and did not strongly diverge from atmospheric equilibrium indicating that sampled marine regions were neither a strong source nor sink of N2O to the atmosphere. Multivariate analysis of data describing water column biogeochemistry and its links to N-cycling activity failed to explain the observed variance in rates of N-regeneration and N-assimilation, possibly due to the limited number of process rate observations. In the surface waters of five further stations, ocean acidification (OA) bioassay experiments were conducted to investigate the response of NH4+ oxidising and regenerating organisms to simulated OA conditions, including the implications for [N2O]. Multivariate analysis was undertaken which considered the complete bioassay data set of measured variables describing changes in N-regeneration rate, [N2O] and the biogeochemical composition of seawater. While anticipating biogeochemical differences between locations, we aimed to test the hypothesis that the underlying mechanism through which pelagic N-regeneration responded to simulated OA conditions was independent of location. Our objective was to develop a mechanistic understanding of how NH4+ regeneration, NH4+ oxidation and N2O production responded to OA. Results indicated that N-regeneration process responses to OA treatments were location specific; no mechanistic understanding of how N-regeneration processes respond to OA in the surface ocean of the NW European shelf sea could be developed.

A global two component signal transduction system that integrates the control of photosynthesis, carbon dioxide assimilation, and nitrogen fixation

Photosynthesis, biological nitrogen fixation, and carbon dioxide assimilation are three fundamental biological processes catalyzed by photosynthetic bacteria. In the present study, it is shown that mutant strains of the nonsulfur purple photosynthetic bacteria Rhodospirillum rubrum and Rhodobacter sphaeroides, containing a blockage in the primary CO2 assimilatory pathway, derepress the synthesis of components of the nitrogen fixation enzyme complex and abrogate normal control mechanisms. The absence of the Calvin–Benson–Bassham (CBB) reductive pentose phosphate CO2 fixation pathway removes an important route for the dissipation of excess reducing power. Thus, the mutant strains develop alternative means to remove these reducing equivalents, resulting in the synthesis of large amounts of nitrogenase even in the presence of ammonia. This response is under the control of a global two-component signal transduction system previously found to regulate photosystem biosynthesis and the transcription of genes required for CO2 fixation through the CBB pathway and alternative routes. In addition, this two-component system directly controls the ability of these bacteria to grow under nitrogen-fixing conditions. These results indicate that there is a molecular link between the CBB and nitrogen fixation process, allowing the cell to overcome powerful control mechanisms to remove excess reducing power generated by photosynthesis and carbon metabolism. Furthermore, these results suggest that the two-component system integrates the expression of genes required for the three processes of photosynthesis, nitrogen fixation, and carbon dioxide fixation.

Nitrogen isotopic composition and chemical composition of altered oceanic crust from ODP Legs 129 and 185

Knowledge of the subduction input flux of nitrogen (N) in altered oceanic crust (AOC) is critical in any attempt to mass-balance N across arc-trench systems on a global or individual-margin basis. We have employed sealed-tube, carrier-gas-based methods to examine the N concentrations and isotopic compositions of AOC. Analyses of 53 AOC samples recovered on DSDP/ODP legs from the North and South Pacific, the North Atlantic, and the Antarctic oceans (with larger numbers of samples from Site 801 outboard of the Mariana trench and Site 1149 outboard of the Izu trench), and 14 composites for the AOC sections at Site 801, give N concentrations of 1.3 to 18.2 ppm and d15N_air of -11.6? to +8.3?, indicating significant N enrichment probably during the early stages of hydrothermal alteration of the oceanic basalts. The N-d15N modeling for samples from Sites 801 and 1149 (n=39) shows that the secondary N may come from (1) the sedimentary N in the intercalated sediments and possibly overlying sediments via fluid-sediment/rock interaction, and (2) degassed mantle N2 in seawater via alteration-related abiotic reduction processes. For all Site 801 samples, weak correlation of N and K2O contents indicates that the siting of N in potassic alteration phases strongly depends on N availability and is possibly influenced by highly heterogeneous temperature and redox conditions during hydrothermal alteration. The upper 470-m AOC recovered by ODP Legs 129 and 185 delivers approximately 800 kg/km N annually into the Mariana margin. If the remaining less-altered oceanic crust (assuming 6.5 km, mostly dikes and gabbros) has MORB-like N of 1.5 ppm, the entire oceanic crust transfers 5100 kg/km N annually into that trench. This N input flux is twice as large as the annual N input of 2500 kg/km in seafloor sediments subducting into the same margin, demonstrating that the N input in oceanic crust, and its isotopic consequences, must be considered in any assessment of convergent margin N flux.

Effects of nitrogen supply on xylem cytokinin delivery, transpiration and leaf expansion of pea genotypes differing in xylem-cytokinin concentration

The rms2 and rms4 pea ( Pisum sativum L.) branching mutants have higher and lower xylem-cytokinin concentration, respectively, relative to wild type (WT) plants. These genotypes were grown at two levels of nitrogen (N) supply for 18 - 20 d to determine whether or not xylem-cytokinin concentration (X-CK) or delivery altered the transpiration and leaf growth responses to N deprivation. Xylem sap was collected by pressurising de-topped root systems. As sap-flow rate increased, X-CK declined in WT and rms2, but did not change in rms4. When grown at 5.0 mM N, X-CKs of rms2 and rms4 were 36% higher and 6-fold lower, respectively, than WT at sap-flow rates equivalent to whole-plant transpiration. Photoperiod cytokinin (CK) delivery rates ( the product of transpiration and X-CK) decreased more than 6-fold in rms4. Growth of plants at 0.5 mM N had negligible (< 10%) effects on transpiration rates expressed on a leaf area basis in WT and rms4, but decreased transpiration rates of rms2. The low-N treatment decreased leaf expansion by 20 - 25% and expanding leaflet N concentration by 15%. These changes were similar in all genotypes. At sap-flow rates equivalent to whole-plant transpiration, the low N treatment decreased X-CK in rms2 but had no discernible effect in WT and rms4. Since the low N treatment decreased transpiration of all genotypes, photoperiod CK delivery rates also decreased in all genotypes. The similar leaf growth response of all genotypes to N deprivation despite differences in both absolute and relative X-CKs and deliveries suggests that shoot N status is more important in regulating leaf expansion than xylem-supplied cytokinins. The decreased X-CK and transpiration rate of rms2 following N deprivation suggests that changes in xylem-supplied CKs may modify water use.

Ocean acidification slows nitrogen fixation and growth in the dominant diazotroph Trichodesmium under low-iron conditions

Dissolution of anthropogenic CO(2) increases the partial pressure of CO(2) (pCO(2)) and decreases the pH of seawater. The rate of Fe uptake by the dominant N(2)-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 pCO(2)/decreasing pH causes a decrease in the rates of N(2) 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 pCO(2) and pH were varied independently showed that the decrease in N(2) fixation is caused by decreasing pH rather than by increasing pCO(2) and corresponds to a lower efficiency of the nitrogenase enzyme. To compensate partially for the loss of N(2) 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 pCO(2) 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 N(2) fixation in Fe-limited Trichodesmium.

Interactive effects of ocean acidification and nitrogen limitation on the diatom Phaeodactylum tricornutum

Climate change is expected to bring about alterations in the marine physical and chemical environment that will induce changes in the concentration of dissolved CO2 and in nutrient availability. These in turn are expected to affect the physiological performance of phytoplankton. In order to learn how phytoplankton respond to the predicted scenario of increased CO2 and decreased nitrogen in the surface mixed layer, we investigated the diatom Phaeodactylum tricornutum as a model organism. The cells were cultured in both low CO2 (390 µatm) and high CO2 (1000 µatm) conditions at limiting (10 µmol/L) or enriched (110 µmol/L) nitrate concentrations. Our study shows that nitrogen limitation resulted in significant decreases in cell size, pigmentation, growth rate and effective quantum yield of Phaeodactylum tricornutum, but these parameters were not affected by enhanced dissolved CO2 and lowered pH. However, increased CO2 concentration induced higher rETRmax and higher dark respiration rates and decreased the CO2 or dissolved inorganic carbon (DIC) affinity for electron transfer (shown by higher values for K1/2 DIC or K1/2 CO2). Furthermore, the elemental stoichiometry (carbon to nitrogen ratio) was raised under high CO2 conditions in both nitrogen limited and nitrogen replete conditions, with the ratio in the high CO2 and low nitrate grown cells being higher by 45% compared to that in the low CO2 and nitrate replete grown ones. Our results suggest that while nitrogen limitation had a greater effect than ocean acidification, the combined effects of both factors could act synergistically to affect marine diatoms and related biogeochemical cycles in future oceans.