Abstract Iron (Fe) and phosphorus (P) availability constrain the growth and N 2 fixation of diazotrophic cyanobacteria in the global ocean. However, how Fe and P limitation may modulate the effects of ocean acidification on the unicellular diazotrophic cyanobacterium Crocosphaera remains largely unknown. Here, we examined the physiological responses of Crocosphaera watsonii WH8501 to CO 2 enrichment under both nutrient‐replete and steadily Fe‐ or P‐limited conditions. Increased CO 2 (750 μ atm vs. 400 μ atm) reduced the growth and N 2 fixation rates of Crocosphaera , with Fe limitation intensifying the negative effect, whereas CO 2 enrichment had a minimal impact under P limitation. Mechanistically, the high CO 2 treatment may have led to a reallocation of limited Fe to nitrogenase synthesis to compensate for the reduction in nitrogenase efficiency caused by low pH; consequently, other Fe‐requiring metabolic pathways, such as respiration and photosynthesis, were impaired, which in turn amplified the negative effects of acidification. Conversely, under P limitation, CO 2 enrichment had little or no effect on cellular P allocation among major P‐containing molecules (polyphosphate, phospholipids, DNA, and RNA). Cell volumes were significantly reduced in P‐limited and high CO 2 cultures, which increased the surface : volume ratio and could facilitate nutrient uptake, thereby alleviating some of the negative effect of acidification on N 2 fixation. These findings highlight the distinct responses of Crocosphaera to high CO 2 under different nutrient conditions, improving a predictive understanding of global N 2 fixation in future acidified oceans.
Abstract The productivity of diatoms in the Southern Ocean plays a key role in the transfer of carbon (C) from the atmosphere to the ocean's interior, which impacts climate. However, diatom growth in the Southern Ocean is limited by several environmental factors including iron (Fe) and light. Ongoing increases in ocean CO 2 concentrations may increase diatom carbon fixation, but it is uncertain how this will interact with extant Fe and light limitation. Here we grew the Southern Ocean diatom Fragilariopsis cylindrus under a matrix of growth sufficient and limiting Fe and light levels and current and elevated CO 2 concentrations, and found that a decrease in Fe concentration at high light, or in light intensity at high Fe, caused a similar 28–35% decrease in growth rate. Combined low Fe and low light caused a much larger (71–75%) decrease in growth rate than occurred with low Fe or low light alone, indicating Fe and light co‐limitation. At a given concentration of bioavailable dissolved inorganic Fe (Fe′), increasing p CO 2 from 400 to 750 μ atm had no significant effect on growth or C‐fixation rates under any Fe and light conditions. These results suggest that unlike previous measurements in Fe‐ and light‐limited temperate diatoms, increased CO 2 should have little effect on C‐fixation rates in Southern Ocean diatoms. The different physiological responses of cold‐water and temperate diatoms to the changing environment warrant further investigation for understanding and predicting changes in the efficiency of the biological carbon pump and the associated potential feedback to the climate change.
Abstract Nitrous oxide (N 2 O) is a potent greenhouse gas and is depleting the stratospheric ozone layer. Diazotrophic N 2 O assimilation to biomass represents a novel biological N 2 O consumption pathway in addition to canonical denitrification. Thermodynamically, N 2 O assimilation is more favorable than dinitrogen (N 2 ) fixation in natural environments, especially under higher N 2 O concentration and cooler conditions. Via isotopic tracing experiments, N 2 O assimilation was detected on cultured diazotrophs Crocosphaera and Trichodesmium with specific rates from 1.27 ± 0.16 × 10 −4 to 2.00 ± 0.25 × 10 −4 hr −1 under elevated [N 2 O]/[N 2 ] conditions (0.0005–0.01) within 24‐hr incubation. The rates of N 2 O assimilation during the light and dark periods were statistically insignificant compared with N 2 fixation activity. In a eutrophic estuary, N 2 O assimilation was not detected in the absence of diazotrophic activity. A competitive substrate kinetic model with experimentally calibrated parameters successfully quantified rate ratios of N 2 O assimilation and N 2 fixation in varying substrate concentrations. The low [N 2 O]/[N 2 ] ratio in natural conditions leads to N 2 O assimilation rate being <0.1% of N 2 fixation rate, rendering negligible impact of N 2 O assimilation. The model was also used to predict the time required for experimental detection of N 2 O assimilation in isotopic tracing experiments under varying [N 2 O]/[N 2 ] ratios. This study enhances the mechanistic understanding of N 2 O assimilation by diazotrophs, broadening the microbial nitrogen cycle by a potential N 2 O sink and nitrogen source for production.
Nitrogen fixation is critical for the biological productivity of the ocean, but clear mechanistic controls on this process remain elusive. Here, we investigate the abundance, activity, and drivers of nitrogen-fixing diazotrophs across the tropical western North Pacific. We find a basin-scale coherence of diazotroph abundances and N2 fixation rates with the supply ratio of iron:nitrogen to the upper ocean. Across a threshold of increasing supply ratios, the abundance of nifH genes and N2 fixation rates increased, phosphate concentrations decreased, and bioassay experiments demonstrated evidence for N2 fixation switching from iron to phosphate limitation. In the northern South China Sea, supply ratios were hypothesized to fall around this critical threshold and bioassay experiments suggested colimitation by both iron and phosphate. Our results provide evidence for iron:nitrogen supply ratios being the most important factor in regulating the distribution of N2 fixation across the tropical ocean.
Although increasing the p CO 2 for diatoms will presumably down‐regulate the CO 2 ‐concentrating mechanism ( CCM ) to save energy for growth, different species have been reported to respond differently to ocean acidification ( OA ). To better understand their growth responses to OA , we acclimated the diatoms Thalassiosira pseudonana , Phaeodactylum tricornutum , and Chaetoceros muelleri to ambient ( p CO 2 400 μatm, pH 8.1), carbonated ( p CO 2 800 μatm, pH 8.1), acidified ( p CO 2 400 μatm, pH 7.8), and OA ( p CO 2 800 μatm, pH 7.8) conditions and investigated how seawater p CO 2 and pH affect their CCM s, photosynthesis, and respiration both individually and jointly. In all three diatoms, carbonation down‐regulated the CCM s, while acidification increased both the photosynthetic carbon fixation rate and the fraction of CO 2 as the inorganic carbon source. The positive OA effect on photosynthetic carbon fixation was more pronounced in C. muelleri , which had a relatively lower photosynthetic affinity for CO 2 , than in either T. pseudonana or P. tricornutum . In response to OA , T. pseudonana increased respiration for active disposal of H + to maintain its intracellular pH , whereas P. tricornutum and C. muelleri retained their respiration rate but lowered the intracellular pH to maintain the cross‐membrane electrochemical gradient for H + efflux. As the net result of changes in photosynthesis and respiration, growth enhancement to OA of the three diatoms followed the order of C. muelleri > P. tricornutum > T. pseudonana . This study demonstrates that elucidating the separate and joint impacts of increased p CO 2 and decreased pH aids the mechanistic understanding of OA effects on diatoms in the future, acidified oceans.
Abstract The (sub)tropical western North Pacific is potentially an area of intense nitrogen (N 2 ) fixation in the global ocean, despite limited understanding of the flux and controlling factors. We conducted high‐resolution observations from 2016 to 2021 in this region and used machine learning algorithms to simulate N 2 fixation flux. Models estimated an N 2 fixation flux from 5.72 to 6.45 Tg N yr −1 , with strong seasonal variation and peak rates in summer. The western North Pacific Subtropical Gyre and the Kuroshio Current contributed more to N 2 fixation flux than did the adjacent areas. Models suggested that sea surface temperature, photosynthetically available radiation, and nutrient supply were most strongly correlated with seasonal and spatial variations in N 2 fixation. This study provides an improved estimation of N 2 fixation in the western North Pacific and advances our understanding of its role in ocean productivity.
Abstract The response of the prominent marine dinitrogen (N 2 )-fixing cyanobacteria Trichodesmium to ocean acidification (OA) is critical to understanding future oceanic biogeochemical cycles. Recent studies have reported conflicting findings on the effect of OA on growth and N 2 fixation of Trichodesmium . Here, we quantitatively analyzed experimental data on how Trichodesmium reallocated intracellular iron and energy among key cellular processes in response to OA, and integrated the findings to construct an optimality-based cellular model. The model results indicate that Trichodesmium growth rate decreases under OA primarily due to reduced nitrogenase efficiency. The downregulation of the carbon dioxide (CO 2 )-concentrating mechanism under OA has little impact on Trichodesmium , and the energy demand of anti-stress responses to OA has a moderate negative effect. We predict that if anthropogenic CO 2 emissions continue to rise, OA could reduce global N 2 fixation potential of Trichodesmium by 27% in this century, with the largest decrease in iron-limiting regions.
Abstract 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 CO 2 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 to led to increased energy generation, through higher rates of light capture at high light and greater investment in photosynthetic proteins when grown on . Secondary energetic expenditures were adjusted modestly at different culture conditions, except that utilization was systematically reduced by increasing p CO 2 . 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 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.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)
