Seagrasses are often considered “winners” of ocean acidification (OA); however, seagrass productivity responses to OA could be limited by nitrogen availability, since nitrogen-derived metabolites are required for carbon assimilation. We tested nitrogen uptake and assimilation, photosynthesis, growth, and carbon allocation responses of the tropical seagrasses Halodule uninervis and Thalassia hemprichii to OA scenarios (428, 734 and 1213 μatm pCO2) under two nutrients levels (0.3 and 1.9 μM NO3−). Net primary production (measured as oxygen production) and growth in H. uninervis increased with pCO2 enrichment, but were not affected by nitrate enrichment. However, nitrate enrichment reduced whole plant respiration in H. uninervis. Net primary production and growth did not show significant changes with pCO2 or nitrate by the end of the experiment (24 d) in T. hemprichii. However, nitrate incorporation in T. hemprichii was higher with nitrate enrichment. There was no evidence that nitrogen demand increased with pCO2 enrichment in either species. Contrary to our initial hypothesis, nutrient increases to levels approximating present day flood plumes only had small effects on metabolism. This study highlights that the paradigm of increased productivity of seagrasses under ocean acidification may not be valid for all species under all environmental conditions.
Posts Tagged 'nitrogen fixation'
Nitrate fertilisation does not enhance CO2 responses in two tropical seagrass species
Published 23 March 2016 Science ClosedTags: biological response, growth, laboratory, multiple factors, nitrogen fixation, nutrients, phanerogams, photosynthesis, physiology, primary production, respiration, South Pacific
Ocean acidification impacts on nitrogen fixation in the coastal western Mediterranean Sea
Published 14 January 2016 Science ClosedTags: biogeochemistry, biological response, BRcommunity, chemistry, community composition, field, Mediterranean, mesocosms, nitrogen fixation, otherprocess, prokaryotes
The effects of ocean acidification on nitrogen (N2) fixation rates and on the community composition of N2-fixing microbes (diazotrophs) were examined in coastal waters of the North-Western Mediterranean Sea. Nine experimental mesocosm enclosures of ∼50 m3 each were deployed for 20 days during June-July 2012 in the Bay of Calvi, Corsica, France. Three control mesocosms were maintained under ambient conditions of carbonate chemistry. The remainder were manipulated with CO2 saturated seawater to attain target amendments of pCO2 of 550, 650, 750, 850, 1000 and 1250 μatm. Rates of N2 fixation were elevated up to 10 times relative to control rates (2.00 ± 1.21 nmol L-1d-1) when pCO2 concentrations were >1000 μatm and pHT (total scale) < 7.74. Diazotrophic phylotypes commonly found in oligotrophic marine waters, including the Mediterranean, were not present at the onset of the experiment and therefore, the diazotroph community composition was characterised by amplifying partial nifH genes from the mesocosms. The diazotroph community was comprised primarily of cluster III nifH sequences (which include possible anaerobes), and proteobacterial (α and γ) sequences, in addition to small numbers of filamentous (or pseudo-filamentous) cyanobacterial phylotypes. The implication from this study is that there is some potential for elevated N2 fixation rates in the coastal western Mediterranean before the end of this century as a result of increasing ocean acidification. Observations made of variability in the diazotroph community composition could not be correlated with changes in carbon chemistry, which highlights the complexity of the relationship between ocean acidification and these keystone organisms.
Impact of climate change and ocean acidification on the marine nitrogen cycle
Published 13 November 2015 Science ClosedTags: biogeochemistry, biological response, globalmodeling, modeling, multiple factors, nitrogen fixation, otherprocess, primary production, temperature
The marine nitrogen cycle is responsible for two climate feedbacks in the Earth System. Firstly, it modulates the fixed nitrogen pool available for phytoplankton growth and hence it modulates in part the strength of the biological pump, one of the mechanisms contributing to the oceanic uptake of anthropogenic CO2. Secondly, the nitrogen cycle produces a powerful greenhouse gas and ozone (O3) depletion agent called nitrous oxide (N2O). Future changes of the nitrogen cycle in response to global warming, ocean deoxygenation and ocean acidification are largely unknown. Processes such as N2-fixation, nitrification, denitrification and N2O production will experience changes under the simultaneous effect of these three stressors. Global ocean biogeochemical models allow us to study such interactions. Using NEMO-PISCES and the CMIP5 model ensemble we project changes in year 2100 under the business-as-usual high CO2 emissions scenario in global scale N2-fixation rates, nitrification rates, N2O production and N2O sea-to-air fluxes adding CO2 sensitive functions into the model parameterizations. Second order effects due to the combination of global warming in tandem with ocean acidification on the fixed nitrogen pool, primary productivity and N2O radiative forcing feedbacks are also evaluated in this thesis.
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No observed effect of ocean acidification on nitrogen biogeochemistry in a summer Baltic Sea plankton community
Published 30 October 2015 Science ClosedTags: abundance, Baltic, biogeochemistry, biological response, BRcommunity, field, mesocosms, nitrogen fixation, otherprocess, prokaryotes
Nitrogen fixation by filamentous cyanobacteria supplies significant amounts of new nitrogen (N) to the Baltic Sea. This balances N loss processes such as denitrification and anammox and forms an important N source supporting primary and secondary production in N-limited post-spring bloom plankton communities. Laboratory studies suggest that filamentous diazotrophic cyanobacteria growth and N2-fixation rates are sensitive to ocean acidification with potential implications for new N supply to the Baltic Sea. In this study, our aim was to assess the effect of ocean acidification on diazotroph growth and activity as well as the contribution of diazotrophically-fixed N to N supply in a natural plankton assemblage. We enclosed a natural plankton community in a summer season in the Baltic Sea near the entrance to the Gulf of Finland in six large-scale mesocosms (volume ~ 55 m3) and manipulated fCO2 over a range relevant for projected ocean acidification by the end of this century (average treatment fCO2: 365–1231 μatm). The direct response of diazotroph growth and activity was followed in the mesocosms over a 47 day study period during N-limited growth in the summer plankton community. Diazotrophic filamentous cyanobacteria abundance throughout the study period and N2-fixation rates (determined only until day 21 due to subsequent use of contaminated commercial 15N-N2 gas stocks) remained low. Thus estimated new N inputs from diazotrophy were too low to relieve N limitation and stimulate a summer phytoplankton bloom. Instead regeneration of organic N sources likely sustained growth in the plankton community. We could not detect significant CO2-related differences in inorganic or organic N pools sizes, or particulate matter N : P stoichiometry. Additionally, no significant effect of elevated CO2 on diazotroph activity was observed. Therefore, ocean acidification had no observable impact on N cycling or biogeochemistry in this N-limited, post-spring bloom plankton assemblage in the Baltic Sea.
Responses of elevated CO2 on photosynthesis and nitrogen metabolism in Ulva lactuca (Chlorophyta) at different temperature levels
Published 22 September 2015 Science ClosedTags: algae, biological response, growth, laboratory, multiple factors, nitrogen fixation, photosynthesis, physiology, respiration, temperature
Ulva lactuca was cultured in a laboratory at two CO2 levels (390 and 700 μl l−1) and at low (15°C) and high (25°C) temperature levels. Growth, biochemical composition, photosynthesis and nitrogen metabolism were examined to evaluate the impacts of elevated CO2 and temperature on this species. Elevated CO2 had little effect on the relative growth rate (RGR) in both low and high temperature levels. The levels of Chl a, carotenoids, soluble protein (SP) and soluble carbohydrates (SC) were also unaffected by CO2 enrichment. Elevated CO2 decreased the content of total fatty acids (FAs) but the higher temperature increased them. At 15°C, the light-saturated maximal photosynthetic rate (Pm), photosynthetic efficiency (α), and dark respiration (Rd) were barely affected by the increase in CO2 concentration. However, at 25°C the Pm, α and Rd were pronouncedly enhanced by elevated CO2. Additionally, high temperature increased the slopes of curves of electron transport rates (ETR) versus gross photosynthesis (Pg), indicating the loss of correlation between ETR and linear photosynthetic flow. The elevation of temperature significantly increased the nitrogen uptake rates (NUR) during the dark and light period, while elevated CO2 increased NUR only during the light period. Increased CO2 had no marked effect on nitrate reductase (NR) activity at 15°C, but high CO2 could remarkably improve the NR activity at 25°C. These results suggest that the responses to elevated CO2 in U. lactuca were temperature-dependent, and elevated CO2 alone was less effective than temperature levels.
Irreversibly increased nitrogen fixation in Trichodesmium experimentally adapted to elevated carbon dioxide
Published 3 September 2015 Science ClosedTags: adaptation, biological response, growth, laboratory, molecular biology, nitrogen fixation, otherprocess, prokaryotes
Nitrogen fixation rates of the globally distributed, biogeochemically important marine cyanobacterium Trichodesmium increase under high carbon dioxide (CO2) levels in short-term studies due to physiological plasticity. However, its long-term adaptive responses to ongoing anthropogenic CO2 increases are unknown. Here we show that experimental evolution under extended selection at projected future elevated CO2 levels results in irreversible, large increases in nitrogen fixation and growth rates, even after being moved back to lower present day CO2 levels for hundreds of generations. This represents an unprecedented microbial evolutionary response, as reproductive fitness increases acquired in the selection environment are maintained after returning to the ancestral environment. Constitutive rate increases are accompanied by irreversible shifts in diel nitrogen fixation patterns, and increased activity of a potentially regulatory DNA methyltransferase enzyme. High CO2-selected cell lines also exhibit increased phosphorus-limited growth rates, suggesting a potential advantage for this keystone organism in a more nutrient-limited, acidified future ocean.
Ocean acidification and marine microorganisms: responses and consequences
Published 5 August 2015 Science ClosedTags: abundance, biological response, BRcommunity, community composition, methods, nitrogen fixation, otherprocess, physiology, phytoplankton, primary production, prokaryotes, review
Ocean acidification (OA) is one of the global issues caused by rising atmospheric CO2. The rising pCO2 and resulting pH decrease has altered ocean carbonate chemistry. Microbes are key components of marine environments involved in nutrient cycles and carbon flow in marine ecosystems. However, these marine microbes and the microbial processes are sensitive to ocean pH shift. Thus, OA affects the microbial diversity, primary productivity and trace gases emission in oceans. Apart from that, it can also manipulate the microbial activities such as quorum sensing, extracellular enzyme activity and nitrogen cycling. Short-term laboratory experiments, mesocosm studies and changing marine diversity scenarios have illustrated undesirable effects of OA on marine microorganisms and ecosystems. However, from the microbial perspective, the current understanding on effect of OA is based mainly on limited experimental studies. It is challenging to predict response of marine microbes based on such experiments for this complex process. To study the response of marine microbes towards OA, multiple approaches should be implemented by using functional genomics, new generation microscopy, small-scale interaction among organisms and/or between organic matter and organisms. This review focuses on the response of marine microorganisms to OA and the experimental approaches to investigate the effect of changing ocean carbonate chemistry on microbial mediated processes.
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Inter- and intra-specific responses of coccolithophores to CO2-induced ocean acidification
Published 14 January 2015 Science ClosedTags: biogeochemistry, biological response, calcification, growth, laboratory, nitrogen fixation, North Pacific, photosynthesis, physiology, phytoplankton, primary production
Oceanic uptake of anthropogenic carbon dioxide (CO2) is altering the seawater chemistry of the world’s oceans with consequences for marine bioregions, especially calcareous organisms such as corals, foraminifera and coccolithophores. The coccolithophores, one of the most abundant and widespread groups of calcifying plankton, are responsible for a large proportion of modern oceanic carbonate production. However, culture experiments examining the response of coccolithophores to elevated CO2 partial pressure (pCO2) have mostly been based on investigations of a single strain and have yielded contradictory results from different experiments between and even within species. Here, four strains of the coccolithophores Emiliania huxleyi (E. huxleyi) and Gephyrocapsa oceanica (G. oceanica), which contained separately naked and calcifying strains, were investigated simultaneously for the first time in a bubbling batch culture at four CO2 grades ranging from approximately 380 to 2000 μatm. We synchronously determined multiple physiological parameters of four coccolithophore strains involving growth, photosynthesis, nitrogen uptake, elemental compositions and calcification efficiency in the process of cultivation. The results did not show a uniform response from different strains to elevated pCO2 up to 2000 μatm, and the naked strain E. huxleyi (N-E) was seriously suppressed, in sharp contrast to the positive response of the different levels of the other three algae. In addition, we fitted nitrogen uptake rate response curves relative to changing pCO2 for the four strains and applied kinetic constants from the response curves to further analyze the hypostatic difference among different strains, which reflected the same variational trend of the four stains above vs. increasing CO2. We determined that the responses of coccolithophores to ocean acidification are inter- and intra-specific, and this variation may cause changes to biodiversity and other ecosystem processes in the future ocean.
Ocean acidification rapidly reduces dinitrogen fixation associated with the hermatypic coral Seriatopora hystrix
Published 24 November 2014 Science ClosedTags: algae, biological response, laboratory, nitrogen fixation
Since productivity and growth of coral-associated dinoflagellate algae is nitrogen (N)-limited, dinitrogen (N2) fixation by coral-associated microbes is likely crucial for maintaining the coral-dinoflagellate symbiosis. It is thus essential to understand the effects future climate change will have on N2 fixation by the coral holobiont. This laboratory study is the first to investigate short-term effects of ocean acidification on N2 fixation activity associated with the tropical, hermatypic coral Seriatopora hystrix using the acetylene reduction assay in combination with calcification measurements. Findings reveal that simulated ocean acidification ( pCO2 1080 µatm) caused a rapid and significant decrease (53%) in N2 fixation rates associated with S. hystrix compared to the present day scenario ( pCO2 486 µatm). In addition, N2 fixation associated with the coral holobiont showed a positive exponential relationship with its calcification rates. This suggests that even small declines in calcification rates of hermatypic corals under high CO2 conditions may result in decreased N2 fixation activity, since these 2 processes may compete for energy in the coral holobiont. Ultimately, an intensified N limitation in combination with a decline in skeletal growth may trigger a negative feedback loop on coral productivity exacerbating the negative long-term effects of ocean acidification.
Experimental assessment of diazotroph responses to elevated seawater pCO2 in the North Pacific Subtropical Gyre
Published 27 May 2014 Science ClosedTags: biological response, laboratory, molecular biology, nitrogen fixation, North Pacific, primary production, prokaryotes
We examined short-term (24-72 hours) responses of naturally occurring marine N2 fixing microorganisms (termed diazotrophs) to abrupt increases in the partial pressure of carbon dioxide (pCO2) in seawater during 9 incubation experiments conducted between May 2010 and September 2012 at Station ALOHA (22°45’N, 158˚W) in the North Pacific Subtropical Gyre (NPSG). Rates of N2 fixation, nitrogenase (nifH) gene abundances and transcripts of six major groups of cyanobacterial diazotrophs (including both unicellular and filamentous phylotypes), and rates of primary productivity (as measured by 14C-bicarbonate assimilation into plankton biomass) were determined under contemporary (~390 ppm) and elevated pCO2 conditions (~1100 ppm). Quantitative polymerase chain reaction (QPCR) amplification of planktonic nifH genes revealed that unicellular cyanobacteria phylotypes dominated gene abundances during these experiments. In the majority of experiments (7 out of 9), elevated pCO2 did not significantly influence rates of dinitrogen (N2) fixation or primary productivity (two-way ANOVA, P > 0.05). During two experiments, rates of N2 fixation rates and primary productivity were significantly lower (by 79 to 82% and 52 to 72%, respectively) in the elevated pCO2 treatments relative to the ambient controls (two-way ANOVA, P < 0.05). QPCR amplification of nifH genes and gene transcripts revealed that diazotroph abundances and nifH gene expression were largely unchanged by the perturbation of the seawater pCO2. Our results suggest that naturally occurring N2 fixing plankton assemblages in the NPSG are relatively resilient to large, short-term increases in pCO2.
Diversity of ocean acidification effects on marine N2 fixers
Published 21 May 2014 Science ClosedTags: biogeochemistry, biological response, growth, laboratory, nitrogen fixation, primary production, prokaryotes
Considering the important role of N2 fixation for primary productivity and CO2 sequestration, it is crucial to assess the response of diazotrophs to ocean acidification. Previous studies on the genus Trichodesmium suggested a strong sensitivity towards ocean acidification. In view of the large functional diversity in N2 fixers, the objective of this study was to improve our knowledge of the CO2 responses of other diazotrophs. To this end, the single-celled Cyanothece sp. and two heterocystous species, Nodularia spumigena and the symbiotic Calothrix rhizosoleniae, were acclimated to two pCO2 levels (380 vs. 980 μatm). Growth rates, cellular composition (carbon, nitrogen and chlorophyll a) as well as carbon and N2 fixation rates (14C incorporation, acetylene reduction) were measured and compared to literature data on different N2 fixers. The three species investigated in this study responded differently to elevated pCO2, showing enhanced, decreased as well as unaltered growth and production rates. For instance, Cyanothece increased production rates with pCO2, which is in line with the general view that N2 fixers benefit from ocean acidification. Due to lowered growth and production of Nodularia, nitrogen input to the Baltic Sea might decrease in the future. In Calothrix, no significant changes in growth or production could be observed, even though N2 fixation was stimulated under elevated pCO2. Reviewing literature data confirmed a large variability in CO2 sensitivity across diazotrophs. The contrasting response patterns in our and previous studies were discussed with regard to the carbonate chemistry in the respective natural habitats, the mode of N2 fixation as well as differences in cellular energy limitation between the species. The group-specific CO2 sensitivities will impact differently on future biogeochemical cycles of open-ocean environments and systems like the Baltic Sea and should therefore be considered in models estimating climate feedback effects.
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Diversity trumps acidification: Lack of evidence for carbon dioxide enhancement of Trichodesmium community nitrogen or carbon fixation at Station ALOHA
Published 25 April 2014 Science ClosedTags: biological response, BRcommunity, laboratory, light, multiple factors, nitrogen fixation, North Pacific, nutrients, primary production, prokaryotes
We conducted 11 independent short-term carbon dioxide (CO2) manipulation experiments using colonies of the filamentous cyanobacteria Trichodesmium isolated on three cruises in the North Pacific Subtropical Gyre (NPSG). Dinitrogen (N2) and carbon (C) fixation rates of these colonies were compared over CO2 conditions ranging from ∼ 18 Pa (equivalent to last glacial maximum atmospheric ) to ∼ 160 Pa (predicted for ∼ year 2200). Our results indicate that elevated has no consistent significant effect on rates of N2 or C fixation by Trichodesmium colonies in the NPSG under present environmental conditions. Differences between treatments were not modulated by phosphorus amendments, iron amendments, or light level. Sequencing the hetR, nifH, 16S, and internal transcribed spacer genes of Trichodesmium colonies revealed a highly diverse community of Trichodesmium and other N2-fixing colony-associated organisms. The species composition of Trichodesmium demonstrated spatiotemporal variability, but over half of total sequences were phylogenetically closely related (> 99% hetR sequence similarity) to isolate H9-4 of T. erythraeum, which showed no response to elevated in previous laboratory experiments. Our handpicked Trichodesmium colonies included a substantial number of organisms other than Trichodesmium with the metabolic capacity for N2 and C fixation. We suggest that the diverse assemblage of Trichodesmium species and coexisting microorganisms within the colonies can explain the lack of an observed CO2 enhancement of N2 or C fixation rates, because different species are known to have different specific affinities for CO2.
Benthic N2 fixation in coral reefs and the potential effects of human-induced environmental change
Published 1 April 2014 Science ClosedTags: biological response, corals, nitrogen fixation, review
Tropical coral reefs are among the most productive and diverse ecosystems, despite being surrounded by ocean waters where nutrients are in short supply. Benthic dinitrogen (N2) fixation is a significant internal source of “new” nitrogen (N) in reef ecosystems, but related information appears to be sparse. Here, we review the current state (and gaps) of knowledge on N2 fixation associated with coral reef organisms and their ecosystems. By summarizing the existing literature, we show that benthic N2 fixation is an omnipresent process in tropical reef environments. Highest N2 fixation rates are detected in reef-associated cyanobacterial mats and sea grass meadows, clearly showing the significance of these functional groups, if present, to the input of new N in reef ecosystems. Nonetheless, key benthic organisms such as hard corals also importantly contribute to benthic N2 fixation in the reef. Given the usually high coral coverage of healthy reef systems, these results indicate that benthic symbiotic associations may be more important than previously thought. In fact, mutualisms between carbon (C) and N2 fixers have likely evolved that may enable reef communities to mitigate N limitation. We then explore the potential effects of the increasing human interferences on the process of benthic reef N2 fixation via changes in diazotrophic populations, enzymatic activities, or availability of benthic substrates favorable to these microorganisms. Current knowledge indicates positive effects of ocean acidification, warming, and deoxygenation and negative effects of increased ultraviolet radiation on the amount of N fixed in coral reefs. Eutrophication may either boost or suppress N2 fixation, depending on the nutrient becoming limiting. As N2 fixation appears to play a fundamental role in nutrient-limited reef ecosystems, these assumptions need to be expanded and confirmed by future research efforts addressing the knowledge gaps identified in this review.
Responses of filamentous cyanobacteria to natural and anthropogenic changes in the Baltic Sea
Published 3 March 2014 Science ClosedTags: Baltic, biogeochemistry, biological response, BRcommunity, community composition, field, growth, laboratory, multiple factors, nitrogen fixation, nutrients, otherprocess, photosynthesis, phytoplankton, primary production, prokaryotes
The studies concentrated on 2 diazotrophic cyanobacteria species, Nodularia spumigena and Aphanizomenon sp.. The dissertation includes diurnal observations, laboratory and field studies. The experiments aimed to observe variations concerning day/night rhythms and anthropogenic induced changes of pCO2 levels. I showed 2 to 4 fold diurnal variations within DIC, N2, DIP incorporations, related to daytime and bloom development. Further I showed stimulated C and N2 fixation rates with increasing pCO2 at sufficient DIP concentrations. Reduced or depleted DIP concentrations debate this effect.
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Combined effects of different CO2 levels and N sources on the diazotrophic cyanobacterium Trichodesmium
Published 28 February 2014 Science ClosedTags: biological response, laboratory, multiple factors, nitrogen fixation, nutrients, photosynthesis, primary production, prokaryotes, respiration
To predict effects of climate change and possible feedbacks, it is crucial to understand the mechanisms behind pCO2 responses of biogeochemically relevant phytoplankton species. Previous experiments on the abundant N2-fixer Trichodesmium demonstrated strong pCO2 responses, which were attributed to an energy reallocation between its carbon and nitrogen acquisition. Pursuing this hypothesis, we manipulated the cellular energy budget by growing Trichodesmium erythraeum IMS101 under different pCO2 levels (180, 380, 980 and 1400 µatm) and nitrogen sources (N2 and NO3–). Subsequently, biomass production and the main energy-generating processes (photosynthesis and respiration) and energy-consuming processes (N2-fixation and carbon acquisition) were measured. While oxygen fluxes and chlorophyll fluorescence indicated that energy generation and its diurnal cycle was neither affected by pCO2 nor nitrogen source, cells differed in production rates and composition. Elevated pCO2 increased N2-fixation and organic carbon and nitrogen contents. The degree of stimulation was higher for nitrogenase activity than for cell contents, indicating a pCO2 effect on the transfer efficiency from N2 to biomass. pCO2-dependent changes in the diurnal cycle of N2-fixation correlated well with carbon affinities, confirming the interactions between nitrogen and carbon acquisition. Regarding effects of the nitrogen source, production rates were enhanced in NO3– grown cells, which we attribute to the higher N retention and lower ATP demand compared to N2-fixation. pCO2 effects on carbon affinity were less pronounced in NO3– users than N2-fixers. Our study illustrates the necessity to understand energy budgets and fluxes under different environmental conditions for explaining indirect effects of rising pCO2.
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Trichodesmium’s strategies to alleviate P-limitation in the future acidified oceans
Published 27 February 2014 Science ClosedTags: biological response, growth, laboratory, multiple factors, nitrogen fixation, nutrients, physiology, prokaryotes
Global warming may exacerbate inorganic nutrient limitation, including phosphorus (P), in the surface-waters of tropical oceans that are home to extensive blooms of the marine diazotrophic cyanobacterium, Trichodesmium. We examined the combined effects of P-limitation and pCO2, forecast under ocean acidification scenarios, on Trichodesmium erythraeum IMS101 cultures. We measured nitrogen acquisition, glutamine synthetase (GS) activity, C uptake rates, intracellular ATP concentration and the pool sizes of related key proteins. Here we present data supporting the idea that cellular energy reallocation enables the higher growth and N2 fixation rates detected in Trichodesmium cultured under high pCO2. This is reflected in altered protein abundance and metabolic pools. Also modified are particulate organic carbon and nitrogen production rates, enzymatic activities, and cellular ATP concentrations. We suggest that adjusting these cellular pathways to changing environmental conditions enables Trichodesmium to compensate for low P availability and to thrive in acidified oceans. Moreover, elevated pCO2 could provide Trichodesmium with a competitive dominance that would extend its niche, particularly in P-limited regions of the tropical and subtropical oceans.
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Colimitation of the unicellular photosynthetic diazotroph Crocosphaera watsonii by phosphorus, light, and carbon dioxide
Published 29 July 2013 Science ClosedTags: biological response, growth, laboratory, light, morphology, multiple factors, nitrogen fixation, nutrients, primary production, prokaryotes
We describe interactive effects of total phosphorus (total P = 0.1–4.0 µmol L−1; added as H2NaPO4), irradiance (40 and 150 µmol quanta m−2 s−1), and the partial pressure of carbon dioxide (pCO2; 19 and 81 Pa, i.e., 190 and 800 ppm) on growth and CO2– and dinitrogen (N2)-fixation rates of the unicellular N2-fixing cyanobacterium Crocosphaera watsonii (WH0003) isolated from the Pacific Ocean near Hawaii. In semicontinuous cultures of C. watsonii, elevated pCO2 positively affected growth and CO2– and N2-fixation rates under high light. Under low light, elevated pCO2 positively affected growth rates at all concentrations of P, but CO2– and N2-fixation rates were affected by pCO2 elevated only when P was low. In both high-light and low-light cultures, the total P requirements for growth and CO2– and N2-fixation declined as pCO2 increased. The minimum concentration (Cmin) of total P and half-saturation constant (K½) for growth and CO2– and N2-fixation rates with respect to total P were reduced by 0.05 µmol L−1 as a function of elevated pCO2. We speculate that low P requirements under high pCO2 resulted from a lower energy demand associated with carbon-concentrating mechanisms in comparison with low-pCO2 cultures. There was also a 0.10 µmol L−1 increase in Cmin and K½ for growth and N2 fixation with respect to total P as a function of increasing light regardless of pCO2 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 N2-fixation rates of unicellular oxygenic diazotrophs like C. watsonii.
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Taxon-specific response of marine nitrogen fixers to elevated carbon dioxide concentrations
Published 8 July 2013 Science ClosedTags: biological response, growth, laboratory, nitrogen fixation, prokaryotes
Much of the bioavailable nitrogen that supports open ocean food webs and biogeochemical cycles is fixed from the atmosphere by marine cyanobacteria of the genera Trichodesmium and Crocosphaera. In previous experiments carried out with a limited set of cyanobacterial isolates, rates of cyanobacterial nitrogen fixation were shown to increase with carbon dioxide concentrations. Here, we report results from a series of laboratory experiments in which we grew seven strains of Trichodesmium and Crocosphaera from the Atlantic and Pacific oceans under a wide range of carbon dioxide concentrations, and monitored rates of nitrogen fixation and growth. We document large, strain-specific differences in the relationship between nitrogen fixation and carbon dioxide concentration, suggesting that individual strains within each genus are adapted to grow and fix nitrogen at different concentrations of carbon dioxide. We apply kinetic constants from the individual carbon dioxide response curves to an illustrative biogeochemical model of the ocean in 2100, which suggests that strains adapted to high carbon dioxide concentrations could potentially be favoured in a future acidified ocean. We suggest that surface ocean carbon dioxide concentrations could constitute a previously unrecognized selective force that shapes the community composition and diversity of nitrogen-fixing cyanobacteria.
Combined effects of CO2 and light on large and small isolates of the unicellular N2-fixing cyanobacterium Crocosphaera watsonii from the western tropical Atlantic Ocean
Published 19 March 2013 Science ClosedTags: biological response, growth, laboratory, light, multiple factors, nitrogen fixation, photosynthesis, prokaryotes
We examined the combined effects of light and pCO2 on growth, CO2-fixation and N2-fixation rates by strains of the unicellular marine N2-fixing cyanobacterium Crocosphaera watsonii with small (WH0401) and large (WH0402) cells that were isolated from the western tropical Atlantic Ocean. In low-pCO2-acclimated cultures (190 ppm) of WH0401, growth, CO2-fixation and N2-fixation rates were significantly lower than those in cultures acclimated to higher (present-day 
385 ppm, or future 750 ppm) pCO2 treatments. Growth rates were not significantly different, however, in low-pCO2-acclimated cultures of WH0402 in comparison with higher pCO2 treatments. Unlike previous reports for C. watsonii (strain WH8501), N2-fixation rates did not increase further in cultures of WH0401 or WH0402 when acclimated to 750 ppm relative to those maintained at present-day pCO2. Both light and pCO2 had a significant negative effect on gross : net N2-fixation rates in WH0402 and trends were similar in WH0401, implying that retention of fixed N was enhanced under elevated light and pCO2. These data, along with previously reported results, suggest that C. watsonii may have wide-ranging, strain-specific responses to changing light and pCO2, emphasizing the need for examining the effects of global change on a range of isolates within this biogeochemically important genus. In general, however, our data suggest that cellular N retention and CO2-fixation rates of C. watsonii may be positively affected by elevated light and pCO2 within the next 100 years, potentially increasing trophic transfer efficiency of C and N and thereby facilitating uptake of atmospheric carbon by the marine biota.
Response of Nodularia spumigena to pCO2 – Part 2: Exudation and extracellular enzyme activities (update)
Published 29 January 2013 Science ClosedTags: biogeochemistry, biological response, laboratory, multiple factors, nitrogen fixation, nutrients, prokaryotes
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.
