Effects Of Nitrogen Limitation Research Articles

Effect of nitrogen limitation on growth, total lipid accumulation and protein amount in Scenedesmus acutus as biofuel reactor candidate

Effect of nitrogen limitation on growth, total lipid accumulation and protein amount in Scenedesmus acutus as biofuel reactor candidate

Stability of carbohydrate production in continuous microalgal cultivation under nitrogen limitation: effect of irradiation regime and intensity on Tetradesmus obliquus

In this work, the effect of nitrogen limitation on Tetradesmus (Scenedesmus) obliquus cultivation was evaluated in a continuous system operated at steady-state to assess the carbohydrate productivity in a single step. Under continuous irradiance (150 and 650 μmol photons m−2 s−1), with an optimized nitrogen limitation in the reactor inlet stream, a carbohydrate fraction of 45% was achieved, with a remarkable productivity of 0.80 g L−1 day−1. Under simulated day-night irradiation, a cyclic steady state was obtained, as a result of alternating light and dark conditions, with an average carbohydrate production of 0.71 and 0.30 g L−1 day−1 (summer and winter, respectively), with a biomass productivity reduced due to the photoperiod. The uptake of nutrient by the cell appeared to be regulated by the light regime, which determines the carbohydrate accumulation as well. Accordingly, it was possible to obtain a stable carbohydrate production in one-step continuous system with day-night and seasonal irradiation by properly managing the nitrogen inlet.

Proteome response of Dunaliella parva induced by nitrogen limitation

Nitrogen limitation is the most widely used method for directing metabolic flux to lipid biosynthesis in microalgae. Protein level changes induced by environmental stress could be investigated by proteomics analysis, which was a favourable approach for understanding the induction mechanisms. Although, we previously provided the results of transcriptome sequencing of Dunaliella parva under nitrogen limitation stress, the effects of nitrogen limitation on the microalgal proteome were not reported. The impact of nitrogen limitation on the proteome of green alga D. parva was investigated using iTRAQ quantitative proteomics technique. A total of 227 proteins were up-regulated, and 159 proteins were down-regulated under the nitrogen limitation condition. Functional analyses of differentially expressed proteins revealed that nitrogen limitation could induce protein level changes related to photosynthesis, stress response, lipid metabolism, carbohydrate metabolism and nitrogen metabolism. This study, together with our previous transcriptome sequencing of D. parva, therefore provides a foundation for understanding the regulatory mechanism of lipid biosynthesis induced by nitrogen limitation, which could further guide the genetic modification of lipid accumulation in D. parva.

Screening of fatty acid composition in Nitzschia sp.

Abstract Introduction The main target of this study was to compare the effects of nitrogen limitation and light intensities on cell growth, lipid content and fatty acid profile of Nitzschia sp. Methods F/2 medium and N-free F/2 medium were both tested at two different light intensities of 11 and 56 μEm−2s−1 in the internal loop air-lift photobioreactor for Nitzschiasp. Results The presence of nitrogen led to more cells grown efficiently. Furthermore, the increase in chlorophyll content went parallel to the increase in dry weight. The most abundant saturated and monounsaturated fatty acids were pentadecanoic acid (C15:0) and palmitoleic acid (C16:1) which constituted 17%–42% and 15%–48% of total fatty acids for all growth conditions, respectively. It was recorded that palmitoleic acid was present at higher concentrations than palmitic acid. Discussion and conclusion The noteworthy finding was that the lipid content increased with increasing the growth rate of Nitzschia sp. under phototrophic conditions.

Carbohydrate productivity in continuous reactor under nitrogen limitation: Effect of light and residence time on nutrient uptake in Chlorella vulgaris

Abstract Chlorella vulgaris is commonly recognized as an interesting species for bioethanol production due to its carbohydrate content. Carbohydrates accumulation is often obtained under nitrogen starvation, which on the other hand may lead to a reduced biomass production. In this work, the effect of nitrogen limitation in a continuous system was assessed, with the aim of finding an optimal value where biomass productivity and carbohydrate content are well balanced. The effect of light intensity was also investigated, and it was highlighted that in a continuous system light stress is the main variable affecting the carbohydrate content and productivity. It was also evidenced that increasing the residence time is a way to boost nitrogen starvation: the biomass yield on nitrogen noticeably changes with the residence time, thus modifying the elemental composition of the microalgal biomass, and resulting in an accumulation of carbohydrates.

Effects of Nitrogen Limitation on Dunaliella sp.–Alteromonas sp. Interactions: From Mutualistic to Competitive Relationships

Interactions between photosynthetic and non-photosynthetic microorganisms play an essential role in natural aquatic environments and the contribution of bacteria and microalgae to the nitrogen cycle can lead to both competitive and mutualistic relationships. Nitrogen is considered to be, with phosphorus and iron, one of the main limiting nutrients for primary production in the oceans and its availability experiences large temporal and geographical variations. For these reasons, it is important to understand how competitive and mutualistic interactions between photosynthetic and heterotrophic microorganisms are impacted by nitrogen limitation. In a previous study performed in batch cultures, the addition of a selected bacterial strain of Alteromonas sp. resulted in a final biomass increase in the green alga Dunaliella sp. as a result of higher nitrogen incorporation into the algal cells. The present work focuses on testing the potential of the same microalgae–bacteria association and nitrogen interactions in chemostats limited by nitrogen. Axenic and mixed cultures were compared at two dilution rates to evaluate the impact of nitrogen limitation on interactions. The addition of bacteria resulted in increased cell size in the microalgae, as well as decreased carbon incorporation, which was exacerbated by high nitrogen limitation. Biochemical analyses for the different components including microalgae, bacteria, non-living particulate matter, and dissolved organic matter, suggested that bacteria uptake carbon from carbon-rich particulate matter released by microalgae. Dissolved organic nitrogen released by microalgae was apparently not taken up by bacteria, which casts doubt on the remineralization of dissolved organic nitrogen by Alteromonas sp. in chemostats. Dunaliella sp. obtained ammonium-nitrogen more efficiently under lower nitrogen limitation. Overall, we revealed competition between microalgae and bacteria for ammonium when this was in continuous but limited supply. Competition for mineral nitrogen increased with nitrogen limitation. From our study we suggest that competitive or mutualistic relationships between microalgae and bacteria largely depend on the ecophysiological status of the two microorganisms. The outcome of microalgae–bacteria interactions in natural and artificial ecosystems largely depends on environmental factors. Our results indicate the need to improve understanding of the interaction/s between these microbial players.

Nitrogen restrictions buffer modeled interactions of water with the carbon cycle

AbstractTerrestrial carbon and water cycles are coupled at multiple spatiotemporal scales and are crucial to carbon sequestration. Water related climate extremes, such as drought and intense precipitation, can substantially affect the carbon cycle. Meanwhile, nitrogen is a limiting resource to plant and has therefore the potential to alter the coupling of water and carbon cycles on land. Here we assess the effect of nitrogen limitation on the response of the terrestrial carbon cycle to moisture anomalies using Geophysical Fluid Dynamics Laboratory's land surface model LM3V‐N. We analyzed the response of three central carbon fluxes: net primary productivity (NPP), heterotrophic respiration (Rh), and net ecosystem productivity (NEP, the difference between NPP and Rh) and how these fluxes were altered under anomalies of the standardized precipitation and evapotranspiration index (SPEI). We found that globally, the correlations between each of the carbon flux and SPEI depended on the timescale and a strong legacy effect of SPEI anomalies on Rh. Consideration of nitrogen constraints reduced anomalies in carbon fluxes in response to extreme dry/wet events. This nitrogen‐induced buffer constrained the growth of plants under wet extremes and allowed for enhanced growth during droughts. Extra gain of soil moisture from the downregulation of canopy transpiration by nitrogen limitation and shifts in the relative importance of water and nitrogen limitation during dry/wet extreme events are possible mechanisms contributing to the buffering of modeled NPP and NEP. Responses of Rh to moisture anomalies were much weaker compared to NPP, and N buffering effects were less evident.

Increased influence of nitrogen limitation on CO2 emissions from future land use and land use change

AbstractIn the latest projections of future greenhouse gas emissions for the Intergovernmental Panel on Climate Change (IPCC), few Earth System Models included the effect of nitrogen limitation, a key process limiting forest regrowth. Few included forest management (wood harvest). We estimate the impacts of nitrogen limitation on the CO2 emissions from land use and land use change (LULUC), including wood harvest, for the period 1900–2100. We use a land surface model that includes a fully coupled carbon and nitrogen cycle and accounts for forest regrowth processes following agricultural abandonment and wood harvest. Future projections are based on the four Representation Concentration Pathways used in the IPCC Fifth Assessment Report, and we account for uncertainty in future climate for each scenario based on ensembles of climate model outputs. Results show that excluding nitrogen limitation will underestimate global LULUC emissions by 34–52 PgC (20–30%) during the 20th century (range across three different historical LULUC reconstructions) and by 128–187 PgC (90–150%) during the 21st century (range across the four IPCC scenarios). The full range for estimated LULUC emissions during the 21st century including climate model uncertainty is 91 to 227 PgC (with nitrogen limitation included). The underestimation increases with time because (1) projected annual wood harvest rates from forests summed over the 21st century are 380–1080% higher compared to those of the 20th century, resulting in more regrowing secondary forests; (2) nitrogen limitation reduces the CO2 fertilization effect on net primary production of regrowing secondary forests following wood harvest and agricultural abandonment; and (3) nitrogen limitation effect is aggravated by the gradual loss of soil nitrogen from LULUC disturbance. Our study implies that (1) nitrogen limitation of CO2 uptake is substantial and sensitive to nitrogen inputs; (2) if LULUC emissions are larger than previously estimated in studies without nitrogen limitation, then meeting the same climate mitigation target would require an equivalent additional reduction of fossil fuel emissions; (3) the effectiveness of land‐based mitigation strategies will critically depend on the interactions between nutrient limitations and secondary forests resulting from LULUC; and (4) it is important for terrestrial biosphere models to consider nitrogen constraint in estimates of the strength of future land carbon uptake.

Effect of nitrogen limitation on cell growth, lipid accumulation and gene expression in Chlorella sorokiniana

The influence of nitrogen (N) limitation on the cell growth, chlorophyll content, intracellular lipid production and expression levels of two pathway genes in Chlorella sorokiniana was investigated in mixotrophic culture in this study. The maximum biomass concentration of C. sorokinianacultured in modified BG11 medium containing 3, 6, 9, 12 and 15 mM N were 1.60, 2.21, 2.74, 3.18 and 3.21 g/L of dry cell weight, respectively with maximum specific growth rate of 0.180, 0.198, 0.201, 0.203, 0.206 day−1 during culture with an initial N feed of 3, 6, 9, 12 and 15 mM in the first eight days, respectively. The maximum lipid content was 51 % with 3 mM N. However, the maximum lipid productivity of 0.00884 g L−1 day−1 was achieved with 6 mM N. Expression levels of accD (heteromeric acetyl-CoA carboxylase beta subunit) and accI (homomeric acetyl-CoA carboxylase) genes in C. sorokiniana were studied by real-time PCR. Increased expression levels of accD reflected the increased lipid content in stationary phase. In contrast, expression of the acc1 gene always remained low, showing that the gene might not be critical for lipid accumulation.

The effect of nitrogen limitation on the physiology and metabolism of chlorella vulgaris var L3

In this study, the time-course of growth, chemical composition and gene expression were analyzed over four days of nitrogen starvation in Chlorella vulgaris var L3 cultures. This strain exhibited a two-stage response to nitrogen depletion. Early starvation led to a slight biomass increase, a marked reduction of nitrogen rich-molecules and a 4-fold increase in carbohydrate content. During this stage, genes involved in carbohydrate metabolism (AGPase and SP) and fatty acid synthesis (ACL, G6PD, ME and accC) were little affected by nitrogen unavailability but the genes linked to carbon and nitrogen fixation (rbcS, NR, PEPCase) and triglyceride synthesis (DGAT 1) were significantly up-regulated. A more extended N-starvation period resulted in increased lipid content, whereas biomass and carbohydrates remained constant. In this late starvation stage, most of the analyzed genes were strongly down-regulated but the transcript level of the gene rbcS remained unaffected and ME gene was up-regulated. The results suggest that photosynthetically fixed carbon is driven toward lipid synthesis when carbohydrate synthesis is stopped. ME enzyme seems to be the main source of NADPH for reserve lipid synthesis and it might also provide precursors for acetyl-CoA synthesis.

Lipid accumulation in response to nitrogen limitation and variation of temperature in Nannochloropsis salina.

BackgroundThis batch study deals with the relation between lipid as well as triglyceride contents in Nannochloropsis salina and variation in culture conditions such as nitrogen concentration and temperature.ResultsThe tested parameters caused reduction in growth expressed as cell count, optical density and dry weight, as well strongly involved in lipids and triglycerides accumulation and significantly affected the lipid productivity. At the beginning of the work, the concentration of nitrogen in the medium was reduced to three quarter, half and quarter of the original f2 medium while the temperature kept constant. After that, the optimal nitrogen concentration (quarter of the original media) giving high lipid yield was tested with different temperature degrees from 15 to 35°C with five degree intervals. Although the growth was insignificantly influenced, a considerable increase in lipid and triglyceride (56.1 and 15.1% of dry weight respectively) was observed when the concentration of nitrogen in the medium was reduced to the quarter. Moreover, 59.3% lipid and 17.1% triglyceride on the basis of dry weight were obtained by the combination of 25% nitrogen concentration and 30°C. Simple regressions recommended that the interaction effect of nitrogen limitation and temperature on lipid and triglyceride accumulation was not as fundamental as for nitrogen limitation stress.ConclusionThe degree of nitrogen availability in the combination of temperature effect has been identified as the critical determinant for the maximal production of lipid in N. salina. Nevertheless, major advances in this field can be considered by studying more stresses techniques and genetic strategies.Electronic supplementary materialThe online version of this article (doi:10.1186/s40529-015-0085-7) contains supplementary material, which is available to authorized users.

Development and in silico analysis of a new nitrogen-limited feeding strategy for fed-batch cultures of Pichia pastoris based on a simple pH-control system

A novel fed-batch strategy based on a pH control system was devised in order to address some of the disadvantages associated with available control strategies for Pichia pastoris cultures. Changing settings of the controller, the tON/tOFF time ratio, adjusted the speed in which the ammonium hydroxide was added for pH control and the sole nitrogen source. Setting the tON/tOFF ratio to 0.5/0.5, 0.5/10 and 0.5/15 resulted in a decrease in the growth rate as the tOFF value increased. Therefore, the design allows running the culture at lower growth rates in a controlled way. Two high cell density cultures with tON/tOFF ratio of 0.5/10 and 0.5/20 were also fed ammonia with the same controller. Setting tON/tOFF at 0.5/10 and 0.5/20 led to an increase in final biomass and protein as the tOFF decreased. A metabolic model assessed the effect of nitrogen-limitation in high cell density cultures on the yeast metabolism. Model results show that fluxes of intracellular reactions such as glycolysis, the tricarboxylic cycle and the pentose-phosphate pathway are higher at tON/tOFF=0.5/10. Additionally, higher sensitivity of growth and protein production to oxygen at tON/tOFF=0.5/10 proves that oxygen limitation led to higher protein production in this case.

Culturing Neochloris oleoabundans microalga in a nitrogen-limited, heterotrophic fed-batch system to enhance lipid and carbohydrate accumulation

The effects of nitrogen limitation on the specific growth rate (μ) and the accumulation of reserve metabolites in Neochloris oleoabundans during heterotrophic fed-batch cultivation were studied in a system consisting of three stages with an excess of glucose in a 40-L bioreactor. During the first stage, which was a batch cultivation with no carbon or nitrogen limitation, the cells grew at a μ of 0.045h−1, and the major cellular component was protein (44%, w/w) on a dry cell weight (DCW) basis. In the second, which was a fed-batch stage, two different exponential nitrate-feeding profiles were employed at μ values of 0.042 and 0.035h−1. The results show that a nitrogen limitation that causes a small reduction in the μ (only 20%) has a marked effect on the biochemical composition of the microalgal cells, promoting greater accumulation of lipids and carbohydrates. During the third stage, which was a post fed-batch stage, the cells were maintained under nitrogen-starvation and excess glucose conditions. When the glucose was depleted, the cells from the fed-batch culture at 0.042h−1 contained 53.8% lipids, reaching a cell mass of 20.9gDCW L−1, and the biomass, lipid and carbohydrate productivities were 1900, 1020 and 500mgL−1day−1, respectively. Thus far, these are the highest productivity values reported for N. oleoabundans cultures under phototrophic, mixotrophic or heterotrophic growth.

An assessment of the usefulness of the cyanobacterium Synechococcus subsalsus as a source of biomass for biofuel production

Nowadays algal biofuels are considered one of the most promising solutions of global energy crisis and climate change for the years to come. By manipulation of the culture conditions, many algal species can be induced to accumulate high concentrations of particular biomolecules and can be directed to the desired output for each fuel. In this context, the present study involved the assessment of the effects of CO 2 availability and nitrogen starvation on growth and chemical composition of the cyanobacterium Synechococcus subsalsus , testing a fast-growing native strain. The control experiments were performed with Conway culture medium in 12-day batch cultures, in 6-liter flasks and 12 h photoperiod, with addition of 2 L min -1 filtered air to each flask. Other two experimental conditions were also tested: (i) the placement into the cultures of additional dissolved nutrients except nitrogen, one week after the start of growth (N-), and (ii) the input of pure CO 2 into the flasks from the 5th day of growth (C+). In all cultures, daily cell counts were done throughout the cultivation, as well as measurements of pH and cell biovolumes. Maximum cell yield were found in N-experiments, while cell yields of C+ and control were similar. Dissolved nitrogen was exhausted before the end of the experiments, but dissolved phosphorus was not totally consumed. Protein and chlorophyll-a concentrations decreased from the exponential to the stationary growth phase of all experiments, except for protein in the control. In all experiments, carbohydrate, lipid and total carotenoid increased from the exponential to the stationary growth phase, as an effect of nitrogen limitation. Increments in carbohydrate concentrations were remarkable, achieving more than 42% of the dry weight (dw), but concentrations of lipid were always lower than 13% dw. The addition of pure CO 2 did not cause a significant increase in biomass of S. subsalsus nor generated more lipid and carbohydrate than the other treatments. Nitrogen starvation caused an intense accumulation of carbohydrate, but the increments of lipid were small. Despite the fast growth, the cyanobacterium S. subsalsus has a virtually null potential for biodiesel production, given its low lipid concentrations. The high concentrations of carbohydrate combined with fast growth point to the potential use of this species as raw material for other possible biotechnological processes, after a demonstration of technical and economic viability.

The effect of nitrogen limitation on acetyl-CoA carboxylase expression and fatty acid content in Chromera velia and Isochrysis aff. galbana (TISO).

Lipids from microalgae have become a valuable product with applications ranging from biofuels to human nutrition. While changes in fatty acid (FA) content and composition under nitrogen limitation are well documented, the involved molecular mechanisms are poorly understood. Acetyl-CoA carboxylase (ACCase) is a key enzyme in the FA synthesis and elongation pathway. Plastidial and cytosolic ACCases provide malonyl-CoA for de novo FA synthesis in the plastid and FA elongation in the endoplasmic reticulum, respectively. The present study aimed at investigating the expression of plastidial and cytosolic ACCase in Chromera velia and Isochrysis aff. galbana (TISO) and their impact on FA content and elongation level when grown under nitrogen-deplete conditions. In C. velia, plastidial ACCase was significantly upregulated during nitrogen starvation and with culture age, strongly correlating with increased FA content. Conversely, plastidial ACCase of I. aff. galbana was not differentially expressed in nitrogen-deplete cultures, but upregulated during the logarithmic phase of nitrogen-replete cultures. In contrast to plastidial ACCase, the cytosolic ACCase of C. velia was downregulated with culture age and nitrogen-starvation, strongly correlating with an increase in medium-chain FAs. In conclusion, the expression of plastidial and cytosolic ACCase changed with growth phase and nutrient status in a species-specific manner and nitrogen limitation did not always result in FA accumulation.

Nitrogen stress triggered biochemical and morphological changes in the microalgae Scenedesmus sp. CCNM 1077

The aim of present study was to investigate the effects of nitrogen limitation as well as sequential nitrogen starvation on morphological and biochemical changes in Scenedesmus sp. CCNM 1077. The results revealed that the nitrogen limitation and sequential nitrogen starvation conditions significantly decreases the photosynthetic activity as well as crude protein content in the organism, while dry cell weight and biomass productivity are largely unaffected up to nitrate concentration of about 30.87mg/L and 3 days nitrate limitation condition. Nitrate stress was found to have a significant effect on cell morphology of Scenedesmus sp. CCNM 1077. Total removal of nitrate from the growth medium resulted in highest lipid (27.93%) and carbohydrate content (45.74%), making it a potential feed stock for biodiesel and bio-ethanol production. This is a unique approach to understand morphological and biochemical changes in freshwater microalgae under nitrate limitation as well as sequential nitrate removal conditions.

The effect of nitrogen limitation on lipid productivity and cell composition in Chlorella vulgaris

Chlorella vulgaris accumulates lipid under nitrogen limitation, but at the expense of biomass productivity. Due to this tradeoff, improved lipid productivity may be compromised, despite higher lipid content. To determine the optimal degree of nitrogen limitation for lipid productivity, batch cultures of C. vulgaris were grown at different nitrate concentrations. The growth rate, lipid content, lipid productivity and biochemical and elemental composition of the cultures were monitored for 20days. A starting nitrate concentration of 170mgL(-1) provided the optimal tradeoff between biomass and lipid production under the experimental conditions. Volumetric lipid yield (in milligram lipid per liter algal culture) was more than double that under nitrogen-replete conditions. Interpolation of the data indicated that the highest volumetric lipid concentration and lipid productivity would occur at nitrate concentrations of 305 and 241mgL(-1), respectively. There was a strong correlation between the nitrogen content of the cells and the pigment, protein and lipid content, as well as biomass and lipid productivity. Knowledge of the relationships between cell nitrogen content, growth, and cell composition assists in the prediction of the nitrogen regime required for optimal productivity in batch or continuous culture. In addition to enhancing lipid productivity, nitrogen limitation improves the lipid profile for biodiesel production and reduces the requirement for nitrogen fertilizers, resulting in cost and energy savings and a reduction in the environmental burden of the process.

Effects of nitrogen limitation on hydrological processes in CLM4‐CN

The role of nitrogen limitation on photosynthesis downregulation and stomatal conductance has a significant influence on evapotranspiration and runoff. In the current Community Land Model with coupled Carbon and Nitrogen cycles (CLM4‐CN), however, the carbon and water coupling in stomata is not linked to nitrogen limitation. We modify the incomplete linkages between carbon, nitrogen, and water, and examine how nitrogen limitation affects hydrological processes in CLM4‐CN. We then evaluate if the modification can improve the simulation of carbon and water fluxes. Applying the effects of nitrogen limitation on stomatal conductance significantly decreases leaf photosynthesis. It leads to a reduction in canopy transpiration, thereby increasing total runoff, mainly due to increasing subsurface runoff. More available soil water for vegetation from the reduced transpiration helps increase gross primary productivity (GPP) in the relatively moisture‐limited regions of grassland/steppe and savanna. However, in the tropics and boreal forest regions, changes in soil water by nitrogen limitation are insignificant, and GPP decreases directly by down‐regulated leaf photosynthesis. Decreasing canopy transpiration and increasing runoff from nitrogen limitation improve simulating latent heat flux and runoff by reducing high biases for latent heat flux in the tropics and low biases for runoff in the tropics and northern high latitudes. In addition, the CLM4‐CN with leaf‐level nitrogen limitation reduces model biases in tropical GPP. Nitrogen limitation on the leaf‐level significantly affects hydrological processes in CLM4‐CN and improves the simulation of carbon and water fluxes. This process should be included with other recent improvements to reduce model biases as much as possible.

Nitrogen Limitation in Neochloris oleoabundans: A Reassessment of Its Effect on Cell Growth and Biochemical Composition

The aim of this work was to reassess the effect of nitrogen limitation (from 0 to 1 mM nitrate), on the growth and the biochemical composition of Neochloris oleoabundans cultures, where only the CO2 available in the air was provided. Slight differences in the initial nitrate concentration, even minimal increments of 0.2 mM, significantly modify the microalgal response towards nitrogen limitation. This stress condition reduced cell proliferation, but increased cell mass values due to the simultaneous accumulation of two storage compounds: lipids, which contained up to a 55.9 % of total fatty acids; and carbohydrates, which may be primarily composed by starch. The highest biomass and lipid productivities of 98.24 and 43.24 mg/l/day, respectively, were attained at an initial nitrate concentration of 0.6 mM. The theoretical annual projection, based on these productivities, allowed the estimation of the liquid fuel energy yields, which are comparable or even higher than those calculated for several biomass feedstocks such as corn, oil palm, sugarcane, or even fast growing grasses, confirming the potential of nitrogen-limited N. oleoabundans biomass as an appropriate feedstock for biofuel purposes.

Impacts of nitrogen limitation on the sinking rate of the coccolithophorid Emiliania huxleyi (Prymnesiophyceae)

Pantorno A., Holland D.P., Stojkovic S. and Beardall B. 2013. Impacts of nitrogen limitation on the sinking rate of the coccolithophorid Emiliania huxleyi (Prymnesiophyceae). Phycologia 52: 288–294. DOI: 10.2216/12–064.1The biological carbon pump in the ocean plays an important role in controlling atmospheric CO2 levels. Approximately 1% of the yearly 50–60 Pg C of marine primary production settles in the deep ocean, where it is effectively sequestered for centuries to millennia. Central to the strength of the pump is the sinking velocity of phytoplankton cells and other organic debris. Stokes' Law indicates that the sinking velocity of a spherical cell will depend on its size and density, where larger, heavier cells will sink at a faster rate. Given that growth conditions can result in changes in cell size and macromolecular composition of phytoplankton, it might be expected that such changes could cause alterations in sinking velocity and carbon drawdown via the biological carbon pump. In the future, phytoplankton cells in the open ocean are predicted to be more subject to nutrient limitation. In this study we examined the effects of nitrogen limitation on the sinking velocity of Emiliania huxleyi, a coccolithophore responsible for significant phytoplankton blooms and biological drawdown of carbon. Nitrogen limitation caused changes in macromolecular composition, especially lipid content and coccosphere thickness. However, the overall density of the cells remained similar, and, as a consequence, cell size was the major determinant of sinking rate with N-limited cells in exponential phase sinking more slowly than N-replete cells. Cells in stationary phase showed the reverse trend with N-limited cells sinking faster, although not as fast as N-replete cells in exponential phase. N-limited cells produced more transparent exopolymers, suggesting an increased capacity for marine snow formation. These observations have implications for the efficiency of the biological carbon pump in areas that have increased nutrient stress caused by global climate change.