Heatwave responses of Arctic phytoplankton communities are driven by combined impacts of warming and cooling.

Marine heatwaves are increasing in frequency and intensity as climate change progresses, especially in the highly productive Arctic regions. Although their effects on primary producers will largely determine the impacts on ecosystem services, mechanistic understanding on phytoplankton responses to these extreme events is still very limited. We experimentally exposed Arctic phytoplankton assemblages to stable warming, as well as to repeated heatwaves, and measured temporally resolved productivity, physiology, and composition. Our results show that even extreme stable warming increases productivity, while the response to heatwaves depends on the specific scenario applied and is not predictable from stable warming responses. This appears to be largely due to the underestimated impact of the cool phase following a heatwave, which can be at least as important as the warm phase for the overall response. We show that physiological and compositional adjustments to both warm and cool phases drive overall phytoplankton productivity and need to be considered mechanistically to predict overall ecosystem impacts.

Interaction matters: Bottom-up driver interdependencies alter the projected response of phytoplankton communities to climate change.

Phytoplankton growth is controlled by multiple environmental drivers, which are all modified by climate change. While numerous experimental studies identify interactive effects between drivers, large-scale ocean biogeochemistry models mostly account for growth responses to each driver separately and leave the results of these experimental multiple-driver studies largely unused. Here, we amend phytoplankton growth functions in a biogeochemical model by dual-driver interactions (CO2 and temperature, CO2 and light), based on data of a published meta-analysis on multiple-driver laboratory experiments. The effect of this parametrization on phytoplankton biomass and community composition is tested using present-day and future high-emission (SSP5-8.5) climate forcing. While the projected decrease in future total global phytoplankton biomass in simulations with driver interactions is similar to that in control simulations without driver interactions (5%-6%), interactive driver effects are group-specific. Globally, diatom biomass decreases more with interactive effects compared with the control simulation (-8.1% with interactions vs. no change without interactions). Small-phytoplankton biomass, by contrast, decreases less with on-going climate change when the model accounts for driver interactions (-5.0% vs. -9.0%). The response of global coccolithophore biomass to future climate conditions is even reversed when interactions are considered (+33.2% instead of -10.8%). Regionally, the largest difference in the future phytoplankton community composition between the simulations with and without driver interactions is detected in the Southern Ocean, where diatom biomass decreases (-7.5%) instead of increases (+14.5%), raising the share of small phytoplankton and coccolithophores of total phytoplankton biomass. Hence, interactive effects impact the phytoplankton community structure and related biogeochemical fluxes in a future ocean. Our approach is a first step to integrate the mechanistic understanding of interacting driver effects on phytoplankton growth gained by numerous laboratory experiments into a global ocean biogeochemistry model, aiming toward more realistic future projections of phytoplankton biomass and community composition.

Abrupt and acclimation responses to changing temperature elicit divergent physiological effects in the diatom Phaeodactylum tricornutum.

Growth rates and other biomass traits of phytoplankton are strongly affected by temperature. We hypothesized that resulting phenotypes originate from deviating temperature sensitivities of underlying physiological processes. We used membrane-inlet mass spectrometry to assess photosynthetic and respiratory O2 and CO2 fluxes in response to abrupt temperature changes as well as after acclimation periods in the diatom Phaeodactylum tricornutum. Abrupt temperature changes caused immediate over- or undershoots in most physiological processes, that is, photosynthetic oxygen release ( PS O 2 ), photosynthetic carbon uptake ( PS CO 2 ), and respiratory oxygen release ( R O 2 ). Over acclimation timescales, cells were, however, able to re-adjust their physiology and revert to phenotypic 'sweet spots'. Respiratory CO2 release ( R CO 2 ) was generally inhibited under high temperature and stimulated under low-temperature settings, on abrupt as well as acclimation timescales. Such behavior may help mitochondria to stabilize plastidial ATP : NADPH ratios and thus maximize photosynthetic carbon assimilation.

Revealing environmentally driven population dynamics of an Arctic diatom using a novel microsatellite PoolSeq barcoding approach.

Ecological stability under environmental change is determined by both interspecific and intraspecific processes. Particularly for planktonic microorganisms, it is challenging to follow intraspecific dynamics over space and time. We propose a new method, microsatellite PoolSeq barcoding (MPB), for tracing allele frequency changes in protist populations. We successfully applied this method to experimental community incubations and field samples of the diatom Thalassiosira hyalina from the Arctic, a rapidly changing ecosystem. Validation of the method found compelling accuracy in comparison with established genotyping approaches within different diversity contexts. In experimental and environmental samples, we show that MPB can detect meaningful patterns of population dynamics, resolving allelic stability and shifts within a key diatom species in response to experimental treatments as well as different bloom phases and years. Through our novel MPB approach, we produced a large dataset of populations at different time-points and locations with comparably little effort. Results like this can add insights into the roles of selection and plasticity in natural protist populations under stable experimental but also variable field conditions. Especially for organisms where genotype sampling remains challenging, MPB holds great potential to efficiently resolve eco-evolutionary dynamics and to assess the mechanisms and limits of resilience to environmental stressors.

Meta-analysis of multiple driver effects on marine phytoplankton highlights modulating role of pCO2.

Responses of marine primary production to a changing climate are determined by a concert of multiple environmental changes, for example in temperature, light, pCO2 , nutrients, and grazing. To make robust projections of future global marine primary production, it is crucial to understand multiple driver effects on phytoplankton. This meta-analysis quantifies individual and interactive effects of dual driver combinations on marine phytoplankton growth rates. Almost 50% of the single-species laboratory studies were excluded because central data and metadata (growth rates, carbonate system, experimental treatments) were insufficiently reported. The remaining data (42 studies) allowed for the analysis of interactions of pCO2 with temperature, light, and nutrients, respectively. Growth rates mostly respond non-additively, whereby the interaction with increased pCO2 profusely dampens growth-enhancing effects of high temperature and high light. Multiple and single driver effects on coccolithophores differ from other phytoplankton groups, especially in their high sensitivity to increasing pCO2 . Polar species decrease their growth rate in response to high pCO2 , while temperate and tropical species benefit under these conditions. Based on the observed interactions and projected changes, we anticipate primary productivity to: (a) first increase but eventually decrease in the Arctic Ocean once nutrient limitation outweighs the benefits of higher light availability; (b) decrease in the tropics and mid-latitudes due to intensifying nutrient limitation, possibly amplified by elevated pCO2 ; and (c) increase in the Southern Ocean in view of higher nutrient availability and synergistic interaction with increasing pCO2 . Growth-enhancing effect of high light and warming to coccolithophores, mainly Emiliania huxleyi, might increase their relative abundance as long as not offset by acidification. Dinoflagellates are expected to increase their relative abundance due to their positive growth response to increasing pCO2 and light levels. Our analysis reveals gaps in the knowledge on multiple driver responses and provides recommendations for future work on phytoplankton.

Ocean acidification increases domoic acid contents during a spring to summer succession of coastal phytoplankton.

Enrichment of the oceans with CO2 may be beneficial for some marine phytoplankton, including harmful algae. Numerous laboratory experiments provided valuable insights into the effects of elevated pCO2 on the growth and physiology of harmful algal species, including the production of phycotoxins. Experiments close to natural conditions are the next step to improve predictions, as they consider the complex interplay between biotic and abiotic factors that can confound the direct effects of ocean acidification. We therefore investigated the effect of ocean acidification on the occurrence and abundance of phycotoxins in bulk plankton samples during a long-term mesocosm experiment in the Gullmar Fjord, Sweden, an area frequently experiencing harmful algal blooms. During the experimental period, a total of seven phycotoxin-producing harmful algal genera were identified in the fjord, and in accordance, six toxin classes were detected. However, within the mesocosms, only domoic acid and the corresponding producer Pseudo-nitzschia spp. was observed. Despite high variation within treatments, significantly higher particulate domoic acid contents were measured in the mesocosms with elevated pCO2. Higher particulate domoic acid contents were additionally associated with macronutrient limitation. The risks associated with potentially higher phycotoxin levels in the future ocean warrants attention and should be considered in prospective monitoring strategies for coastal marine waters.

Phenotypic plasticity of carbon fixation stimulates cyanobacterial blooms at elevated CO2.

Although phenotypic plasticity is a widespread phenomenon, its implications for species responses to climate change are not well understood. For example, toxic cyanobacteria can form dense surface blooms threatening water quality in many eutrophic lakes, yet a theoretical framework to predict how phenotypic plasticity affects bloom development at elevated pCO2 is still lacking. We measured phenotypic plasticity of the carbon fixation rates of the common bloom-forming cyanobacterium Microcystis. Our results revealed a 1.8- to 5-fold increase in the maximum CO2 uptake rate of Microcystis at elevated pCO2, which exceeds CO2 responses reported for other phytoplankton species. The observed plasticity was incorporated into a mathematical model to predict dynamic changes in cyanobacterial abundance. The model was successfully validated by laboratory experiments and predicts that acclimation to high pCO2 will intensify Microcystis blooms in eutrophic lakes. These results indicate that this harmful cyanobacterium is likely to benefit strongly from rising atmospheric pCO2.

Higher sensitivity towards light stress and ocean acidification in an Arctic sea-ice-associated diatom compared to a pelagic diatom.

Thalassiosira hyalina and Nitzschia frigida are important members of Arctic pelagic and sympagic (sea-ice-associated) diatom communities. We investigated the effects of light stress (shift from 20 to 380 µmol photons m-2  s-1 , resembling upwelling or ice break-up) under contemporary and future pCO2 (400 vs 1000 µatm). The responses in growth, elemental composition, pigmentation and photophysiology were followed over 120 h and are discussed together with underlying gene expression patterns. Stress response and subsequent re-acclimation were efficiently facilitated by T. hyalina, which showed only moderate changes in photophysiology and elemental composition, and thrived under high light after 120 h. In N. frigida, photochemical damage and oxidative stress appeared to outweigh cellular defenses, causing dysfunctional photophysiology and reduced growth. pCO2 alone did not specifically influence gene expression, but amplified the transcriptomic reactions to light stress, indicating that pCO2 affects metabolic equilibria rather than sensitive genes. Large differences in acclimation capacities towards high light and high pCO2 between T. hyalina and N. frigida indicate species-specific mechanisms in coping with the two stressors, which may reflect their respective ecological niches. This could potentially alter the balance between sympagic and pelagic primary production in a future Arctic.

Ocean acidification has little effect on the biochemical composition of the coccolithophore Emiliania huxleyi.

Owing to the hierarchical organization of biology, from genomes over transcriptomes and proteomes down to metabolomes, there is continuous debate about the extent to which data and interpretations derived from one level, e.g. the transcriptome, are in agreement with other levels, e.g. the metabolome. Here, we tested the effect of ocean acidification (OA; 400 vs. 1000 µatm CO2) and its modulation by light intensity (50 vs. 300 µmol photons m-2 s-1) on the biomass composition (represented by 75 key metabolites) of diploid and haploid life-cycle stages of the coccolithophore Emiliania huxleyi (RCC1216 and RCC1217) and compared these data with interpretations from previous physiological and gene expression screenings. The metabolite patterns showed minor responses to OA in both life-cycle stages. Whereas previous gene expression analyses suggested that the observed increased biomass buildup derived from lipid and carbohydrate storage, this dataset suggests that OA slightly increases overall biomass of cells, but does not significantly alter their metabolite composition. Generally, light was shown to be a more dominant driver of metabolite composition than OA, increasing the relative abundances of amino acids, mannitol and storage lipids, and shifting pigment contents to accommodate increased irradiance levels. The diploid stage was shown to contain vastly more osmolytes and mannitol than the haploid stage, which in turn had a higher relative content of amino acids, especially aromatic ones. Besides the differences between the investigated cell types and the general effects on biomass buildup, our analyses indicate that OA imposes only negligible effects on E. huxleyi´s biomass composition.

Company matters: The presence of other genotypes alters traits and intraspecific selection in an Arctic diatom under climate change.

Arctic phytoplankton and their response to future conditions shape one of the most rapidly changing ecosystems on the planet. We tested how much the phenotypic responses of strains from the same Arctic diatom population diverge and whether the physiology and intraspecific composition of multistrain populations differs from expectations based on single strain traits. To this end, we conducted incubation experiments with the diatom Thalassiosira hyalina under present-day and future temperature and pCO2 treatments. Six fresh isolates from the same Svalbard population were incubated as mono- and multistrain cultures. For the first time, we were able to closely follow intraspecific selection within an artificial population using microsatellites and allele-specific quantitative PCR. Our results showed not only that there is substantial variation in how strains of the same species cope with the tested environments but also that changes in genotype composition, production rates, and cellular quotas in the multistrain cultures are not predictable from monoculture performance. Nevertheless, the physiological responses as well as strain composition of the artificial populations were highly reproducible within each environment. Interestingly, we only detected significant strain sorting in those populations exposed to the future treatment. This study illustrates that the genetic composition of populations can change on very short timescales through selection from the intraspecific standing stock, indicating the potential for rapid population level adaptation to climate change. We further show that individuals adjust their phenotype not only in response to their physicochemical but also to their biological surroundings. Such intraspecific interactions need to be understood in order to realistically predict ecosystem responses to global change.

N2 fixation in free-floating filaments of Trichodesmium is higher than in transiently suboxic colony microenvironments.

To understand the role of micrometer-scale oxygen (O2 ) gradients in facilitating dinitrogen (N2 ) fixation, we characterized O2 dynamics in the microenvironment around free-floating trichomes and colonies of Trichodesmium erythraeum IMS101. Diurnal and spatial variability in O2 concentrations in the bulk medium, within colonies, along trichomes and within single cells were determined using O2 optodes, microsensors and model calculations. Carbon (C) and N2 fixation as well as O2 evolution and uptake under different O2 concentrations were analyzed by stable isotope incubations and membrane inlet mass spectrometry. We observed a pronounced diel rhythm in O2 fluxes, with net O2 evolution restricted to short periods in the morning and evening, and net O2 uptake driven by dark respiration and light-dependent O2 uptake during the major part of the light period. Remarkably, colonies showed lower N2 fixation and C fixation rates than free-floating trichomes despite the long period of O2 undersaturation in the colony microenvironment. Model calculations demonstrate that low permeability of the cell wall in combination with metabolic heterogeneity between single cells allows for anoxic intracellular conditions in colonies but also free-floating trichomes of Trichodesmium. Therefore, whereas colony formation must have benefits for Trichodesmium, it does not favor N2 fixation.

Effects of ocean acidification on primary production in a coastal North Sea phytoplankton community.

We studied the effect of ocean acidification (OA) on a coastal North Sea plankton community in a long-term mesocosm CO2-enrichment experiment (BIOACID II long-term mesocosm study). From March to July 2013, 10 mesocosms of 19 m length with a volume of 47.5 to 55.9 m3 were deployed in the Gullmar Fjord, Sweden. CO2 concentrations were enriched in five mesocosms to reach average CO2 partial pressures (pCO2) of 760 µatm. The remaining five mesocosms were used as control at ambient pCO2 of 380 µatm. Our paper is part of a PLOS collection on this long-term mesocosm experiment. Here, we here tested the effect of OA on total primary production (PPT) by performing 14C-based bottle incubations for 24 h. Furthermore, photoacclimation was assessed by conducting 14C-based photosynthesis-irradiance response (P/I) curves. Changes in chlorophyll a concentrations over time were reflected in the development of PPT, and showed higher phytoplankton biomass build-up under OA. We observed two subsequent phytoplankton blooms in all mesocosms, with peaks in PPT around day 33 and day 56. OA had no significant effect on PPT, except for a marginal increase during the second phytoplankton bloom when inorganic nutrients were already depleted. Maximum light use efficiencies and light saturation indices calculated from the P/I curves changed simultaneously in all mesocosms, and suggest that OA did not alter phytoplankton photoacclimation. Despite large variability in time-integrated productivity estimates among replicates, our overall results indicate that coastal phytoplankton communities can be affected by OA at certain times of the seasonal succession with potential consequences for ecosystem functioning.

Combined Effects of Ocean Acidification and Light or Nitrogen Availabilities on 13C Fractionation in Marine Dinoflagellates.

Along with increasing oceanic CO2 concentrations, enhanced stratification constrains phytoplankton to shallower upper mixed layers with altered light regimes and nutrient concentrations. Here, we investigate the effects of elevated pCO2 in combination with light or nitrogen-limitation on 13C fractionation (εp) in four dinoflagellate species. We cultured Gonyaulax spinifera and Protoceratium reticulatum in dilute batches under low-light ('LL') and high-light ('HL') conditions, and grew Alexandrium fundyense and Scrippsiella trochoidea in nitrogen-limited continuous cultures ('LN') and nitrogen-replete batches ('HN'). The observed CO2-dependency of εp remained unaffected by the availability of light for both G. spinifera and P. reticulatum, though at HL εp was consistently lower by about 2.7‰ over the tested CO2 range for P. reticulatum. This may reflect increased uptake of (13C-enriched) bicarbonate fueled by increased ATP production under HL conditions. The observed CO2-dependency of εp disappeared under LN conditions in both A. fundyense and S. trochoidea. The generally higher εp under LN may be associated with lower organic carbon production rates and/or higher ATP:NADPH ratios. CO2-dependent εp under non-limiting conditions has been observed in several dinoflagellate species, showing potential for a new CO2-proxy. Our results however demonstrate that light- and nitrogen-limitation also affect εp, thereby illustrating the need to carefully consider prevailing environmental conditions.

Acidification, not carbonation, is the major regulator of carbon fluxes in the coccolithophore Emiliania huxleyi.

A combined increase in seawater [CO2 ] and [H(+) ] was recently shown to induce a shift from photosynthetic HCO3 (-) to CO2 uptake in Emiliania huxleyi. This shift occurred within minutes, whereas acclimation to ocean acidification (OA) did not affect the carbon source. To identify the driver of this shift, we exposed low- and high-light acclimated E. huxleyi to a matrix of two levels of dissolved inorganic carbon (1400, 2800 µmol kg(-1) ) and pH (8.15, 7.85) and directly measured cellular O2 , CO2 and HCO3 (-) fluxes under these conditions. Exposure to increased [CO2 ] had little effect on the photosynthetic fluxes, whereas increased [H(+) ] led to a significant decline in HCO3 (-) uptake. Low-light acclimated cells overcompensated for the inhibition of HCO3 (-) uptake by increasing CO2 uptake. High-light acclimated cells, relying on higher proportions of HCO3 (-) uptake, could not increase CO2 uptake and photosynthetic O2 evolution consequently became carbon-limited. These regulations indicate that OA responses in photosynthesis are caused by [H(+) ] rather than by [CO2 ]. The impaired HCO3 (-) uptake also provides a mechanistic explanation for lowered calcification under OA. Moreover, it explains the OA-dependent decrease in photosynthesis observed in high-light grown phytoplankton.

CO2-dependent carbon isotope fractionation in dinoflagellates relates to their inorganic carbon fluxes.

Carbon isotope fractionation (εp) between the inorganic carbon source and organic matter has been proposed to be a function of pCO2. To understand the CO2-dependency of εp and species-specific differences therein, inorganic carbon fluxes in the four dinoflagellate species Alexandrium fundyense, Scrippsiella trochoidea, Gonyaulax spinifera and Protoceratium reticulatum have been measured by means of membrane-inlet mass spectrometry. In-vivo assays were carried out at different CO2 concentrations, representing a range of pCO2 from 180 to 1200 µatm. The relative bicarbonate contribution (i.e. the ratio of bicarbonate uptake to total inorganic carbon uptake) and leakage (i.e. the ratio of CO2 efflux to total inorganic carbon uptake) varied from 0.2 to 0.5 and 0.4 to 0.7, respectively, and differed significantly between species. These ratios were fed into a single-compartment model, and εp values were calculated and compared to carbon isotope fractionation measured under the same conditions. For all investigated species, modeled and measured εp values were comparable (A. fundyense, S. trochoidea, P. reticulatum) and/or showed similar trends with pCO2 (A. fundyense, G. spinifera, P. reticulatum). Offsets are attributed to biases in inorganic flux measurements, an overestimated fractionation factor for the CO2-fixing enzyme RubisCO, or the fact that intracellular inorganic carbon fluxes were not taken into account in the model. This study demonstrates that CO2-dependency in εp can largely be explained by the inorganic carbon fluxes of the individual dinoflagellates.

Environmental stability affects phenotypic evolution in a globally distributed marine picoplankton.

Marine phytoplankton can evolve rapidly when confronted with aspects of climate change because of their large population sizes and fast generation times. Despite this, the importance of environment fluctuations, a key feature of climate change, has received little attention-selection experiments with marine phytoplankton are usually carried out in stable environments and use single or few representatives of a species, genus or functional group. Here we investigate whether and by how much environmental fluctuations contribute to changes in ecologically important phytoplankton traits such as C:N ratios and cell size, and test the variability of changes in these traits within the globally distributed species Ostreococcus. We have evolved 16 physiologically distinct lineages of Ostreococcus at stable high CO2 (1031±87 µatm CO2, SH) and fluctuating high CO2 (1012±244 µatm CO2, FH) for 400 generations. We find that although both fluctuation and high CO2 drive evolution, FH-evolved lineages are smaller, have reduced C:N ratios and respond more strongly to further increases in CO2 than do SH-evolved lineages. This indicates that environmental fluctuations are an important factor to consider when predicting how the characteristics of future phytoplankton populations will have an impact on biogeochemical cycles and higher trophic levels in marine food webs.

Ocean acidification decreases the light-use efficiency in an Antarctic diatom under dynamic but not constant light.

There is increasing evidence that different light intensities strongly modulate the effects of ocean acidification (OA) on marine phytoplankton. The aim of the present study was to investigate interactive effects of OA and dynamic light, mimicking natural mixing regimes. The Antarctic diatom Chaetoceros debilis was grown under two pCO2 (390 and 1000 µatm) and light conditions (constant and dynamic), the latter yielding the same integrated irradiance over the day. To characterize interactive effects between treatments, growth, elemental composition, primary production and photophysiology were investigated. Dynamic light reduced growth and strongly altered the effects of OA on primary production, being unaffected by elevated pCO2 under constant light, yet significantly reduced under dynamic light. Interactive effects between OA and light were also observed for Chl production and particulate organic carbon quotas. Response patterns can be explained by changes in the cellular energetic balance. While the energy transfer efficiency from photochemistry to biomass production (Φe,C ) was not affected by OA under constant light, it was drastically reduced under dynamic light. Contrasting responses under different light conditions need to be considered when making predictions regarding a more stratified and acidified future ocean.

Cellular inorganic carbon fluxes in Trichodesmium: a combined approach using measurements and modelling.

To predict effects of climate change on phytoplankton, it is crucial to understand how their mechanisms for carbon acquisition respond to environmental conditions. Aiming to shed light on the responses of extra- and intracellular inorganic C (Ci) fluxes, the cyanobacterium Trichodesmium erythraeum IMS101 was grown with different nitrogen sources (N2 vs NO3 (-)) and pCO2 levels (380 vs 1400 µatm). Cellular Ci fluxes were assessed by combining membrane inlet mass spectrometry (MIMS), (13)C fractionation measurements, and modelling. Aside from a significant decrease in Ci affinity at elevated pCO2 and changes in CO2 efflux with different N sources, extracellular Ci fluxes estimated by MIMS were largely unaffected by the treatments. (13)C fractionation during biomass production, however, increased with pCO2, irrespective of the N source. Strong discrepancies were observed in CO2 leakage estimates obtained by MIMS and a (13)C-based approach, which further increased under elevated pCO2. These offsets could be explained by applying a model that comprises extracellular CO2 and HCO3 (-) fluxes as well as internal Ci cycling around the carboxysome via the CO2 uptake facilitator NDH-14. Assuming unidirectional, kinetic fractionation between CO2 and HCO3 (-) in the cytosol or enzymatic fractionation by NDH-14, both significantly improved the comparability of leakage estimates. Our results highlight the importance of internal Ci cycling for (13)C composition as well as cellular energy budgets of Trichodesmium, which ought to be considered in process studies on climate change effects.

Emiliania huxleyi endures N-limitation with an efficient metabolic budgeting and effective ATP synthesis.

BACKGROUND: Global change will affect patterns of nutrient upwelling in marine environments, potentially becoming even stricter regulators of phytoplankton primary productivity. To better understand phytoplankton nutrient utilization on the subcellular basis, we assessed the transcriptomic responses of the life-cycle stages of the biogeochemically important microalgae Emiliania huxleyi to nitrogen-limitation. Cells grown in batch cultures were harvested at 'early' and 'full' nitrogen-limitation and were compared with non-limited cells. We applied microarray-based transcriptome profilings, covering ~10.000 known E. huxleyi gene models, and screened for expression patterns that indicate the subcellular responses. RESULTS: The diploid life-cycle stage scavenges nitrogen from external organic sources and -like diatoms- uses the ornithine-urea cycle to rapidly turn over cellular nitrogen. The haploid stage reacts similarly, although nitrogen scavenging is less pronounced and lipid oxidation is more prominent. Generally, polyamines and proline appear to constitute major organic pools that back up cellular nitrogen. Both stages induce a malate:quinone-oxidoreductase that efficiently feeds electrons into the respiratory chain and drives ATP generation with reduced respiratory carbon throughput. CONCLUSIONS: The use of the ornithine-urea cycle to budget the cellular nitrogen in situations of limitation resembles the responses observed earlier in diatoms. This suggests that underlying biochemical mechanisms are conserved among distant clades of marine phototrophic protists. The ornithine-urea cycle and proline oxidation appear to constitute a sensory-regulatory system that monitors and controls cellular nitrogen budgets under limitation. The similarity between the responses of the life-cycle stages, despite the usage of different genes, also indicates a strong functional consistency in the responses to nitrogen-limitation that appears to be owed to biochemical requirements. The malate:quinone-oxidoreductase is a genomic feature that appears to be absent from diatom genomes, and it is likely to strongly contribute to the uniquely high endurance of E. huxleyi under nutrient limitation.

The impact of iron limitation on the physiology of the Antarctic diatom Chaetoceros simplex.

Iron availability strongly governs the growth of Southern Ocean phytoplankton. To investigate how iron limitation affects photosynthesis as well as the uptake of carbon and iron in the Antarctic diatom Chaetocerossimplex, a combination of chlorophyll a fluorescence measurements and radiotracer incubations in the presence and absence of chemical inhibitors was conducted. Iron limitation in C. simplex led to a decline in growth rates, photochemical efficiency and structural changes in photosystem II (PSII), including a reorganisation of photosynthetic units in PSII and an increase in size of the functional absorption cross section of PSII. Iron-limited cells further exhibited a reduced plastoquinone pool and decreased photosynthetic electron transport rate, while non-photochemical quenching and relative xanthophyll pigment content were strongly increased, suggesting a photoprotective response. Additionally, iron limitation resulted in a strong decline in carbon fixation and thus the particulate organic carbon quotas. Inhibitor studies demonstrated that, independent of the iron supply, carbon fixation was dependent on internal, but not on extracellular carbonic anhydrase activity. Orthovanadate more strongly inhibited iron uptake in iron-limited cells, indicating that P-type ATPase transporters are involved in iron uptake. The stronger reduction in iron uptake by ascorbate in iron-limited cells suggests that the re-oxidation of iron is required before it can be taken up and further supports the presence of a high-affinity iron transport pathway. The measured changes to photosystem architecture and shifts in carbon and iron uptake strategies in C. simplex as a result of iron limitation provide evidence for a complex interaction of these processes to balance the iron requirements for photosynthesis and carbon demand for sustained growth in iron-limited waters.