Two Southern Ocean diatoms are more sensitive to ocean acidification and changes in irradiance than the prymnesiophyte Phaeocystis antarctica

To better understand the impact of ocean acidification (OA) and changes in light availability on Southern Ocean phytoplankton physiology, we investigated the effects of pCO₂ (380 and 800 µatm) in combination with low and high irradiance (20 or 50 and 200 µmol photons m^?2 s^?1) on growth, particulate organic carbon (POC) fixation and photophysiology in the three ecologically relevant species Chaetoceros debilis, Fragilariopsis kerguelensis and Phaeocystis antarctica. Irrespective of the light scenario, neither growth nor POC per cell was stimulated by OA in any of the tested species and the two diatoms even displayed negative responses in growth (e.g. C. debilis) or POC content (e.g. F. kerguelensis) under OA in conjunction with high light. For both diatoms, also maximum quantum yields of photosystem II (F_v/F_m) were decreased under these conditions, indicating lowered photochemical efficiencies. To counteract the negative effects by OA and high light, the two diatoms showed diverging photoacclimation strategies. While cellular chlorophyll a (Chl a) and fucoxanthin contents were enhanced in C. debilis to potentially maximize light absorption, F. kerguelensis exhibited reduced Chl a per cell, increased disconnection of antennae from photosystem II reaction centers and strongly lowered absolute electron transport rates (ETR). The decline in ETRs in F. kerguelensis might be explained in terms of different species‐specific strategies for tuning the available flux of adenosine triphosphate and nicotinamide adenine dinucleotide phosphate. Overall, our results revealed that P. antarctica was more tolerant to OA and changes in irradiance than the two diatoms, which may have important implications for biogeochemical cycling.     

MORPHOLOGICAL AND PHYSIOLOGICAL EFFECTS IN PROBOSCIA ALATA (BACILLARIOPHYCEAE) GROWN UNDER DIFFERENT LIGHT AND CO2 CONDITIONS OF THE MODERN SOUTHERN OCEAN1

The combined effects of different light and aqueous CO₂ conditions were assessed for the Southern Ocean diatom Proboscia alata (Brightwell) Sundström in laboratory experiments. Selected culture conditions (light and CO_2(aq)) were representative for the natural ranges in the modern Southern Ocean. Light conditions were 40 (low) and 240 (high) μmol photons · m^?2 · s^?1. The three CO_2(aq) conditions ranged from 8 to 34 μmol · kg^?1 CO_2(aq) (equivalent to a pCO₂ from 137 to 598 μatm, respectively). Clear morphological changes were induced by these different CO_2(aq) conditions. Cells in low [CO_2(aq)] formed spirals, while many cells in high [CO_2(aq)] disintegrated. Cell size and volume were significantly affected by the different CO_2(aq) concentrations. Increasing CO_2(aq) concentrations led to an increase in particulate organic carbon concentrations per cell in the high light cultures, with exactly the opposite happening in the low light cultures. However, other parameters measured were not influenced by the range of CO_2(aq) treatments. This included growth rates, chlorophyll a concentration and photosynthetic yield (F_V/F_M). Different light treatments had a large effect on nutrient uptake. High light conditions caused an increased nutrient uptake rate compared to cells grown in low light conditions. Light and CO₂ conditions co‐determined in various ways the response of P. alata to changing environmental conditions. Overall P. alata appeared to be well adapted to the natural variability in light availability and CO_2(aq) concentration of the modern Southern Ocean. Nevertheless, our results showed that P. alata is susceptible to future changes in inorganic carbon concentrations in the Southern Ocean.     

Iron Limitation Modulates Ocean Acidification Effects on Southern Ocean Phytoplankton Communities

The potential interactive effects of iron (Fe) limitation and Ocean Acidification in the Southern Ocean (SO) are largely unknown. Here we present results of a long-term incubation experiment investigating the combined effects of CO₂ and Fe availability on natural phytoplankton assemblages from the Weddell Sea, Antarctica. Active Chl a fluorescence measurements revealed that we successfully cultured phytoplankton under both Fe-depleted and Fe-enriched conditions. Fe treatments had significant effects on photosynthetic efficiency (F_v/F_m; 0.3 for Fe-depleted and 0.5 for Fe-enriched conditions), non-photochemical quenching (NPQ), and relative electron transport rates (rETR). pCO₂ treatments significantly affected NPQ and rETR, but had no effect on F_v/F_m. Under Fe limitation, increased pCO₂ had no influence on C fixation whereas under Fe enrichment, primary production increased with increasing pCO₂ levels. These CO₂-dependent changes in productivity under Fe-enriched conditions were accompanied by a pronounced taxonomic shift from weakly to heavily silicified diatoms (i.e. from Pseudo-nitzschia sp. to Fragilariopsis sp.). Under Fe-depleted conditions, this functional shift was absent and thinly silicified species dominated all pCO₂ treatments (Pseudo-nitzschia sp. and Synedropsis sp. for low and high pCO₂, respectively). Our results suggest that Ocean Acidification could increase primary productivity and the abundance of heavily silicified, fast sinking diatoms in Fe-enriched areas, both potentially leading to a stimulation of the biological pump. Over much of the SO, however, Fe limitation could restrict this possible CO₂ fertilization effect.     

ELEVATED CARBON DIOXIDE DIFFERENTIALLY ALTERS THE PHOTOPHYSIOLOGY OF THALASSIOSIRA PSEUDONANA (BACILLARIOPHYCEAE) AND EMILIANIA HUXLEYI (HAPTOPHYTA)1

Increasing anthropogenic carbon dioxide is causing changes to ocean chemistry, which will continue in a predictable manner. Dissolution of additional atmospheric carbon dioxide leads to increased concentrations of dissolved carbon dioxide and bicarbonate and decreased pH in ocean water. The concomitant effects on phytoplankton ecophysiology, leading potentially to changes in community structure, are now a focus of concern. Therefore, we grew the coccolithophore Emiliania huxleyi (Lohmann) W. W. Hay et H. Mohler and the diatom strains Thalassiosira pseudonana (Hust.) Hasle et Heimdal CCMP 1014 and T. pseudonana CCMP 1335 under low light in turbidostat photobioreactors bubbled with air containing 390 ppmv or 750 ppmv CO₂. Increased pCO₂ led to increased growth rates in all three strains. In addition, protein levels of RUBISCO increased in the coastal strains of both species, showing a larger capacity for CO₂ assimilation at 750 ppmv CO₂. With increased pCO₂, both T. pseudonana strains displayed an increased susceptibility to PSII photoinactivation and, to compensate, an augmented capacity for PSII repair. Consequently, the cost of maintaining PSII function for the diatoms increased at increased pCO₂. In E. huxleyi, PSII photoinactivation and the counter‐acting repair, while both intrinsically larger than in T. pseudonana, did not change between the current and high‐pCO₂ treatments. The content of the photosynthetic electron transport intermediary cytochrome b6/f complex increased significantly in the diatoms under elevated pCO₂, suggesting changes in electron transport function.     

Distribution of biogenic silica and particulate organic matter in coastal and oceanic surface waters off the South Shetland Islands in summer

Biogenic silica (BSi), lithogenic silica (LSi), particiulate organic carbon (POC) and nitrogen (PON), and chlorophyll a (Chl a) concentration levels were measured in the surface waters (<100 m) off the northern coast of the South Shetland Islands in summer 1991. High concentration levels of BSi and LSi were recorded in the oceanic area and the coastal area, respectively. However, marked regional differences were not observed for POC, PON and Chl a concentrations. The mean BSi/POC atomic ratio (±SD) in the oceanic area (0.27±0.17) was 6 times that in the coastal area (0.045±0.020), except for the bloom situation (0.19±0.029). In contrast, the mean POC/PON atomic ratio was not significantly different in the coastal area (5.9±1.4) and the oceanic area (5.2±1.7). Nitzschia spp. were the dominant diatoms in the oceanic area but not in the coastal area. High BSi/POC ratios have been reported for blooms dominated by Nitzschia spp. even in the coastal regions of the Antarctic Ocean. The area difference in the BSi/POC ratios was probably related to the difference in species composition of phytoplankton and not to regional contrast. This species contributes significantly to high BSi/POC ratios in the Antarctic Ocean.     

Acclimation conditions modify physiological response of the diatom Thalassiosira pseudonana to elevated CO2 concentrations in a nitrate‐limited chemostat

Diatoms are responsible for a large proportion of global carbon fixation, with the possibility that they may fix more carbon under future levels of high CO₂. To determine how increased CO₂ concentrations impact the physiology of the diatom Thalassiosira pseudonana Hasle et Heimdal, nitrate‐limited chemostats were used to acclimate cells to a recent past (333 ± 6 μatm) and two projected future concentrations (476 ± 18 μatm, 816 ± 35 μatm) of CO₂. Samples were harvested under steady‐state growth conditions after either an abrupt (15–16 generations) or a longer acclimation process (33–57 generations) to increased CO₂ concentrations. The use of un‐bubbled chemostat cultures allowed us to calculate the uptake ratio of dissolved inorganic carbon relative to dissolved inorganic nitrogen (DIC:DIN), which was strongly correlated with fCO₂ in the shorter acclimations but not in the longer acclimations. Both CO₂ treatment and acclimation time significantly affected the DIC:DIN uptake ratio. Chlorophyll a per cell decreased under elevated CO₂ and the rates of photosynthesis and respiration decreased significantly under higher levels of CO₂. These results suggest that T. pseudonana shifts carbon and energy fluxes in response to high CO₂ and that acclimation time has a strong effect on the physiological response.     

Antarctic emerald rockcod have the capacity to compensate for warming when uncoupled from CO2‐acidification

Increases in atmospheric CO₂ levels and associated ocean changes are expected to have dramatic impacts on marine ecosystems. Although the Southern Ocean is experiencing some of the fastest rates of change, few studies have explored how Antarctic fishes may be affected by co‐occurring ocean changes, and even fewer have examined early life stages. To date, no studies have characterized potential trade‐offs in physiology and behavior in response to projected multiple climate change stressors (ocean acidification and warming) on Antarctic fishes. We exposed juvenile emerald rockcod Trematomus bernacchii to three PCO₂ treatments (~450, ~850, and ~1,200 μatm PCO₂) at two temperatures (?1 or 2°C). After 2, 7, 14, and 28 days, metrics of physiological performance including cardiorespiratory function (heart rate [f_H] and ventilation rate [f_V]), metabolic rate (), and cellular enzyme activity were measured. Behavioral responses, including scototaxis, activity, exploration, and escape response were assessed after 7 and 14 days. Elevated PCO₂ independently had little impact on either physiology or behavior in juvenile rockcod, whereas warming resulted in significant changes across acclimation time. After 14 days, f_H, f_V and significantly increased with warming, but not with elevated PCO₂. Increased physiological costs were accompanied by behavioral alterations including increased dark zone preference up to 14%, reduced activity by 12%, as well as reduced escape time suggesting potential trade‐offs in energetics. After 28 days, juvenile rockcod demonstrated a degree of temperature compensation as f_V, , and cellular metabolism significantly decreased following the peak at 14 days; however, temperature compensation was only evident in the absence of elevated PCO₂. Sustained increases in f_V and after 28 days exposure to elevated PCO₂ indicate additive (f_V) and synergistic () interactions occurred in combination with warming. Stressor‐induced energetic trade‐offs in physiology and behavior may be an important mechanism leading to vulnerability of Antarctic fishes to future ocean change.     

The physiological response of marine diatoms to ocean acidification: differential roles of seawater pCO2 and pH

Although increasing the pCO₂ for diatoms will presumably down‐regulate the CO₂‐concentrating mechanism (CCM) to save energy for growth, different species have been reported to respond differently to ocean acidification (OA). To better understand their growth responses to OA, we acclimated the diatoms Thalassiosira pseudonana, Phaeodactylum tricornutum, and Chaetoceros muelleri to ambient (pCO₂ 400 μatm, pH 8.1), carbonated (pCO₂ 800 μatm, pH 8.1), acidified (pCO₂ 400 μatm, pH 7.8), and OA (pCO₂ 800 μatm, pH 7.8) conditions and investigated how seawater pCO₂ and pH affect their CCMs, photosynthesis, and respiration both individually and jointly. In all three diatoms, carbonation down‐regulated the CCMs, while acidification increased both the photosynthetic carbon fixation rate and the fraction of CO₂ as the inorganic carbon source. The positive OA effect on photosynthetic carbon fixation was more pronounced in C. muelleri, which had a relatively lower photosynthetic affinity for CO₂, than in either T. pseudonana or P. tricornutum. In response to OA, T. pseudonana increased respiration for active disposal of H⁺ to maintain its intracellular pH, whereas P. tricornutum and C. muelleri retained their respiration rate but lowered the intracellular pH to maintain the cross‐membrane electrochemical gradient for H⁺ efflux. As the net result of changes in photosynthesis and respiration, growth enhancement to OA of the three diatoms followed the order of C. muelleri > P. tricornutum > T. pseudonana. This study demonstrates that elucidating the separate and joint impacts of increased pCO₂ and decreased pH aids the mechanistic understanding of OA effects on diatoms in the future, acidified oceans.     

Interacting effects of atmospheric CO2 enrichment and solar radiation on growth of the aquatic fern Azolla filiculoides

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THE ROLE OF PHYTOPLANKTON SIZE ON PHOTOCHEMICAL RECOVERY DURING THE SOUTHERN OCEAN IRON EXPERIMENT1

Phytoplankton primary productivity in the Southern Ocean is controlled by complex interactions among iron, light, and grazing. This project interfaced with the Southern Ocean iron experiment (SOFeX) that created two iron‐enriched patches north and south of the Polar Front each with distinct silicic acid concentrations. We used pulse amplitude modulated fluorometry and measured the recovery of the maximum quantum yield of photochemistry (F_v/F_m) for three size fractions (whole, <5, <20 μm) and light adapted quantum yield (ΔF/F′_m) for single phytoplankton cells. The rates of recovery from iron stress were found to be unrelated to average cell size for both size‐fractioned and single‐celled measurements. The smallest cells appeared to exhibit more severe iron stress at the onset of the experiment than the larger taxa. The largest response detected in regression parameters was that of the pennate diatoms, which took only ～3.4 days to reach the maximum quantum yield, whereas the centric diatom Asteromphalus sp. reached maximum ΔF/F′_m after ～10.4 days. The north patch measurements showed a different response; the smallest cells never reached maximum ΔF/F′_m, whereas the size fraction containing the largest cells did. Single‐celled measurements made nearly 30 days after the initial iron enrichment suggested that diatoms were experiencing either silicic acid or iron limitation, whereas measurements of Phaeocystis sp. did not. These data represent the first study of in situ recovery rates of PSII for groups of diatoms, and may help elucidate the mechanisms of species change in response to environmental perturbation.     

High CO2 promotes the production of paralytic shellfish poisoning toxins by Alexandrium catenella from Southern California waters

In many dinoflagellates, cellular toxin levels have been demonstrated to increase when growth is limited by essential nutrients such as phosphorus. Despite the recognized importance of nutrient limitation to dinoflagellate toxicity, interactions with current and future global environmental change variables have been relatively unexplored. This is a critical question, as dissolution of anthropogenic CO₂ emissions into seawater is leading to progressively lower pH values, or ocean acidification. Sea surface temperatures are concurrently increasing, a trend that is also projected to continue in the future. We conditioned a clonal culture of paralytic shellfish poisoning toxin producing Alexandrium catenella (A-11c) isolated from coastal Southern California to factorial combinations of two temperatures, two pCO₂ levels, and two phosphate concentrations for a period of eight months. Interactions between these variables influenced growth and carbon fixation rates and although these treatments only elicited minor differences in toxin profile, total cellular toxicity was dramatically affected. Cells conditioned to high pCO₂ (levels projected for year 2075) and low phosphate at low temperature (15 °C) were the most toxic, while lower pCO₂, higher phosphate levels, and warmer temperature (19 °C) alleviated this toxicity to varying degrees. Overall increased pCO₂ generally led to enhanced potency. Our results suggest that future increased ocean acidification may exacerbate the toxic threat posed by this toxic dinoflagellate, especially when combined with nutrient limitation, but that future warmer temperatures could also offset some of this enhanced toxicity.     

One‐year experiment on the physiological response of the Mediterranean crustose coralline alga,Lithophyllum cabiochae,to elevated pCO2 and temperature

The response of respiration, photosynthesis, and calcification to elevated pCO₂ and temperature was investigated in isolation and in combination in the Mediterranean crustose coralline alga Lithophyllum cabiochae. Algae were maintained in aquaria during 1 year at near‐ambient conditions of irradiance, at ambient or elevated temperature (+3°C), and at ambient (ca. 400 μatm) or elevated pCO₂ (ca. 700 μatm). Respiration, photosynthesis, and net calcification showed a strong seasonal pattern following the seasonal variations of temperature and irradiance, with higher rates in summer than in winter. Respiration was unaffected by pCO₂ but showed a general trend of increase at elevated temperature at all seasons, except in summer under elevated pCO₂. Conversely, photosynthesis was strongly affected by pCO₂ with a decline under elevated pCO₂ in summer, autumn, and winter. In particular, photosynthetic efficiency was reduced under elevated pCO₂. Net calcification showed different responses depending on the season. In summer, net calcification increased with rising temperature under ambient pCO₂ but decreased with rising temperature under elevated pCO₂. Surprisingly, the highest rates in summer were found under elevated pCO₂ and ambient temperature. In autumn, winter, and spring, net calcification exhibited a positive or no response at elevated temperature but was unaffected by pCO₂. The rate of calcification of L. cabiochae was thus maintained or even enhanced under increased pCO₂. However, there is likely a trade‐off with other physiological processes. For example, photosynthesis declines in response to increased pCO₂ under ambient irradiance. The present study reports only on the physiological response of healthy specimens to ocean warming and acidification, however, these environmental changes may affect the vulnerability of coralline algae to other stresses such as pathogens and necroses that can cause major dissolution, which would have critical consequence for the sustainability of coralligenous habitats and the budgets of carbon and calcium carbonate in coastal Mediterranean ecosystems.     

Growth form analysis of epiphytic diatom communities of Terra Nova Bay (Ross Sea, Antarctica)

Diatoms have been long collected from the Southern Ocean but almost no data exist for epiphytic communities, despite their high ecological significance as an important food source in Antarctic coastal food chains. Here, we present a first growth form analysis of diatoms associated with rhodophyte hosts from Terra Nova Bay, Ross Sea, Antarctica. We performed this study to gather baseline information on the species composition of epiphytic diatom communities, determine the influence of some environmental variables on the diatom distribution patterns, and assess the caveats that must be taken into account in terms of sampling design. Macroalgal material was collected during the Italian Antarctic expeditions between 1990 and 2004. Epiphytic diatoms were studied by means of scanning electron microscopy. In terms of growth forms, there were no significant differences between the diatom communities on the different macroalgal host species. Motile (mainly small-celled Navicula perminuta and other Navicula spp.) and adnate (Cocconeis spp.) diatoms dominated the community throughout the study period. Many of the macroalgal blades examined were also covered by epiphytic animals (calcareous bryozoans, hydroids) over most of their surface, with a significant effect on the associated diatom community structure. Our findings suggest that the bio-physicochemical characteristics of each sampling site affected the epiphytic diatom communities more than the substrate type provided by the macroalgal host or the sampling depth.     

The relative availability of inorganic carbon and inorganic nitrogen influences the response of the dinoflagellate Protoceratium reticulatum to elevated CO2

This work originates from three facts: (i) changes in CO₂ availability influence metabolic processes in algal cells; (ii) Spatial and temporal variations of nitrogen availability cause repercussions on phytoplankton physiology; (iii) Growth and cell composition are dependent on the stoichiometry of nutritional resources. In this study, we assess whether the impact of rising pCO₂ is influenced by N availability, through the impact that it would have on the C/N stoichiometry, in conditions of N sufficiency. Our experiments used the dinoflagellate Protoceratium reticulatum, which we cultured under three CO₂ regimes (400, 1,000, and 5,000 ppmv, pH of 8.1) and either variable (the NO₃^? concentration was always 2.5 mmol · L^?1) or constant (NO₃^? concentration varied to maintain the same C_i/NO₃^? ratio at all pCO₂) C_i/NO₃^? ratio. Regardless of N availability, cells had higher specific growth rates, but lower cell dry weight and C and N quotas, at elevated CO₂. The carbohydrate pool size and the C/N was unaltered in all treatments. The lipid content only decreased at high pCO₂ at constant C_i/NO₃^? ratio. In the variable C_i/NO₃^? conditions, the relative abundance of Rubisco (and other proteins) also changed; this did not occur at constant C_i/NO₃^?. Thus, the biomass quality of P. reticulatum for grazers was affected by the C_i/NO₃^? ratio in the environment and not only by the pCO₂, both with respect to the size of the main organic pools and the composition of the expressed proteome.     

No stimulation of nitrogen fixation by non‐filamentous diazotrophs under elevated CO2 in the South Pacific

Nitrogen fixation by diazotrophic cyanobacteria is a critical source of new nitrogen to the oligotrophic surface ocean. Research to date indicates that some diazotroph groups may increase nitrogen fixation under elevated pCO₂. To test this in natural plankton communities, four manipulation experiments were carried out during two voyages in the South Pacific (30–35^oS). High CO₂ treatments, produced using 750 ppmv CO₂ to adjust pH to 0.2 below ambient, and ‘Greenhouse’ treatments (0.2 below ambient pH and ambient temperature +3 °C), were compared with Controls in trace metal clean deckboard incubations in triplicate. No significant change was observed in nitrogen fixation in either the High CO₂ or Greenhouse treatments over 5 day incubations. qPCR measurements and optical microscopy determined that the diazotroph community was dominated by Group A unicellular cyanobacteria (UCYN‐A), which may account for the difference in response of nitrogen fixation under elevated CO₂ to that reported previously for Trichodesmium. This may reflect physiological differences, in that the greater cell surface area:volume of UCYN‐A and its lack of metabolic pathways involved in carbon fixation may confer no benefit under elevated CO₂. However, multiple environmental controls may also be a factor, with the low dissolved iron concentrations in oligotrophic surface waters limiting the response to elevated CO₂. If nitrogen fixation by UCYN‐A is not stimulated by elevated pCO₂, then future increases in CO₂ and warming may alter the regional distribution and dominance of different diazotroph groups, with implications for dissolved iron availability and new nitrogen supply in oligotrophic regions.     

Sustained effects of atmospheric [CO2] and nitrogen availability on forest soil CO2 efflux

Soil CO₂ efflux (F_soil) is the largest source of carbon from forests and reflects primary productivity as well as how carbon is allocated within forest ecosystems. Through early stages of stand development, both elevated [CO₂] and availability of soil nitrogen (N; sum of mineralization, deposition, and fixation) have been shown to increase gross primary productivity, but the long‐term effects of these factors on F_soil are less clear. Expanding on previous studies at the Duke Free‐Air CO₂ Enrichment (FACE) site, we quantified the effects of elevated [CO₂] and N fertilization on F_soil using daily measurements from automated chambers over 10 years. Consistent with previous results, compared to ambient unfertilized plots, annual F_soil increased under elevated [CO₂] (ca. 17%) and decreased with N (ca. 21%). N fertilization under elevated [CO₂] reduced F_soil to values similar to untreated plots. Over the study period, base respiration rates increased with leaf productivity, but declined after productivity saturated. Despite treatment‐induced differences in aboveground biomass, soil temperature and water content were similar among treatments. Interannually, low soil water content decreased annual F_soil from potential values – estimated based on temperature alone assuming nonlimiting soil water content – by ca. 0.7% per 1.0% reduction in relative extractable water. This effect was only slightly ameliorated by elevated [CO₂]. Variability in soil N availability among plots accounted for the spatial variability in F_soil, showing a decrease of ca. 114 g C m^?2 yr^?1 per 1 g m^?2 increase in soil N availability, with consistently higher F_soil in elevated [CO₂] plots ca. 127 g C per 100 ppm [CO₂] over the +200 ppm enrichment. Altogether, reflecting increased belowground carbon partitioning in response to greater plant nutritional needs, the effects of elevated [CO₂] and N fertilization on F_soil in this stand are sustained beyond the early stages of stand development and through stabilization of annual foliage production.     

The role of external carbonic anhydrase in photosynthesis during growth of the marine diatom Chaetoceros muelleri

Carbon dioxide concentrating mechanisms (CCMs) act to improve the supply of CO₂ at the active site of ribulose‐1,5‐bisphosphate carboxylase/oxygenase. There is substantial evidence that in some microalgal species CCMs involve an external carbonic anhydrase (CA_ext) and that CA_ext activity is induced by low CO₂ concentrations in the growth medium. However, much of this work has been conducted on cells adapted to air‐equilibrium concentrations of CO₂, rather than to changing CO₂ conditions caused by growing microalgal populations. We investigated the role of CA_ext in inorganic carbon (C_i) acquisition and photosynthesis at three sampling points during the growth cycle of the cosmopolitan marine diatom Chaetoceros muelleri. We observed that CA_ext activity increased with decreasing C_i, particularly CO₂, concentration, supporting the idea that CA_ext is modulated by external CO₂ concentration. Additionally, we found that the contribution of CA_ext activity to carbon acquisition for photosynthesis varies over time, increasing between the first and second sampling points before decreasing at the last sampling point, where external pH was high. Lastly, decreases in maximum quantum yield of photosystem II (F_v/F_m), chlorophyll, maximum relative electron transport rate, light harvesting efficiency (α) and maximum rates of C_i‐ saturated photosynthesis (V_max) were observed over time. Despite this decrease in photosynthetic capacity an up‐regulation of CCM activity, indicated by a decreasing half‐saturation constant for CO₂ (K_0.5CO₂), occurred over time. The flexibility of the CCM during the course of growth in C. muelleri may contribute to the reported dominance and persistence of this species in phytoplankton blooms.     

The 24 hour recovery kinetics from n starvation in Phaeodactylum tricornutum and Emiliania huxleyi

The response of N (nitrate) starved cells of the diatom Phaeodactylum tricornutum and the coccolithophore Emiliania huxleyi to a pulse of new N were measured to investigate rapid cellular and photosynthetic recovery kinetics. The changes of multiple parameters were followed over 24 h. In P. tricornutum, the recovery of F_v/F_m (the maximum quantum yield of PS II) and σ_PSII (the functional absorption cross‐section for PSII) started within the first hour, much earlier than other parameters. Cellular pigments did not recover during the 24 h but the chlorophyll (chl) a/carotenoid ratios increased to levels measured in the controls. Cell division was independent of the recovery of chl a. In E. huxleyi, the recovery of F_v/F_m and σ_PSII started after an hour, synchronous with the increase in cellular organic N and chl a with pigments fully recovered within 14 h. P. tricornutum prioritized the recovery of its photosynthetic functions and cell divisions while E. huxleyi did not follow this pattern. We hypothesize that the different recovery strategies between the two species allow P. tricornutum to be more competitive when N pulses are introduced into N‐limited water while E. huxleyi is adapted to N scarce waters where such pulses are infrequent. These findings are consistent with successional patterns observed in coastal environments. This is one of only a few studies exploring recovery kinetics of cellular functions and photosynthesis after nitrogen stress in phytoplankton. Our results can be used to enhance ecological models linking phytoplankton traits to species diversity and community structure.     

Effect of eutrophication on air–sea CO2 fluxes in the coastal Southern North Sea: a model study of the past 50 years

The RIVERSTRAHLER model, an idealized biogeochemical model of the river system, has been coupled to MIRO‐CO₂, a complex biogeochemical model describing diatom and Phaeocystis blooms and carbon and nutrient cycles in the marine domain, to assess the dual role of changing nutrient loads and increasing atmospheric CO₂ as drivers of air–sea CO₂ exchanges in the Southern North Sea with a focus on the Belgian coastal zone (BCZ). The whole area, submitted to the influence of two main rivers (Seine and Scheldt), is characterized by variable diatom and Phaeocystis colonies blooms which impact on the trophic status and air–sea CO₂ fluxes of the coastal ecosystem. For this application, the MIRO‐CO₂ model is implemented in a 0D multibox frame covering the eutrophied Eastern English Channel and Southern North Sea and receiving loads from the rivers Seine and Scheldt. Model simulations are performed for the period between 1951 and 1998 using real forcing fields for sea surface temperature, wind speed and atmospheric CO₂ and RIVERSTRAHLER simulations for river carbon and nutrient loads. Model results suggest that the BCZ shifted from a source of CO₂ before 1970 (low eutrophication) towards a sink during the 1970–1990 period when anthropogenic DIN and P loads increased, stimulating C fixation by autotrophs. In agreement, a shift from net annual heterotrophy towards autotrophy in BCZ is simulated from 1980. The period after 1990 is characterized by a progressive decrease of P loads concomitant with a decrease of primary production and of the CO₂ sink in the BCZ. At the end of the simulation period, the BCZ ecosystem is again net heterotroph and acts as a source of CO₂ to the atmosphere. R‐MIRO‐CO₂ scenarios testing the relative impact of temperature, wind speed, atmospheric CO₂ and river loads variability on the simulated air–sea CO₂ fluxes suggest that the trend in air–sea CO₂ fluxes simulated between 1951 and 1998 in the BCZ was mainly controlled by the magnitude and the ratio of inorganic nutrient river loads. Quantitative nutrient changes control the level of primary production while qualitative changes modulate the relative contribution of diatoms and Phaeocystis to this flux and hence the sequestration of atmospheric CO₂.