Particulate Organic Carbon Production Research Articles

Particulate inorganic to organic carbon production as a predictor for coccolithophorid sensitivity to ongoing ocean acidification

AbstractOcean acidification (OA) can induce shifts in plankton community composition, with coccolithophores being mostly negatively impacted. This is likely to change particulate inorganic and organic carbon (PIC and POC, respectively) production, with impacts on the biological carbon pump. Hence, assessing and, most importantly, understanding species‐specific sensitivities of coccolithophores is paramount. In a multispecies comparison, spanning more than two orders of magnitude in terms of POC and PIC production rates, among Calcidiscus leptoporus, Coccolithus pelagicus subsp. braarudii, Emiliania huxleyi, Gephyrocapsa oceanica, and Scyphosphaera apsteinii, we found that cellular PIC : POC was a good predictor for a species’ OA sensitivity. This is likely related to the need for cellular pH homeostasis, which is challenged by the process of calcification producing protons internally, especially when seawater pH decreases in an OA scenario. With higher PIC : POC, species and strains being more sensitive to OA coccolithophores may shift toward less calcified varieties in the future.

The impacts of iron limitation and ocean acidification on the cellular stoichiometry, photophysiology, and transcriptome of Phaeocystis antarctica

Abstract Phaeocystis antarctica is an integral player of the phytoplankton community of the Southern Ocean (SO), the world's largest high‐nutrient low‐chlorophyll region, and faces chronic iron (Fe) limitation. As the SO is responsible for 40% of anthropogenic CO2 uptake, P. antarctica must also deal with ocean acidification (OA). However, mechanistic studies investigating the effects of Fe limitation and OA on trace metal (TM) stoichiometry, transcriptomic, and photophysiological responses of this species, as well as on the Fe chemistry, are lacking. This study reveals that P. antarctica responded strongly to Fe limitation by reducing its growth rate and particulate organic carbon (POC) production. Cellular concentrations of all TMs, not just Fe, were greatly reduced, suggesting that Fe limitation may drive cells into secondary limitation by another TM. P. antarctica was able to adjust its photophysiology in response to Fe limitation, resulting in similar absolute electron transport rates across PSII. Even though OA‐stimulated growth in Fe‐limited and ‐replete treatments, the slight reduction in cellular POC resulted in no net effect on POC production. In addition, relatively few genes were differentially expressed due to OA. Finally, this study demonstrates that, under our culture conditions, OA did not affect inorganic Fe or humic‐acid‐like substances in seawater but triggered the production of humic‐acid‐like substances by P. antarctica. This species is well adapted to OA under all Fe conditions, giving it a competitive advantage over more sensitive species in a future ocean.

Warming enhances sedimentation and decomposition of organic carbon in shallow macrophyte-dominated systems with zero net effect on carbon burial.

Temperatures have been rising throughout recent decades and are predicted to rise further in the coming century. Global warming affects carbon cycling in freshwater ecosystems, which both emit and bury substantial amounts of carbon on a global scale. Currently, most studies focus on the effect of warming on overall carbon emissions from freshwater ecosystems, while net effects on carbon budgets may strongly depend on burial in sediments. Here, we tested whether year-round warming increases the production, sedimentation, or decomposition of particulate organic carbon and eventually alters the carbon burial in a typical shallow freshwater system. We performed an indoor experiment in eight mesocosms dominated by the common submerged aquatic plant Myriophyllum spicatum testing two temperature treatments: a temperate seasonal temperature control and a warmed (+4°C) treatment (n=4). During a full experimental year, the carbon stock in plant biomass, dissolved organic carbon in the water column, sedimented organic matter, and decomposition of plant detritus were measured. Our results showed that year-round warming nearly doubled the final carbon stock in plant biomass from 6.9±1.1g C in the control treatment to 12.8±0.6g C (mean±SE), mainly due to a prolonged growing season in autumn. DOC concentrations did not differ between the treatments, but organic carbon sedimentation increased by 60% from 96±9.6 to 152±16gCm-2 yaer-1 (mean±SE) from control to warm treatments. Enhanced decomposition of plant detritus in the warm treatment, however, compensated for the increased sedimentation. As a result, net carbon burial was 40±5.7gCm-2 year-1 in both temperature treatments when fluxes were combined into a carbon budget model. These results indicate that warming can increase the turnover of organic carbon in shallow macrophyte-dominated systems, while not necessarily affecting net carbon burial on a system scale.

Population-specific responses in physiological rates of <i>Emiliania huxleyi</i> to a broad CO<sub>2</sub> range

Abstract. Although coccolithophore physiological responses to CO2-induced changes in seawater carbonate chemistry have been widely studied in the past, there is limited knowledge on the variability of physiological responses between populations from different areas. In the present study, we investigated the specific responses of growth, particulate organic (POC) and inorganic carbon (PIC) production rates of three populations of the coccolithophore Emiliania huxleyi from three regions in the North Atlantic Ocean (Azores: six strains, Canary Islands: five strains, and Norwegian coast near Bergen: six strains) to a CO2 partial pressure (pCO2) range from 120 to 2630 µatm. Physiological rates of each population and individual strain increased with rising pCO2 levels, reached a maximum and declined thereafter. Optimal pCO2 for growth, POC production rates, and tolerance to low pH (i.e., high proton concentration) was significantly higher in an E. huxleyi population isolated from the Norwegian coast than in those isolated near the Azores and Canary Islands. This may be due to the large environmental variability including large pCO2 and pH fluctuations in coastal waters off Bergen compared to the rather stable oceanic conditions at the other two sites. Maximum growth and POC production rates of the Azores and Bergen populations were similar and significantly higher than that of the Canary Islands population. This pattern could be driven by temperature–CO2 interactions where the chosen incubation temperature (16 ∘C) was slightly below what strains isolated near the Canary Islands normally experience. Our results indicate adaptation of E. huxleyi to their local environmental conditions and the existence of distinct E. huxleyi populations. Within each population, different growth, POC, and PIC production rates at different pCO2 levels indicated strain-specific phenotypic plasticity. Accounting for this variability is important to understand how or whether E. huxleyi might adapt to rising CO2 levels.

Physiological and biochemical responses of Thalassiosira punctigera to nitrate limitation

Marine diatoms play an important role in the global primary production and contribute significantly to carbon export in upwelling regions. However, their carbon fixation is frequently limited by nutrient availability, which varies regionally and temporally. In general, as a result of global warming, coastal waters are predicted to become more eutrophic, while nutrient limitation in the open ocean would be further enhanced. For a better understanding on how diatoms respond to nitrogen supply in the future ocean, we cultivated Thalassiosira punctigera at different levels of nitrate (from 1 to 100 µmol L−1). Results showed that the half-saturation value for growth rate (Kµ = 3.06 µmol L−1) corroborates the values obtained from a size-dependent model (3.03 µmol L−1), suggesting that nutrient storage in this large diatom has equilibrated with the bulk medium added during culturing. Measured physiological parameters, including growth rate, cellular Chl a concentration, and photosynthesis, were all significantly reduced as the extent of nitrogen limitation increased. However, the responses of elemental composition did not vary linearly with respect to the different degrees of nitrate limitation. Specifically, the elemental ratios of C:N and Si:N all increased under moderate nitrate limitation (2–10 µmol L−1), but further decreased when nitrate became extremely limited (1 µmol L−1). The particulate organic nitrogen production rate was sensitive to nitrate limitation, showing a significant decrease when nitrate was below 10 µmol L−1. However, particulate organic carbon production rate remained stable until nitrate was reduced to 2 µmol L−1. The diverse responses of physiological processes and elemental ratios to various nitrate levels indicate the fact that there is a large uncertainty about the biogeochemical consequences of changes in the future ocean.

Adaptive evolution in the coccolithophore Gephyrocapsa oceanica following 1,000 generations of selection under elevated CO2.

Coccolithophores are important oceanic primary producers not only in terms of photosynthesis but also because they produce calcite plates called coccoliths. Ongoing ocean acidification associated with changing seawater carbonate chemistry may impair calcification and other metabolic functions in coccolithophores. While short-term ocean acidification effects on calcification and other properties have been examined in a variety of coccolithophore species, long-term adaptive responses have scarcely been documented, other than for the single species Emiliania huxleyi. Here, we investigated the effects of ocean acidification on another ecologically important coccolithophore species, Gephyrocapsa oceanica, following 1,000 generations of growth under elevated CO2 conditions (1,000μatm). High CO2 -selected populations exhibited reduced growth rates and enhanced particulate organic carbon (POC) and nitrogen (PON) production, relative to populations selected under ambient CO2 (400μatm). Particulate inorganic carbon (PIC) and PIC/POC ratios decreased progressively throughout the selection period in high CO2 -selected cell lines. All of these trait changes persisted when high CO2 -grown populations were moved back to ambient CO2 conditions for about 10 generations. The results suggest that the calcification of some coccolithophores may be more heavily impaired by ocean acidification than previously predicted based on short-term studies, with potentially large implications for the ocean's carbon cycle under accelerating anthropogenic influences.

Phosphorus limitation and heat stress decrease calcification in <i>Emiliania huxleyi</i>

Abstract. Calcifying haptophytes (coccolithophores) sequester carbon in the form of organic and inorganic cellular components (coccoliths). We examined the effect of phosphorus (P) limitation and heat stress on particulate organic and inorganic carbon (calcite) production in the coccolithophore Emiliania huxleyi. Both environmental stressors are related to rising CO2 levels and affect carbon production in marine microalgae, which in turn impacts biogeochemical cycling. Using semi-continuous cultures, we show that P limitation and heat stress decrease the calcification rate in E. huxleyi. However, using batch cultures, we show that different culturing approaches (batch versus semi-continuous) induce different physiologies. This affects the ratio of particulate inorganic (PIC) to organic carbon (POC) and complicates general predictions on the effect of P limitation on the PIC ∕ POC ratio. We found heat stress to increase P requirements in E. huxleyi, possibly leading to lower standing stocks in a warmer ocean, especially if this is linked to lower nutrient input. In summary, the predicted rise in global temperature and resulting decrease in nutrient availability may decrease CO2 sequestration by E. huxleyi through lower overall carbon production. Additionally, the export of carbon may be diminished by a decrease in calcification and a weaker coccolith ballasting effect.

Photochemical degradation of dissolved organic matter reduces the availability of phosphorus for aquatic primary producers

In situ experiments were done to determine the effects of the photochemical degradation of dissolved organic matter (DOM) and subsequent formation of particulate matter on dissolved phosphorus (P) concentrations in surface waters. Filtered (1.2 or 0.4 μm) headwaters (DOM of 8.1–26 mg L−1; P of 22–43 μg L−1) were exposed to solar radiation in quartz bottles located 5 cm below the water surface for 7–10 days. Dark controls were wrapped in aluminum foil. After incubation, particulate organic carbon (POC) and particulate phosphorus (PP) were determined in both the filtrate and newly formed particles. The results revealed increasing concentrations of PP and POC in exposed samples with increasing exposure time (cumulative irradiation energy). At the end of experiments, PP concentrations were from 5 to 20 μg L−1 in the exposed samples. Based on an enumeration of bacteria in the samples, we estimated the contribution of biotic and abiotic processes to the PP production. The abiotic PP formation ranged from 56 to 83% and 50–95% of the total PP in the exposed and control samples, respectively. The remainder was assumed to be bacterial P uptake. Despite the overlapping intervals, biotic and abiotic PP productions were usually higher in exposed samples than in controls. The PP and POC production was affected by the properties of DOM, such as its humic content and freshness index. We hypothesize that the observed immobilization of dissolved P in bacteria and on photochemically-formed particles can contribute to a P limitation of primary production in headwater environments that receive waters rich in soil DOM.

Iron sources alter the response of Southern Ocean phytoplankton to ocean acidification

The rise in anthropogenic CO2 and the associated ocean acidification (OA) will change trace metal solubility and speciation, potentially altering Southern Ocean (SO) phytoplankton productivity and species composition. As iron (Fe) sources are important determinants of Fe bioavailability, we assessed the effect of Fe-laden dust versus inorganic Fe (FeCl3) enrichment under ambient and high pCO2 levels (390 and 900 μatm) in a naturally Fe-limited SO phytoplankton community. Despite similar Fe chemical speciation and net particulate organic carbon (POC) production rates, CO2-dependent species shifts were controlled by Fe sources. Final phytoplankton communities of both control and dust treatments were dominated by the same species, with an OA-dependent shift from the diatom Pseudo nitzschia prolongatoides towards the prymnesiophyte Phaeocystis antarctica. Addition of FeCl3 resulted in high abundances of Nitzschia lecointei and Chaetoceros neogracilis under ambient and high pCO2, respectively. These findings reveal that both the characterization of the phytoplankton community at the species level and the use of natural Fe sources are essential for a realistic projection of the biological carbon pump in the Fe-limited pelagic SO under OA. As dust deposition represents a more realistic scenario for the Fe-limited pelagic SO under OA, unaffected net POC production and dominance of P. antarctica can potentially weaken the export of carbon and silica in the future.

Elevated alkaline phosphatase activity in a phosphate‐replete environment: Influence of sinking particles

AbstractAlkaline phosphatase activity (APA) is traditionally a proxy for phosphate (DIP)‐limitation because it is induced by DIP‐limited microbes to access the labile ester fraction of the organic phosphorus (OP) pool. Here, we present multi‐year summertime depth distributions of APA and enzyme kinetics in the DIP‐replete Celtic Sea. Our findings support the cumulating body of evidence that APA has a potentially widespread role in OP remineralization through the water column. APA and Vmax were positively correlated with depth and DIP, with total APA being threefold higher below (0.93 ± 0.32 nM P h−1) compared to above the thermocline (0.30 ± 0.24 nM P h−1, p < 0.001). Separation of particles by sinking speed demonstrated that APA was eightfold higher on fast sinking (Ffast) particles compared to slow sinking particles (Fslow; p < 0.05). When normalized to particulate organic carbon (POC) and bacterial production (BP), APAPOC and APABP associated with Ffast (0.76 ± 0.10 nmol P μmol C−1 h−1, 21.13 ± 2.2 nmol P nmol C−1, respectively) were fourfold and 25‐fold higher compared to the combined APA associated with dissolved plus suspended (Fsusp) and Fslow fractions (0.19 ± 0.06 nmol P μmol C−1 h−1 and 0.84 ± 0.23 nmol P nmol C−1, respectively). We postulate that this may reflect enhanced ectoenzyme activity associated with bacteria colonizing particle surfaces and/or release by zooplankton via faecal pellet excretion. Knowledge of the disparity between APA and BP associated with particle and dissolved phases is required to accurately define the O2 : P ratio of regenerated P derived from sinking particles as a result of AP‐facilitated remineralization.

Effects of ocean acidification on the physiological performance and carbon production of the Antarctic sea ice diatom Nitzschia sp. ICE-H

Ocean acidification (OA) resulting from increasing atmospheric CO2 strongly influences marine ecosystems, particularly in the polar ocean due to greater CO2 solubility. Here, we grew the Antarctic sea ice diatom Nitzschia sp. ICE-H in a semicontinuous culture under low (~400ppm) and high (1000ppm) CO2 levels. Elevated CO2 resulted in a stimulated physiological response including increased growth rates, chlorophyll a contents, and nitrogen and phosphorus uptake rates. Furthermore, high CO2 enhanced cellular particulate organic carbon production rates, indicating a greater shift from inorganic to organic carbon. However, the cultures grown in high CO2 conditions exhibited a decrease in both extracellular and intracellular carbonic anhydrase activity, suggesting that the carbon concentrating mechanisms of Nitzschia sp. ICE-H may be suppressed by elevated CO2. Our results revealed that OA would be beneficial to the survival of this sea ice diatom strain, with broad implications for global carbon cycles in the future ocean.

Independence of nutrient limitation and carbon dioxide impacts on the Southern Ocean coccolithophore Emiliania huxleyi

Future oceanic conditions induced by anthropogenic greenhouse gas emissions include warming, acidification and reduced nutrient supply due to increased stratification. Some parts of the Southern Ocean are expected to show rapid changes, especially for carbonate mineral saturation. Here we compare the physiological response of the model coccolithophore Emiliania huxleyi (strain EHSO 5.14, originating from 50oS, 149oE) with pH/CO2 gradients (mimicking ocean acidification ranging from 1 to 4 × current pCO2 levels) under nutrient-limited (nitrogen and phosphorus) and -replete conditions. Both nutrient limitations decreased per cell photosynthesis (particulate organic carbon (POC) production) and calcification (particulate inorganic carbon (PIC) production) rates for all pCO2 levels, with more than 50% reductions under nitrogen limitation. These impacts, however, became indistinguishable from nutrient-replete conditions when normalized to cell volume. Calcification decreased three-fold and linearly with increasing pCO2 under all nutrient conditions, and was accompanied by a smaller ~30% nonlinear reduction in POC production, manifested mainly above 3 × current pCO2. Our results suggest that normalization to cell volume allows the major impacts of nutrient limitation (changed cell sizes and reduced PIC and POC production rates) to be treated independently of the major impacts of increasing pCO2 and, additionally, stresses the importance of including cell volume measurements to the toolbox of standard physiological analysis of coccolithophores in field and laboratory studies.

Physiological responses and lipid storage of the coral Lophelia pertusa at varying food density

ABSTRACTDespite the importance of the cold-water coral Lophelia pertusa to deep-sea reef ecosystem functioning, current knowledge of key physiological responses to available food resources is scarce. Scenarios with varying food density may help to understand how corals deal with seasonal variations in the dark ocean and might be used to study consequences of anthropogenic activities potentially affecting food availability. Thus, the physiological responses of L. pertusa to varying food (Artemia salina nauplii) concentration, ranging from 20% to 300% of carbon equivalent turned over by basal coral respiration, were investigated. A starvation group was also included. Measurements of respiration, growth, mucus production, and energy reserves (storage fatty acids) were performed at several time intervals over 26 weeks. In general, data showed a stronger effect of experimental time on measured responses, but no significant influence of food density treatment. In starved corals, respiration rate declined to 52% of initial respiration, while skeleton growth rate was maintained at the same rate as Artemia-fed corals throughout the investigation. Mucus production measured as the sum of dissolved organic carbon (DOC) and particulate organic carbon (POC) was also similar across food treatments, but POC production exceeded that of DOC at the highest food density. No marked effect was observed on storage fatty acids. These results confirm that L. pertusa is highly resilient to environmental conditions with suboptimal food densities over a time scale of months. Regulation of several physiological processes, including respiration and mucus production, possibly in combination with an opportunistic feeding strategy, contributed to this tolerance to maintain viable corals. Thus, it appears that L. pertusa is well adapted to life in the deep sea.

Dynamics and production of dissolved organic carbon in a large continental shelf system under the influence of both river plume and coastal upwelling

AbstractWe examined the dynamics and production of dissolved organic carbon (DOC) on a large continental shelf in the northern South China Sea, which is largely shaped by a river plume and coastal upwelling, based on a cruise in summer 2008. The plume water extended from the mouth of the Pearl River estuary to the middle shelf and was characterized by high DOC concentrations, while the upwelled water occupying the nearshore area featured low DOC concentrations. Biological production of DOC was observed in both the river plume and the coastal upwelling zones with different behavior between regions. The system appeared to be autotrophic in terms of DOC throughout the plume, while in the upwelling circulation, the metabolism of DOC was mixed trophic. Nevertheless, the integrated net DOC production rate of 11.5 ± 6.9 mmol C m−2 d−1 in the upwelling zone was comparable to that in the plume (7.1 ± 7.0 mmol C m−2 d−1). The net DOC production correlated strongly with net consumption of dissolved inorganic carbon (DIC) and inorganic nutrients, suggesting that the net DOC production was highly coupled to net community production (NCP) in both the plume and upwelling zones. Both regimes had similar DOC/NCP partitioning, with 19–27% of NCP in the plume and 24–26% of NCP in the upwelling zones converted to DOC. A positive correlation was also found between particulate organic carbon (POC) and net DIC consumption, with higher POC production in the upwelling zones where large phytoplankton prevailed. Most NCP removal occurred through POC sinking and/or the diffusion and horizontal transport of DOC.

Mixed‐layer carbon cycling at the Kuroshio Extension Observatory

AbstractSeven years of data from the NOAA Kuroshio Extension Observatory (KEO) surface mooring, located in the North Pacific Ocean carbon sink region, were used to evaluate drivers of mixed‐layer carbon cycling. A time‐dependent mass balance approach relying on two carbon tracers was used to diagnostically evaluate how surface ocean processes influence mixed‐layer carbon concentrations over the annual cycle. Results indicate that the annual physical carbon input is predominantly balanced by biological carbon uptake during the intense spring bloom. Net annual gas exchange that adds carbon to the mixed layer and the opposing influence of net precipitation that dilutes carbon concentrations make up smaller contributions to the annual mixed‐layer carbon budget. Decomposing the biological term into annual net community production (aNCP) and calcium carbonate production (aCaCO3) yields 7 ± 3 mol C m−2 yr−1 aNCP and 0.5 ± 0.3 mol C m−2 yr−1 aCaCO3, giving an annually integrated particulate inorganic carbon to particulate organic carbon production ratio of 0.07 ± 0.05, as a lower limit. Although we find that vertical physical processes dominate carbon input to the mixed layer at KEO, it remains unclear how horizontal features, such as eddies, influence carbon production and export by altering nutrient supply as well as the depth of winter ventilation. Further research evaluating linkages between Kuroshio Extension jet instabilities, eddy activity, and nutrient supply mechanisms is needed to adequately characterize the drivers and sensitivities of carbon cycling near KEO.

Stable carbon isotope signals in particulate organic and inorganic carbon of coccolithophores – A numerical model study for Emiliania huxleyi

A recent numerical cell model, which explains observed light and carbonate system effects on particulate organic and inorganic carbon (POC and PIC) production rates under the assumption of internal pH homeostasis, is extended for stable carbon isotopes (12C, 13C). Aim of the present study is to mechanistically understand the stable carbon isotopic fractionation signal (ε) in POC and PIC and furthermore the vital effect(s) included in measured εPIC values. The virtual cell is divided into four compartments, for each of which the 12C as well as the 13C carbonate system kinetics are implemented. The compartments are connected to each other via trans-membrane fluxes. In contrast to existing carbon fractionation models, the presented model calculates the disequilibrium state for both carbonate systems and for each compartment. It furthermore calculates POC and PIC production rates as well as εPOC and εPIC as a function of given light conditions and the compositions of the external carbonate system. Measured POC and PIC production rates as well as εPIC values are reproduced well by the model (comparison with literature data). The observed light effect on εPOC (increase of εPOC with increasing light intensities), however, is not reproduced by the basic model set-up, which is solely based on RubisCO fractionation. When extending the latter set-up by assuming that biological fractionation includes further carbon fractionation steps besides the one of RubisCO, the observed light effect on εPOC is also reproduced. By means of the extended model version, four different vital effects that superimpose each other in a real cell can be detected. Finally, we discuss potential limitations of the εPIC proxy.

Light Intensity Modulates the Response of Two Antarctic Diatom Species to Ocean Acidification

It is largely unknown how rising atmospheric CO2 concentrations and changes in the upper mixed layer depth, with its subsequent effects on light availability will affect phytoplankton physiology in the Southern Ocean. Linking seasonal variations in the availability of CO2 and light to abundances and physiological traits of key phytoplankton species could aid to understand their abilities to acclimate to predicted future climatic conditions. To investigate the combined effects of CO2 and light on two ecologically relevant Antarctic diatoms (Fragilariopsis curta and Odontella weisflogii) a matrix of three light intensities (LL=20, ML=200, HL=500 µmol photons m-2 s-1) and three pCO2 levels (low=180, ambient=380, high=1000 µatm) was applied assessing their effects on growth, particulate organic carbon (POC) fixation and photophysiology. Under ambient pCO2, POC production rates were highest already at low light in Fragilariopsis, indicating saturation of photosynthesis, while in Odontella highest rates were only reached at medium irradiances. In both species ocean acidification did not stimulate, but rather inhibited, growth and POC production under low and medium light. This effect was, however, amended under high growth irradiances. Low pCO2 levels inhibited growth and POC production in both species at low and medium light, and further decreased absETRs under high light. Our results suggest that Southern Ocean diatoms were sensitive to changes in pCO2, showing species-specific responses, which were further modulated by light intensity. The two diatom species represent distinct ecotypes and revealed discrete physiological traits that matched their seasonal occurrence with the related physical conditions in Antarctic coastal waters.

Intraspecific Differences in Biogeochemical Responses to Thermal Change in the Coccolithophore Emiliania huxleyi.

The species concept in marine phytoplankton is defined based on genomic, morphological, and functional properties. Reports of intraspecific diversity are widespread across major phytoplankton groups but the impacts of this variation on ecological and biogeochemical processes are often overlooked. Intraspecific diversity is well known within coccolithophores, which play an important role in the marine carbon cycle via production of particulate inorganic carbon. In this study, we investigated strain-specific responses to temperature in terms of morphology, carbon production, and carbonate mineralogy using a combination of microscopy, elemental analysis, flow cytometry, and nuclear magnetic resonance. Two strains of the cosmopolitan coccolithophore E. huxleyi isolated from different regions (subtropical, CCMP371; temperate, CCMP3266) were cultured under a range of temperature conditions (10°C, 15°C, and 20°C) using batch cultures and sampled during both exponential and stationary growth. Results for both strains showed that growth rates decreased at lower temperatures while coccosphere size increased. Between 15°C and 20°C, both strains produced similar amounts of total carbon, but differed in allocation of that carbon between particulate inorganic carbon (PIC) and particulate organic carbon (POC), though temperature effects were not detected. Between 10°C and 20°C, temperature effects on daily production of PIC and POC, as well as the cellular quota of POC were detected in CCMP3266. Strain-specific differences in coccolith shedding rates were found during exponential growth. In addition, daily shedding rates were negatively related to temperature in CCMP371 but not in CCMP3266. Despite differences in rates of particulate inorganic carbon production, both strains were found to produce coccoliths composed entirely of pure calcite, as established by solid-state 13C and 43Ca NMR and X-ray diffraction measurements. These results highlight the limitations of the species concept and the need for a trait-based system to better quantify diversity within marine phytoplankton communities.

H+ -driven increase in CO2 uptake and decrease in HCO3− uptake explain coccolithophores' acclimation responses to ocean acidification

Recent ocean acidification (OA) studies revealed that seawater [H+] rather than [CO2] or [HCO3-] regulate short-term responses in carbon fluxes of Emiliania huxleyi. Here, we investigated whether acclimation to altered carbonate chemistry modulates this regulation pattern and how the carbon supply for calcification is affected by carbonate chemistry. We acclimated E. huxleyi to present-day (ambient [CO2], [HCO3-], and pH) and OA conditions (high [CO2], ambient [HCO3-], low pH). To differentiate between the CO2 and pH/H+ effects, we also acclimated cells to carbonation (high [CO2] and [HCO3-], ambient pH) and acidification (ambient [CO2], low [HCO3-], and pH). Under these conditions, growth, production of particulate inorganic and organic carbon, as well as carbon and oxygen fluxes were measured. Under carbonation, photosynthesis and calcification were stimulated due to additional inline image uptake, whereas growth was unaffected. Such stimulatory effects are not apparent after short-term carbonation, indicating that cells adjusted their carbon acquisition during acclimation. Being driven by H+, these regulations can, however, not explain typical OA effects. Under acidification and OA, photosynthesis stayed constant, whereas calcification and growth decreased. Similar to the short-term responses toward high [H+], CO2 uptake significantly increased, but inline image uptake decreased. This antagonistic regulation in CO2 and inline image uptake can explain why photosynthesis, being able to use CO2 and inline image, often benefits from OA, whereas calcification, being mostly dependent on inline image, often decreases. We identified H+ as prime driver of coccolithophores' acclimation responses toward OA. Acidified conditions seem to put metabolic burdens on the cells that result in decreased growth.

The Growth Response of Two Diatom Species to Atmospheric Dust from the Last Glacial Maximum.

Relief of iron (Fe) limitation in the surface Southern Ocean has been suggested as one driver of the regular glacial-interglacial cycles in atmospheric carbon dioxide (CO2). The proposed cause is enhanced deposition of Fe-bearing atmospheric dust to the oceans during glacial intervals, with consequent effects on export production and the carbon cycle. However, understanding the role of enhanced atmospheric Fe supply in biogeochemical cycles is limited by knowledge of the fluxes and ‘bioavailability’ of atmospheric Fe during glacial intervals. Here, we assess the effect of Fe fertilization by dust, dry-extracted from the Last Glacial Maximum portion of the EPICA Dome C Antarctic ice core, on the Antarctic diatom species Eucampia antarctica and Proboscia inermis. Both species showed strong but differing reactions to dust addition. E. antarctica increased cell number (3880 vs. 786 cells mL-1), chlorophyll a (51 vs. 3.9 μg mL-1) and particulate organic carbon (POC; 1.68 vs. 0.28 μg mL-1) production in response to dust compared to controls. P. inermis did not increase cell number in response to dust, but chlorophyll a and POC per cell both strongly increased compared to controls (39 vs. 15 and 2.13 vs. 0.95 ng cell-1 respectively). The net result of both responses was a greater production of POC and chlorophyll a, as well as decreased Si:C and Si:N incorporation ratios within cells. However, E, antarctica decreased silicate uptake for the same nitrate and carbon uptake, while P. inermis increased carbon and nitrate uptake for the same silicate uptake. This suggests that nutrient utilization changes in response to Fe addition could be driven by different underlying mechanisms between different diatom species. Enhanced supply of atmospheric dust to the surface ocean during glacial intervals could therefore have driven nutrient-utilization changes which could permit greater carbon fixation for lower silica utilization. Additionally, both species responded more strongly to lower amounts of direct Fe chloride addition than they did to dust, suggesting that not all the Fe released from dust was in a bioavailable form available for uptake by diatoms.