Seawater carbonate chemistry and carbon allocation, growth and morphology of the coccolithophore Emiliania huxleyi (calcifying strain CCMP 371) during experiments, 2011

Coccolithophores are unicellular phytoplankton that produce calcium carbonate coccoliths as an exoskeleton. Emiliania huxleyi, the most abundant coccolithophore in the world's ocean, plays a major role in the global carbon cycle by regulating the exchange of CO2 across the ocean-atmosphere interface through photosynthesis and calcium carbonate precipitation. As CO2 concentration is rising in the atmosphere, the ocean is acidifying and ammonium (NH4) concentration of future ocean water is expected to rise. The latter is attributed to increasing anthropogenic nitrogen (N) deposition, increasing rates of cyanobacterial N2 fixation due to warmer and more stratified oceans, and decreased rates of nitrification due to ocean acidification. Thus future global climate change will cause oceanic phytoplankton to experience changes in multiple environmental parameters including CO2, pH, temperature and nitrogen source. This study reports on the combined effect of elevated pCO2 and increased NH4 to nitrate (NO3) ratio (NH4/NO3) on E. huxleyi, maintained in continuous cultures for more than 200 generations under two pCO2 levels and two different N sources. Here we show that NH4 assimilation under N-replete conditions depresses calcification at both low and high pCO2, alters coccolith morphology, and increases primary production. We observed that N source and pCO2 synergistically drive growth rates, cell size and the ratio of inorganic to organic carbon. These responses to N source suggest that, compared to increasing CO2 alone, a greater disruption of the organic carbon pump could be expected in response to the combined effect of increased NH4/NO3 ratio and CO2 level in the future acidified ocean. Additional experiments conducted under lower nutrient conditions are needed prior to extrapolating our findings to the global oceans. Nonetheless, our results emphasize the need to assess combined effects of multiple environmental parameters on phytoplankton biology in order to develop accurate predictions of phytoplankton responses to ocean acidification.

Grazing rate of microzooplankton measured experimentally in the North Atlantic during the Maria S. Merian cruise MSM26, spring 2013

The microzooplankton grazing dilution experiments were conducted at stations 126, 127, 131 and 133-137, following Landry & Hassett (1982). Seawater samples (whole seawater - WSW) were taken via Niskin bottles mounted on to a CTD Rosette out of the chlorophyll maximum at each station. Four different dilution levels were prepared with WSW and GF/F filtered seawater - 100% WSW, 75% WSW, 50% WSW and 25% WSW. The diluted WSW was filled in 2.4 L polycarbonate bottles (two replicates for every dilution level). Three subsamples (250 - 500 mL depending on in situ chlorophyll) of the 100% WSW were filtered on to GF/F filters (25 mm diameter) and chlorophyll was extracted in 5 mL 96% ethanol for 12-24 hours. Afterwards it was measured fluorometrically before and after the addition of HCl with a Turner fluorometer according to Jespersen and Christoffersen (1987) on board of the ship. In addition, one 250 mL subsample of the 100% WSW was fixed in 2% Lugol (final concentration), to determine the microzooplankton community when back at the Institute for Hydrobiology and Fisheries Science in Hamburg. Also, one 50 mL subsample of the 100% WSW was fixed in 1 mL glutaraldehyde, to quantify bacteria abundance. The 2.4 L bottles were put in black mesh-bags, which reduced incoming radiation to approximately 50% (to minimize chlorophyll bleaching). The bottles were incubated for 24 hours in a tank on deck with flow-through water, to maintain in situ temperature. An additional experiment was carried out to test the effect of temperature on microzooplankton grazing in darkness. Therefore, 100% WSW was incubated in the deck tank and in two temperature control rooms of 5 and 15°C in darkness (two bottles each). The same was done with bottles where copepods were added (five copepods of Calanus finmarchicus in each bottle; males and females were randomly picked and divided onto the bottles). In addition, two 100% WSW bottles with five copepods each were incubated at in situ temperature at 100% light level (without mesh-bags). All experiments were incubated for 24 hours and afterwards two subsamples of each bottle were filtered on to GF/F filters (25 mm diameter); 500 - 1000 mL depending on in situ chlorophyll. One 250 mL subsample of one of the two replicates of each dilution level and each additional experiment (temperature and temperature/copepods) was fixed in 5 mL lugol for microzooplankton determination. One 50 mL subsample of one of the two 100% WSW bottles as well as of one of the additional experiments without copepods was fixed in 1 mL glutaraldehyde for bacteria determination later on. Copepods were fixed in 4% formaldehyde for length measurements and sex determination.

Direct visualization of solute locations in laboratory ice samples, links to videos

Many important chemical reactions occur in polar snow, where solutes may be present in several reservoirs, including at the air-ice interface and in liquid-like regions within the ice matrix. Some recent laboratory studies suggest chemical reaction rates may differ in these two reservoirs. While investigations have examined where solutes are found in natural snow and ice, similar research has not identified solute locations in laboratory samples, nor the possible factors controlling solute segregation. To address this, we examined solute locations in ice samples prepared from either aqueous cesium chloride (CsCl) or Rose Bengal solutions that were frozen using several different methods. Samples frozen in a laboratory freezer had the largest liquid-like inclusions and air bubbles, while samples frozen in a custom freeze chamber had somewhat smaller air bubbles and inclusions; in contrast, samples frozen in liquid nitrogen showed much smaller concentrated inclusions and air bubbles, only slightly larger than the resolution limit of our images (~2 µm). Freezing solutions in plastic versus glass vials had significant impacts on the sample structure, perhaps because the poor heat conductivity of plastic vials changes how heat is removed from the sample as it cools. Similarly, the choice of solute had a significant impact on sample structure, with Rose Bengal solutions yielding smaller inclusions and air bubbles compared to CsCl solutions frozen using the same method. Additional experiments using higher-resolution imaging of an ice sample show that CsCl moves in a thermal gradient, supporting the idea that the solutes in ice are present in liquid-like regions. Our work shows that the structure of laboratory ice samples, including the location of solutes, is sensitive to freezing method, sample container, and solute characteristics, requiring careful experimental design and interpretation of results.

Summary of physical properties on cores used for petrofabric analyses triaxial experiments at DSDP Leg 67 Holes

Deformation features within the cores are studied with a view towards elucidating the structure of the Middle America Trench along the transect drilled during Leg 67. Where possible, inferences are made as to the physical environment of deformation. Extensional tectonics prevails in the area of the seaward slope and trench. Fracturing and one well-preserved normal fault are found mostly within the lower Miocene chalks, at the base of the sedimentary section. These chalks have high porosities (40%-60%) and water content (30%-190%, based on % dry wt.). Experimental triaxial compression tests conducted on both dry and water-saturated samples of chalk from Holes 495 and 499B show that only in the saturated samples is more brittle behavior observed. Brittle failure of the chalks is greatly facilitated by pore fluid pressures that lead to low effective pressures. Additional embrittlement (weakening) can take place as a result of the imposed extensional stress resulting from bending of a subducting elastic oceanic plate. The chalks exhibit, in a landward direction, an increase in density and mechanical strength and a decrease in water content. These changes are attributed to mechanical compaction that may have resulted from tectonic horizontal compression. The structure of the landward slope is not well understood because the slope sites had to be abandoned due to the presence of gas hydrate. The relationship of the chaotic, brittle deformation (observed in the cores from Hole 494A) at the base of the landward slope to tectonic processes remains unclear. The deformation observed on the slope sites (Holes 496 and 497) is mostly fracturing and near-vertical sigmoidal veinlets. These are interpreted as being the result of gas/fluid overpressurization due to the decomposition of the gas hydrate, and not due to tectonic loading of accreted sediments. Aside from four small displacement (less than 1cm) reverse faults observed in the lower Miocene chalks (which may be the product of soft-sediment deformation), there is a noticeable absence of structures reflecting a dominance of horizontal (tectonic) compression along the transect drilled. The absence of such features, the lack of continuity of sediment types across the trench-landward slope, and the normal stratigraphic sequence in Hole 494A do not support any known accretionary model.

Model results of sensitivity experiments for marine nitrogen cycle in four NetCDF files

The marine nitrogen (N) inventory is thought to be stabilized by negative feedback mechanisms that reduce N inventory excursions relative to the more slowly overturning phosphorus inventory. Using a global biogeochemical ocean circulation model we show that negative feedbacks stabilizing the N inventory cannot persist if a close spatial association of N2 fixation and denitrification occurs. In our idealized model experiments, nitrogen deficient waters, generated by denitrification, stimulate local N2 fixation activity. But, because of stoichiometric constraints, the denitrification of newly fixed nitrogen leads to a net loss of N. This can enhance the N deficit, thereby triggering additional fixation in a vicious cycle, ultimately leading to a runaway N loss. To break this vicious cycle, and allow for stabilizing negative feedbacks to occur, inputs of new N need to be spatially decoupled from denitrification. Our idealized model experiments suggest that factors such as iron limitation or dissolved organic matter cycling can promote such decoupling and allow for negative feedbacks that stabilize the N inventory. Conversely, close spatial co-location of N2 fixation and denitrification could lead to net N loss.

Seawater carbonate chemistry and boron incorporation of planktic foraminifer Orbulina universa during experiments, 2011

Culture experiments with living planktic foraminifers reveal that the ratio of boron to calcium (B/Ca) in Orbulina universa increases from 56 to 92 µmol mol-1 when pH is raised from 7.61 +/- 0.02 to 8.67 +/- 0.03 (total scale). Across this pH range, the abundances of carbonate, bicarbonate, and borate ions also change (+ 530, - 500, and + 170 µmol kg-1, respectively). Thus specific carbonate system control(s) on B/Ca remain unclear, complicating interpretation of paleorecords. B/Ca in cultured O. universa also increases with salinity (55-72 µmol mol-1 from 29.9-35.4 per mil) and seawater boron concentration (62-899 µmol mol-1 from 4-40 ppm B), suggesting that these parameters may need to be taken into account for paleorecords spanning large salinity changes (~ 2 per mil) and for samples grown in seawater whose boron concentration ([B]SW) differs from modern by more than 0.25 ppm. While our results are consistent with the predominant incorporation of the charged borate species B(OH)4 into foraminiferal calcite, the behavior of the partition coefficient KD (defined as [B/Ca]calcite/B(OH)4/HCO3seawater) cannot be explained by borate incorporation alone, and suggests the involvement of other pH-sensitive ions such as CO3 For a given increase in seawater B(OH)4, the corresponding increase in B/Ca is stronger when B(OH)4 is raised by increasing [B]SW than when it is raised by increasing pH. These results suggest that B incorporation controls should be reconsidered. Additional insight is gained from laser-ablation ICP-MS profiles, which reveal variable B/Ca distributions within individual shells.

Seawater carbonate chemistry and shell length of northern abalone (Haliotis kamtschatkana) during experiments, 2011

Increasing levels of anthropogenic carbon dioxide in the world's oceans are resulting in a decrease in the availability of carbonate ions and a drop in seawater pH. This process, known as ocean acidification, is a potential threat to marine populations via alterations in survival and development. To date, however, little research has examined the effects of ocean acidification on rare or endangered species. To begin to assess the impacts of acidification on endangered northern abalone (Haliotis kamtschatkana) populations, we exposed H. kamtschatkana larvae to various levels of CO2 [400 ppm (ambient), 800 ppm, and 1800 ppm CO2] and measured survival, settlement, shell size, and shell development. Larval survival decreased by ca. 40% in elevated CO2 treatments relative to the 400 ppm control. However, CO2 had no effect on the proportion of surviving larvae that metamorphosed at the end of the experiment. Larval shell abnormalities became apparent in approximately 40% of larvae reared at 800 ppm CO2, and almost all larvae reared at 1800 ppm CO2 either developed an abnormal shell or lacked a shell completely. Of the larvae that did not show shell abnormalities, shell size was reduced by 5% at 800 ppm compared to the control. Overall, larval development of H. kamtschatkana was found to be sensitive to ocean acidification. Near future levels of CO2 will likely pose a significant additional threat to this species, which is already endangered with extinction due in part to limited reproductive output and larval recruitment.

Seawater carbonate chemistry, nutrients and growth rate of coral (Acropora intermedia) and coral-reef seaweed (Lobophora papenfussii) during experiments, 2011

Space competition between corals and seaweeds is an important ecological process underlying coral-reef dynamics. Processes promoting seaweed growth and survival, such as herbivore overfishing and eutrophication, can lead to local reef degradation. Here, we present the case that increasing concentrations of atmospheric CO2 may be an additional process driving a shift from corals to seaweeds on reefs. Coral (Acropora intermedia) mortality in contact with a common coral-reef seaweed (Lobophora papenfussii) increased two- to threefold between background CO2 (400 ppm) and highest level projected for late 21st century (1140 ppm). The strong interaction between CO2 and seaweeds on coral mortality was most likely attributable to a chemical competitive mechanism, as control corals with algal mimics showed no mortality. Our results suggest that coral (Acropora) reefs may become increasingly susceptible to seaweed proliferation under ocean acidification, and processes regulating algal abundance (e.g. herbivory) will play an increasingly important role in maintaining coral abundance.

Seawater carbonate chemistry and nutrient fluxes during experiments with brittlestar Amphiura filiformis, 2009

Rising levels of atmospheric carbon dioxide and the concomitant increased uptake of this by the oceans is resulting in hypercapnia-related reduction of ocean pH. Research focussed on the direct effects of these physicochemical changes on marine invertebrates has begun to improve our understanding of impacts at the level of individual physiologies. However, CO2-related impairment of organisms' contribution to ecological or ecosystem processes has barely been addressed. The burrowing ophiuroid Amphiura filiformis, which has a physiology that makes it susceptible to reduced pH, plays a key role in sediment nutrient cycling by mixing and irrigating the sediment, a process known as bioturbation. Here we investigate the role of A. filiformis in modifying nutrient flux rates across the sediment-water boundary and the impact of CO2- related acidification on this process. A 40 day exposure study was conducted under predicted pH scenarios from the years 2100 (pH 7.7) and 2300 (pH 7.3), plus an additional treatment of pH 6.8. This study demonstrated strong relationships between A. filiformis density and cycling of some nutrients; activity increases the sediment uptake of phosphate and the release of nitrite and nitrate. No relationship between A. filiformis density and the flux of ammonium or silicate were observed. Results also indicated that, within the timescale of this experiment, effects at the individual bioturbator level appear not to translate into reduced ecosystem influence. However, long term survival of key bioturbating species is far from assured and changes in both bioturbation and microbial processes could alter key biogeochemical processes in future, more acidic oceans.

Seawater carbonate chemistry and biological processes of foraminifera, Globigerinoides ruber and Globigerinella siphonifera during experiments, 2011

Specimens of two species of planktic foraminifera, Globigerinoides ruber and Globigerinella siphonifera, were grown under controlled laboratory conditions at a range of temperatures (18-31 °C), salinities (32-44 psu) and pH levels (7.9-8.4). The shells were examined for their calcium isotope compositions (d44/40Ca) and strontium to calcium ratios (Sr/Ca) using Thermal Ionization Mass Spectrometry and Inductively Coupled Plasma Mass Spectrometry. Although the total variation in d44/40Ca (~0.3 per mill) in the studied species is on the same order as the external reproducibility, the data set reveals some apparent trends that are controlled by more than one environmental parameter. There is a well-defined inverse linear relationship between d44/40Ca and Sr/Ca in all experiments, suggesting similar controls on these proxies in foraminiferal calcite independent of species. Analogous to recent results from inorganically precipitated calcite, we suggest that Ca isotope fractionation and Sr partitioning in planktic foraminifera are mainly controlled by precipitation kinetics. This postulation provides us with a unique tool to calculate precipitation rates and draws support from the observation that Sr/Ca ratios are positively correlated with average growth rates. At 25 °C water temperature, precipitation rates in G. siphonifera and G. ruber are calculated to be on the order of 2000 and 3000 µmol/m**2/h, respectively. The lower d44/40Ca observed at 29 °C in both species is consistent with increased precipitation rates at high water temperatures. Salinity response of d44/40Ca (and Sr/Ca) in G. siphonifera implies that this species has the highest precipitation rates at the salinity of its natural habitat, whereas increasing salinities appear to trigger higher precipitation rates in G. ruber. Isotope effects that cannot be explained by precipitation rate in planktic foraminifera can be explained by a biological control, related to a vacuolar pathway for supply of ions during biomineralization and a pH regulation mechanism in these vacuoles. In case of an additional pathway via cross-membrane transport, supplying light Ca for calcification, the d44/40Ca of the reservoir is constrained as -0.2 per mill relative to seawater. Using a Rayleigh distillation model, we calculate that calcification occurs in a semi-open system, where less than half of the Ca supplied by vacuolization is utilized for calcite precipitation. Our findings are relevant for interpreting paleo-proxy data on d44/40Ca and Sr/Ca in foraminifera as well as understanding their biomineralization processes.

Seawater carbonate chemistry and giant kelp Macrocystis pyrifera reproduction processes during experiments, 2011

The worldwide effects of ocean acidification (OA) on marine species are a growing concern. In temperate coastal seas, seaweeds are dominant primary producers that create complex habitats and supply energy to higher trophic levels. Studies on OA and macroalgae have focused on calcifying species and adult stages but, critically, they have overlooked the microscopic stages of the reproductive life cycle, which, for other anthropogenic stress e.g. UV-B radiation, are the most susceptible life-history phase. Also, environmental cues and stressors can cause changes in the sex ratio which has implications for the mating system and recruitment success. Here, we report the effects of pH (7.59-8.50) on meiospore germination and sex determination for the giant kelp, Macrocystis pyrifera (Laminariales), in the presence and absence of additional dissolved inorganic carbon (DIC). Lowered pH (7.59-7.60, using HCl-only) caused a significant reduction in germination, while added DIC had the opposite effect, indicating that increased CO2 at lower pH ameliorates physiological stress. This finding also highlights the importance of appropriate manipulation of seawater carbonate chemistry when testing the effects of ocean acidification on photosynthetic organisms. The proportion of male to female gametophytes did not vary significantly between treatments suggesting that pH was not a primary environmental modulator of sex. Relative to the baseline (pH 8.19), gametophytes were 32% larger under moderate OA (pH 7.86) compared to their size (10% increase) under extreme OA (pH 7.61). This study suggests that metabolically-active cells can compensate for the acidification of seawater. This homeostatic function minimises the negative effects of lower pH (high H+ ions) on cellular activity. The 6-9% reduction in germination success under extreme OA suggests that meiospores of M.pyrifera may be resistant to future ocean acidification.

Seawater carbonate chemistry and biological processes during experiments with the spider crab Hyas araneus in Kongsfjorden, Svalbard, 2009

With global climate change, ocean warming and acidification occur concomitantly. In this study, we tested the hypothesis that increasing CO2 levels affect the acid-base balance and reduce the activity capacity of the Arctic spider crab Hyas araneus, especially at the limits of thermal tolerance. Crabs were acclimated to projected oceanic CO2 levels for 12 days (today: 380, towards the year 2100: 750 and 1,120 and beyond: 3,000 ?atm) and at two temperatures (1 and 4 °C). Effects of these treatments on the righting response (RR) were determined (1) at acclimation temperatures followed by (2) righting when exposed to an additional acute (15 min) heat stress at 12 °C. Prior to (resting) and after the consecutive stresses of combined righting activity and heat exposure, acid-base status and lactate contents were measured in the haemolymph. Under resting conditions, CO2 caused a decrease in haemolymph pH and an increase in oxygen partial pressure. Despite some buffering via an accumulation of bicarbonate, the extracellular acidosis remained uncompensated at 1 °C, a trend exacerbated when animals were acclimated to 4 °C. The additional combined exposure to activity and heat had only a slight effect on blood gas and acid-base status. Righting activity in all crabs incubated at 1 and 4 °C was unaffected by elevated CO2 levels or acute heat stress but was significantly reduced when both stressors acted synergistically. This impact was much stronger in the group acclimated at 1 °C where some individuals acclimated to high CO2 levels stopped responding. Lactate only accumulated in the haemolymph after combined righting and heat stress. In the group acclimated to 1 °C, lactate content was highest under normocapnia and lowest at the highest CO2 level in line with the finding that RR was largely reduced. In crabs acclimated to 4 °C, the RR was less affected by CO2 such that activity caused lactate to increase with rising CO2 levels. In line with the concept of oxygen and capacity limited thermal tolerance, all animals exposed to temperature extremes displayed a reduction in scope for performance, a trend exacerbated by increasing CO2 levels. Additionally, the differences seen between cold- and warm-acclimated H. araneus after heat stress indicate that a small shift to higher acclimation temperatures also alleviates the response to temperature extremes, indicating a shift in the thermal tolerance window which reduces susceptibility to additional CO2 exposure.

Viral, bacterial and ciliate abundance and viral-bacterial diversity in mesocosm experiments, 2007-2008, Kongsfjorden

Two mesocosm experiments, PAME-I and PAME-II were conducted in 2007 and 2008 to investigate fate of organic carbon in the arctic microbial food web. Mesocosms were nutrient fertilized initially to induce phytoplankton bloom development. In PAME-I eight units (each 700 L) formed two four point gradients of additional DOC in form of glucose (0, 0.5, 1 and 3 times Redfield ratio in terms of carbon relative to the nitrogen and phosphorus additions) (Fig. 1). All the eight units also got a daily dose of NH4+ and PO4**3- in Redfield ratio. Two gradients were set up, one with silicate addition, performed in the Arctic location Ny Ålesund, Svalbard, have previously been reported to give different food-web level responses to similar nutrient perturbations. In PAME-II all ten units (each 900 L) formed two four point gradients of additional DOC in form of glucose (0, 0.5, 1, 2 and 3 times Redfield ratio in terms of carbon relative to nitrogen and phosphorus additions). The two gradients in glucose were kept silicate replete. NH4+ was used as the DIN source in one gradient (units 1 to 5) and NO3- in the other (units 6-9). All units got a daily dose of PO4**3- in Redfield ratio. Prokaryotes and viruses were measured by flow cytometry, while ciliate abundances were counted using a Flow Cam. Viral and bacterial diversity was measured by PFGE and DGGE, respectively. In PAME-II the abundance of ciliates was lower than in PAME-I, presumably caused by higher copepod grazing. The abundances of prokaryotes and viruses were also lower in PAME-II compared to PAME-I. Further, less diversity was detected in the viral community (FCM and PFGE) in PAME-II, and no response was observed in the bacterial community structure due to addition of organic carbon.

Seawater carbonate chemistry and processes during experiments with coral Acropora eurystoma, 2006

The rise in atmospheric CO2 has caused significant decrease in sea surface pH and carbonate ion (CO3-2) concentration. This decrease has a negative effect on calcification in hermatypic corals and other calcifying organisms. We report the results of three laboratory experiments designed specifically to separate the effects of the different carbonate chemistry parameters (pH, CO3-2, CO2 [aq], total alkalinity [AT], and total inorganic carbon [CT]) on the calcification, photosynthesis, and respiration of the hermatypic coral Acropora eurystoma. The carbonate system was varied to change pH (7.9-8.5), without changing CT; CT was changed keeping the pH constant, and CT was changed keeping the pCO2 constant. In all of these experiments, calcification (both light and dark) was positively correlated with CO3-2 concentration, suggesting that the corals are not sensitive to pH or CT but to the CO3-2 concentration. A decrease of ~30% in the CO3-2 concentration (which is equivalent to a decrease of about 0.2 pH units in seawater) caused a calcification decrease of about 50%. These results suggest that calcification in today's ocean (pCO2 = 370 ppm) is lower by ~20% compared with preindustrial time (pCO2 = 280 ppm). An additional decrease of ~35% is expected if atmospheric CO2 concentration doubles (pCO2 = 560 ppm). In all of these experiments, photosynthesis and respiration did not show any significant response to changes in the carbonate chemistry of seawater. Based on this observation, we propose a mechanism by which the photosynthesis of symbionts is enhanced by coral calcification at high pH when CO2(aq) is low. Overall it seems that photosynthesis and calcification support each other mainly through internal pH regulation, which provides CO3-2 ions for calcification and CO2(aq) for photosynthesis.

Seawater carbonate chemistry and hydrogen ions and carbonate chemistry in calcifying fluid of coral Astrangia poculata during experiments, 2011

A generalized physicochemical model of the response of marine organisms' calcifying fluids to CO2-induced ocean acidification is proposed. The model is based upon the hypothesis that some marine calcifiers induce calcification by elevating pH, and thus Omega aragonite, of their calcifying fluid by removing protons (H+). The model is explored through two end-member scenarios: one in which a fixed number of H+ is removed from their calcifying fluid, regardless of atmospheric pCO2, and another in which a fixed external-internal proton ratio ([H+]E/[H+]I) is maintained. The model is able to generate the full range of calcification response patterns observed in prior ocean acidification experiments and is consistent with the assertion that organisms' calcification response to ocean acidification is more negative for marine calcifiers that exert weaker control over their calcifying fluid pH. The model is empirically evaluated for the temperate scleractinian coral Astrangia poculata with in situ pH microelectrode measurements of the coral's calcifying fluid under control and acidified conditions. These measurements reveal that (1) the pH of the coral's calcifying fluid is substantially elevated relative to its external seawater under both control and acidified conditions, (2) the coral's [H+]E/[H+]I remains constant under control and acidified conditions, and (3) the coral removes fewer H+ from its calcifying fluid under acidified conditions than under control conditions. Thus, the carbonate system dynamics of A. poculata's calcifying fluid appear to be most consistent with the fixed [H+]E/[H+]I end-member scenario. Similar microelectrode experiments performed on additional taxa are required to assess the model's general applicability.