Microsensor studies on Padina from a natural CO2 seep: implications of morphology on acclimation to low pH

Low seawater pH can be harmful to many calcifying marine organisms, but the calcifying macroalgae Padina spp. flourish at natural submarine carbon dioxide seeps where seawater pH is low. We show that the microenvironment created by the rolled thallus margin of Padina australis facilitates supersaturation of CaCO₃ and calcifi‐cation via photosynthesis‐induced elevated pH. Using microsensors to investigate oxygen and pH dynamics in the microenvironment of P. australis at a shallow CO₂ seep, we found that, under saturating light, the pH inside the microenvironment (pH_ME) was higher than the external seawater (pH_SW) at all pH_SW levels investigated, and the difference (i.e., pH_ME ? pH_SW) increased with decreasing pH_SW (0.9 units at pH_SW 7.0). Gross photosynthesis (P_g) inside the microenvironment increased with decreasing pH_SW, but algae from the control site reached a threshold at pH 6.5. Seep algae showed no pH threshold with respect to P_g within the pH_SW range investigated. The external carbonic anhydrase (CA) inhibitor, acetazolamide, strongly inhibited P_g of P. australis at pH_SW 8.2, but the effect was diminished under low pH_SW (6.4–7.5), suggesting a greater dependence on membrane‐bound CA for the dehydration of HCO₃^? ions during dissolved inorganic carbon uptake at the higher pH_SW. In comparison, a calcifying green alga, Halimeda cuneata f. digitata, was not inhibited by AZ, suggesting efficient bicarbonate transport. The ability of P. australis to elevate pH_ME at the site of calcification and its strong dependence on CA may explain why it can thrive at low pH_SW.     

Photosynthesis and calcification in the calcifying algae Halimeda discoidea studied with microsensors

With microsensors, we measured the steady‐state microprofiles of O₂, pH and Ca²⁺ on the topside of young segments of Halimeda discoidea, as well as the surface dynamics upon light–dark shifts. The effect of several inhibitors was studied. The steady‐state measurements showed that under high light intensity, calcium and protons were taken up, while O₂ was produced. In the dark, O₂ was consumed, the pH decreased to below seawater level and Ca²⁺ uptake was reduced to 50%. At low light intensity (12 mmol photons m^‐2 s^‐1), Ca²⁺ efflux was observed. Upon light–dark shifts, a complicated pattern of both the pH and calcium surface dynamics was observed. Illumination caused an initial pH decrease, followed by a gradual pH increase: this indicated that the surface pH of H. discoidea is determined by more than one light‐induced process. When photosynthesis was inhibited by dichlorophenyl dimethyl urea (DCMU), a strong acidification was observed upon illumination. The nature and physiological function of this putative pump is not known. The calcium dynamics followed all pH dynamics closely, both in the presence and absence of DCMU. The Ca‐channel blockers verapamil and nifedipine had no effect on the Ca²⁺ dynamics and steady‐state profiles. Thus, in H. discoidea, calcification is not regulated by the alga, but is a consequence of pH increase during photosynthesis. Acetazolamide had no effect on photosynthesis, whereas ethoxyzolamide inhibited photosynthesis at higher light intensities. Therefore, all carbonic anhydrase activity is intracellular. Carbonic anhydrase is required to alleviate the CO₂ limitation. Calcification cannot supply sufficient protons and CO₂ to sustain photosynthesis.     

Growth inhibition of hybridoma cells by ammonium ion: correlation with effects on intracellular pH

The effect of NH₄Cl addition on intracellular pH (pH _i ) was determined by flow cytometric measurements of the fluorescence of a pH-sensitive dye. The effects of NH₄Cl on growth were determined for batch growth of cells in flasks in an incubator. The addition of NH₄Cl caused a cytoplasmic acidification. A new lower steady-state value of pH _i was attained within 20–40 min of NH₄Cl addition. A correlation was found between the effects of NH₄Cl on growth and on pH _i : whereas 3 mM NH₄Cl had little effect on growth and on pH _i , 10 mM NH₄Cl caused a substantial growth inhibition and a pH _i decrease of 0.2–0.3 units. The effects of NH₄Cl on growth and on pH _i were found to be independent of the external pH value (pH _e over the range 6.8 to 7.6, except that 10 mM NH₄Cl was more toxic at pH _e 7.6. The addition of NH₄Cl caused an increase in the average cell volume at pH _e 7.6, but had no effect on the average cell volume at pH _e 's 6.8 and 7.2. For comparison, the effects of pH _e alone on growth and on pH _i were determined. There was little difference in cell growth at pH _e 's 6.8, 7.2 and 7.6. At pH _e 6.6, there was a substantial growth inhibition. Some measurements of the effects of pH _e on pH _i were made, although the steady-state value of pH _i as a function of pH _e was not determined due to limitations in the pH _i -measuring technique. These measurements showed that pH _i remained constant from pH _e 7.6 to 6.8, but fell by 0.2 units at pH _e 6.6, in agreement with the growth results.     

Intracellular pH of symbiotic dinoflagellates

Intracellular pH (pH_i) is likely to play a key role in maintaining the functional success of cnidarian–dinoflagellate symbiosis, yet until now the pH_i of the symbiotic dinoflagellates (genus Symbiodinium) has never been quantified. Flow cytometry was used in conjunction with the ratiometric fluorescent dye BCECF to monitor changes in pH_i over a daily light/dark cycle. The pH_i of Symbiodinium type B1 freshly isolated from the model sea anemone Aiptasia pulchella was 7.25 ± 0.01 (mean ± SE) in the light and 7.10 ± 0.02 in the dark. A comparable effect of irradiance was seen across a variety of cultured Symbiodinium genotypes (types A1, B1, E1, E2, F1, and F5) which varied between pH_i 7.21–7.39 in the light and 7.06–7.14 in the dark. Of note, there was a significant genotypic difference in pH_i, irrespective of irradiance.     

Coralline algae elevate pH at the site of calcification under ocean acidification

Coralline algae provide important ecosystem services but are susceptible to the impacts of ocean acidification. However, the mechanisms are uncertain, and the magnitude is species specific. Here, we assess whether species‐specific responses to ocean acidification of coralline algae are related to differences in pH at the site of calcification within the calcifying fluid/medium (pH_cf) using δ¹¹B as a proxy. Declines in δ¹¹B for all three species are consistent with shifts in δ¹¹B expected if B(OH)₄^? was incorporated during precipitation. In particular, the δ¹¹B ratio in Amphiroa anceps was too low to allow for reasonable pH_cf values if B(OH)₃ rather than B(OH)₄^? was directly incorporated from the calcifying fluid. This points towards δ¹¹B being a reliable proxy for pH_cf for coralline algal calcite and that if B(OH)₃ is present in detectable proportions, it can be attributed to secondary postincorporation transformation of B(OH)₄^?. We thus show that pH_cf is elevated during calcification and that the extent is species specific. The net calcification of two species of coralline algae (Sporolithon durum, and Amphiroa anceps) declined under elevated CO₂, as did their pH_cf. Neogoniolithon sp. had the highest pH_cf, and most constant calcification rates, with the decrease in pH_cf being ¼ that of seawater pH in the treatments, demonstrating a control of coralline algae on carbonate chemistry at their site of calcification. The discovery that coralline algae upregulate pH_cf under ocean acidification is physiologically important and should be included in future models involving calcification.     

An increase in the intracellular pH of fertilized eggs of Xenopus laevis is associated with inhibition of protein and DNA syntheses and followed by an arrest of embryonic development

In many systems, events participating in cell division are controlled by intracellular pH (pH_i). In Xenopus eggs, fertilization is accompanied by an increase in pH_i which occurs concomitantly with an increase in protein synthesis and a reinitiation of DNA synthesis, leading the embryo to cell division. In this paper, we have shown that increasing pH_i of fertilized eggs from 7.8 to 8.2 by using weak bases produced an arrest in embryonic development. Such a change in pH_i was accompanied by a severe inhibition of both protein and DNA syntheses. In order to discriminate between a direct effect of pH_i and a pH-independent effect of weak bases on these biosyntheses, the situation was studied in vitro. For this purpose, cytoplasmic extracts were used in which weak base addition did not produce any change in pH. Under these conditions, protein synthesis was not inhibited, suggesting that pH is probably one of the events implicated in the regulation of protein synthesis. On the other hand, DNA synthesis was inhibited by weak bases in vitro, without any change in pH intervening.     

Assessing physiological tipping point of sea urchin larvae exposed to a broad range of pH

Our ability to project the impact of global change on marine ecosystem is limited by our poor understanding on how to predict species sensitivity. For example, the impact of ocean acidification is highly species‐specific, even in closely related taxa. The aim of this study was to test the hypothesis that the tolerance range of a given species to decreased pH corresponds to their natural range of exposure. Larvae of the green sea urchin Strongylocentrotus droebachiensis were cultured from fertilization to metamorphic competence (29 days) under a wide range of pH (from pH_T = 8.0/pCO₂ ≈ 480 μatm to pH_T = 6.5/pCO₂ ≈ 20 000 μatm) covering present (from pH_T 8.7 to 7.6), projected near‐future variability (from pH_T 8.3 to 7.2) and beyond. Decreasing pH impacted all tested parameters (mortality, symmetry, growth, morphometry and respiration). Development of normal, although showing morphological plasticity, swimming larvae was possible as low as pH_T ≥ 7.0. Within that range, decreasing pH increased mortality and asymmetry and decreased body length (BL) growth rate. Larvae raised at lowered pH and with similar BL had shorter arms and a wider body. Relative to a given BL, respiration rates and stomach volume both increased with decreasing pH suggesting changes in energy budget. At the lowest pHs (pH_T ≤ 6.5), all the tested parameters were strongly negatively affected and no larva survived past 13 days post fertilization. In conclusion, sea urchin larvae appeared to be highly plastic when exposed to decreased pH until a physiological tipping point at pH_T = 7.0. However, this plasticity was associated with direct (increased mortality) and indirect (decreased growth) consequences for fitness.     

The intracellular pH of isolated,photosynthetically active Asparagus mesophyll cells

The intracellular pH of isolated, photosynthetically active mesophyll cells of Asparagus sprengeri Regel has been determined, in the light and dark, by the distribution of the weak acid 5,5-dimethyl-[2-¹⁴C]oxazolidine-2,4-dione ([¹⁴C]DMO) between the cells and the liquid medium. [¹⁴C]DMO was taken up rapidly, reaching equilibrium in 7–10 min of incubation, but was not metabolized by the cells, and intracellular binding of the compound was minimal. The intracellular pH, measured at saturating light fluence and 1.5 mM sodium bicarbonate, was found to remain relatively constant at 6.95–7.21 over the external pH range of 5.5–7.2. Illumination of the cells increased the intracellular pH compared to dark controls. The pH of the cytoplasm, excluding and including the chloroplasts (cytoplasmic and bulk cytoplasmic, respectively) was calculated from the experimentally derived intracellular [¹⁴C]DMO concentration and estimates of the vacuolar, chloroplastic and cytoplasmic volumes. The calculated cytoplasmic pH was similar in the light and dark, being 7.75 and 7.74, respectively, while the calculated pH of bulk cytoplasm was 7.85 in the light and 7.49 in the dark. Theoretical analysis indicated that intracellular pH is a good indicator of changes in the bulk cytoplasmic pH but insensitive to changes in vacuolar pH. The external pH optimum for photosynthesis (O₂ evolution) of isolated Asparagus cells was pH 7.2. At pH 8.0 photosynthesis was inhibited by 30% and at pH 5.25 by 45%. Inhibition at alkaline pH may be the result of a decrease in the pH gradient between the cells and the medium, causing CO₂ limitation in the cell. At acid pH, decrease in internal pH caused by substantial accumulation of inorganic carbon may account for the loss in photosynthetic activity.Abbreviations [¹⁴C]DMO 5,5-dimethyl[2-¹⁴C]oxazolidine-2,4-dione - pH_i overall intracellular pH - pH_e pH of external medium     

Near‐future pH conditions severely impact calcification,metabolism and the nervous system in the pteropod Heliconoides inflatus

Shelled pteropods play key roles in the global carbon cycle and food webs of various ecosystems. Their thin external shell is sensitive to small changes in pH, and shell dissolution has already been observed in areas where aragonite saturation state is ~1. A decline in pteropod abundance has the potential to disrupt trophic networks and directly impact commercial fisheries. Therefore, it is crucial to understand how pteropods will be affected by global environmental change, particularly ocean acidification. In this study, physiological and molecular approaches were used to investigate the response of the Mediterranean pteropod, Heliconoides inflatus, to pH values projected for 2100 under a moderate emissions trajectory (RCP6.0). Pteropods were subjected to pH_T 7.9 for 3 days, and gene expression levels, calcification and respiration rates were measured relative to pH_T 8.1 controls. Gross calcification decreased markedly under low pH conditions, while genes potentially involved in calcification were up‐regulated, reflecting the inability of pteropods to maintain calcification rates. Gene expression data imply that under low pH conditions, both metabolic processes and protein synthesis may be compromised, while genes involved in acid–base regulation were up‐regulated. A large number of genes related to nervous system structure and function were also up‐regulated in the low pH treatment, including a GABA_A receptor subunit. This observation is particularly interesting because GABA_A receptor disturbances, leading to altered behavior, have been documented in several other marine animals after exposure to elevated CO₂. The up‐regulation of many genes involved in nervous system function suggests that exposure to low pH could have major effects on pteropod behavior. This study illustrates the power of combining physiological and molecular approaches. It also reveals the importance of behavioral analyses in studies aimed at understanding the impacts of low pH on marine animals.     

The acid tolerance response of Bacillus cereus ATCC14579 is dependent on culture pH, growth rate and intracellular pH

The food pathogen Bacillus cereus is likely to encounter acidic environments (i) in food when organic acids are added for preservation purposes, and (ii) during the stomachal transit of aliments. In order to characterise the acid stress response of B. cereus ATCC14579, cells were grown in chemostat at different pH values (pH_o from 9.0 to 5.5) and different growth rates (μ from 0.1 to 0.8 h⁻¹), and were submitted to acid shock at pH 4.0. Cells grown at low pH_o were adapted to acid media and induced a significant acid tolerance response (ATR). The ATR induced was modulated by both pH_o and μ, and the μ effect was more marked at pH_o 5.5. Intracellular pH (pH_i) was affected by both pH_o and μ. At a pH_o above 6, the pH_i decreased with the decrease of pH_o and the increase of μ. At pH_o 5.5, pH_i was higher compared to pH_o 6.0, suggesting that mechanisms of pH_i homeostasis were induced. The acid survival of B. cereus required protein neo-synthesis and the capacity of cells to maintain their pH_i and ΔpH (pH_i - pH_o). Haemolysin BL and non-haemolytic enterotoxin production were both influenced by pH_o and μ.     

Ocean acidification effects on calcification and dissolution in tropical reef macroalgae

Net calcification rates for coral reef and other calcifiers have been shown to decline as ocean acidification (OA) occurs. However, the role of calcium carbonate dissolution in lowering net calcification rates is unclear. The objective of this study was to distinguish OA effects on calcification and dissolution rates in dominant calcifying macroalgae of the Florida Reef Tract, including two rhodophytes (Neogoniolithon strictum, Jania adhaerens) and two chlorophytes (Halimeda scabra, Udotea luna). Two experiments were conducted: (1) to assess the difference in gross (⁴⁵Ca uptake) versus net (total alkalinity anomaly) calcification rates in the light/dark and (2) to determine dark dissolution (⁴⁵CaCO₃), using pH levels predicted for the year 2100 and ambient pH. At low pH in the light, all species maintained gross calcification rates and most sustained net calcification rates relative to controls. Net calcification rates in the dark were ~84% lower than in the light. In contrast to the light, all species had lower net calcification rates in the dark at low pH with chlorophytes exhibiting net dissolution. These data are supported by the relationship (R² = 0.82) between increasing total alkalinity and loss of ⁴⁵Ca from pre-labelled ⁴⁵CaCO₃ thalli at low pH in the dark. Dark dissolution of ⁴⁵CaCO₃-labelled thalli was ~18% higher in chlorophytes than rhodophytes at ambient pH, and ~ twofold higher at low pH. Only Udotea, which exhibited dissolution in the light, also had lower daily calcification rates integrated over 24 h. Thus, if tropical macroalgae can maintain high calcification rates in the light, lower net calcification rates in the dark from dissolution may not compromise daily calcification rates. However, if organismal dissolution in the dark is additive to sedimentary carbonate losses, reef dissolution may be amplified under OA and contribute to erosion of the Florida Reef Tract and other reefs that exhibit net dissolution.

     

Octocoral Tissue Provides Protection from Declining Oceanic pH

Increase in anthropogenic pCO₂ alters seawater chemistry and could lead to reduced calcification or skeleton dissolution of calcifiers and thereby weaken coral-reef structure. Studies have suggested that the complex and diverse responses in stony coral growth and calcification, as a result of elevated pCO₂, can be explained by the extent to which their soft tissues cover the underlying skeleton. This study compared the effects of decreased pH on the microstructural features of both in hospite (within the colony) and isolated sclerites (in the absence of tissue protection) of the zooxanthellate reef-dwelling octocoral Ovabunda macrospiculata. Colonies and isolated sclerites were maintained under normal (8.2) and reduced (7.6 and 7.3) pH conditions for up to 42 days. Both in hospite and isolated sclerites were then examined under SEM and ESEM microscopy in order to detect any microstructural changes. No differences were found in the microstructure of the in hospite sclerites between the control and the pH treatments. In stark contrast, the isolated sclerites revealed dissolution damage related to the acidity of the water. These findings suggest a protective role of the octocoral tissue against adverse pH conditions, thus maintaining them unharmed at high pCO₂. In light of the competition for space with the less resilient reef calcifiers, octocorals may thus have a significant advantage under greater than normal acidic conditions.     

Seawater pH,and not inorganic nitrogen source,affects pH at the blade surface of Macrocystis pyrifera: implications for responses of the giant kelp to future oceanic conditions

Ocean acidification (OA), the ongoing decline in seawater pH, is predicted to have wide‐ranging effects on marine organisms and ecosystems. For seaweeds, the pH at the thallus surface, within the diffusion boundary layer (DBL), is one of the factors controlling their response to OA. Surface pH is controlled by both the pH of the bulk seawater and by the seaweeds' metabolism: photosynthesis and respiration increase and decrease pH within the DBL (pH_DBL), respectively. However, other metabolic processes, especially the uptake of inorganic nitrogen (N_i; NO₃^? and NH₄⁺) may also affect the pH_DBL. Using Macrocystis pyrifera, we hypothesized that (1) NO₃^? uptake will increase the pH_DBL, whereas NH₄⁺ uptake will decrease it, (2) if NO₃^? is cotransported with H⁺, increases in pH_DBL would be greater under an OA treatment (pH = 7.65) than under an ambient treatment (pH = 8.00), and (3) decreases in pH_DBL will be smaller at pH 7.65 than at pH 8.00, as higher external [H⁺] might affect the strength of the diffusion gradient. Overall, N_i source did not affect the pH_DBL. However, increases in pH_DBL were greater at pH 7.65 than at pH 8.00. CO₂ uptake was higher at pH 7.65 than at pH 8.00, whereas HCO₃^? uptake was unaffected by pH. Photosynthesis and respiration control pH_DBL rather than N_i uptake. We suggest that under future OA, Macrocystis pyrifera will metabolically modify its surface microenvironment such that the physiological processes of photosynthesis and N_i uptake will not be affected by a reduced pH.     

Primary Production and Calcification by the Soritid Foraminifer Archais angulatus (Fichtel & Moll)*

SYNOPSIS. Symbiosis between Chlamydomonas hedleyi (Lee, Crockett, Hagen & Stone) and Archais angulatus (Fichtel & Moll) was examined during laboratory studies of primary production and light-enhanced calcification. Photosynthesis and calcification are directly proportional to light intensity in the range of 0–200 μEinsteins m^-2 sec^-1. Calcification in the light is directly proportional to photosynthesis and proceeds at rates that are 2–3 times that observed in the dark. The herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), in concentrations of 1–100 μM, completely inhibits photosynthesis and light-enhanced calcification. Calcification of the foraminiferan test is therefore due to the photosynthetic activity of the symbiote. Calcification rates for foraminifers incubated in the dark or with DCMU are not significantly different from the calcification rates obtained for dead foraminifers. Rates of calcification obtained with ⁴⁵Ca are twice that obtained with ¹⁴C.     

Effects of High Dissolved Inorganic and Organic Carbon Availability on the Physiology of the Hard Coral Acropora millepora from the Great Barrier Reef

Coral reefs are facing major global and local threats due to climate change-induced increases in dissolved inorganic carbon (DIC) and because of land-derived increases in organic and inorganic nutrients. Recent research revealed that high availability of labile dissolved organic carbon (DOC) negatively affects scleractinian corals. Studies on the interplay of these factors, however, are lacking, but urgently needed to understand coral reef functioning under present and near future conditions. This experimental study investigated the individual and combined effects of ambient and high DIC (pCO₂ 403 μatm/ pH_Total 8.2 and 996 μatm/pH_Total 7.8) and DOC (added as Glucose 0 and 294 μmol L^-1, background DOC concentration of 83 μmol L^-1) availability on the physiology (net and gross photosynthesis, respiration, dark and light calcification, and growth) of the scleractinian coral Acropora millepora (Ehrenberg, 1834) from the Great Barrier Reef over a 16 day interval. High DIC availability did not affect photosynthesis, respiration and light calcification, but significantly reduced dark calcification and growth by 50 and 23%, respectively. High DOC availability reduced net and gross photosynthesis by 51% and 39%, respectively, but did not affect respiration. DOC addition did not influence calcification, but significantly increased growth by 42%. Combination of high DIC and high DOC availability did not affect photosynthesis, light calcification, respiration or growth, but significantly decreased dark calcification when compared to both controls and DIC treatments. On the ecosystem level, high DIC concentrations may lead to reduced accretion and growth of reefs dominated by Acropora that under elevated DOC concentrations will likely exhibit reduced primary production rates, ultimately leading to loss of hard substrate and reef erosion. It is therefore important to consider the potential impacts of elevated DOC and DIC simultaneously to assess real world scenarios, as multiple rather than single factors influence key physiological processes in coral reefs.     

Intracellular pH regulation in the mouse lacrimal gland acinar cells

Summary Intracellular pH (pH _i ) of the acinar cells of the isolated, superfused mouse lacrimal gland has been measured using pH-sensitive microelectrodes. Under nonstimulated condition pH _i was 7.25, which was about 0.5 unit higher than the equilibrium pH. Alterations of the external pH by ±0.4 unit shifted pH _i only by ±0.08 unit. The intracellular buffering value determined by applications of 25mm NH ₄ ⁺ and bicarbonate buffer solution gassed with 5% CO₂/95% O₂ was 26 and 46mm/pH, respectively Stimulation with 1 m acetylcholine (ACh) caused a transient, small decrease and then a sustained increase in pH _i . In the presence of amiloride (0.1mm) or the absence of Na⁺, application of ACh caused a significant decrease in pH _i and removal of amiloride or replacement with Na⁺-containing saline, respectively, rapidly increased the pH _i . Pretreatment with DIDS (0.2mm) did not change the pH _i of the nonstimulated conditions; however, it significantly enhanced the increase in pH _i induced by ACh. The present results showed that (i) there is an active acid extrusion mechanism that is stimulated by ACh; (ii) stimulation with ACh enhances the rate of acid production in the acinar cells; and (iii) the acid extrusion mechanism is inhibited by amiloride addition to and Na⁺ removal from the bath solution. We suggest that both Na⁺/H⁺ and HCO ₃ ^– /Cl^– exchange transport mechanisms are taking roles in the intracellular pH regulation in the lacrimal gland acinar cells.     

Effect of intracellular pH on the light-induced potential change and electrogenic activity in tonoplast-free cells of Chora australis

Effects of the intracellular H+ concentration on the membranepotential and membrane resistance of tonoplast-free cells ofChara australis were examined under light and dark conditions.Cells were made tonoplast-free by perfusing the vacuole withmedia containing 5 mM EGTA and 30 mM buffers of various pH values.The electrogenic pump was stopped by removing the intracellularATP. The ATP-dependent part of the membrane potential was largeat pH₁ 6.2 and 6.9, but decreased in both the acidic (pH₁ 5.1,5.7) and alkaline (pH₁ 7.9, 8.7) ranges. Assuming that the putativeelectrogenic H+ pump acts solely as a current source, we estimatedthe current and the work done by the pump. Both the currentand the work were at a maximum at pH₁ 6.9, but decreased tosome extent at pH₁ 7.9–8.7 and decreased greatly at pH₁5.7–5.1. Light caused a significant membrane hyperpolarization at pH₁6.2–7.9, but only a slight hyperpolarization at pH₁ 5.1,5.7 and 8.7. The presence of light-induced hyperpolarizationat pH₁ lower than 6.2 suggests that acidification of the cytoplasmupon illumination does not cause activation of the putativeH+ pump. Membrane resistance was very high at pH₁ 5.1 and 5.7 and verylow at pH₁ 8.7. The very high R_m values at pH₁ 5.1 and 5.6 arebelieved to be related to inhibition of the activity of thepump. (Received June 20, 1979; )     

Models for relating pH measurements in water and calcium chloride for a wide range of pH,soil types and depths

Soil pH is commonly measured in water (pHw) or 0.01 M CaCl₂ (pH_Ca). The need to convert between these methods has led to the publication of linear, quadratic and cubic polynomial relationships for limited suites of soils. Concerns over the applicability of such relationships when mapping a wide range of soils and pH led to the establishment of a database of pH_W and pH_Ca values on each of 7894 samples from soil survey and field experimental sites in Queensland. The relationship between pH_W and pH_Ca across all soils was investigated and preliminary results examining the effect of soil depth and soil type on the relationship are presented.For all soils and depths, a linear regression accounted for 93.2% of the variation but did not predict pH_Ca well at very high or low pH_W values. The inclusion of second and third powers of pH_W accounted for significantly more of the variation (R²=0.94) in pH_Ca and the resultant curve matched the data better at high and low pH.Analysis of surface, sub-surface and subsoil groupings did not reveal any appreciable differences in the relationship between pH_W and pH_Ca attributable to depth. In contrast, differences in the relationship were evident between soil types. Generally, the mildly leached soils had linear relationships, while the weathered soils were distinctly curvilinear at low pH.     

BICARBONATE STIMULATION OF CALCIFICATION AND PHOTOSYNTHESIS IN TWO HERMATYPIC CORALS1

A wide range of bicarbonate concentrations was used to monitor the kinetics of bicarbonate (HCO₃^?) use in both photosynthesis and calcification in two reef‐building corals, Porites porites and Acropora sp. Experiments carried out close to the P. porites collection site in Barbados showed that additions of NaHCO₃ to synthetic seawater proportionally increased the calcification rate of this coral until the concentration exceeded three times that of seawater (6 mM). Photosynthetic rates were also stimulated by HCO₃^? addition, but these became saturated at a lower concentration (4 mM). Similar experiments on aquarium‐acclimated colonies of Indo‐Pacific Acropora sp. showed that calcification and photosynthesis in this coral were enhanced to an even greater extent than P. porites, with calcification continuing to increase above 8 mM HCO₃^?, and photosynthesis saturating at 6 mM. Calcification rates of Acropora sp. were also monitored in the dark, and, although these were lower than in the light for a given HCO₃^? concentration, they still increased dramatically with HCO₃^? addition, showing that calcification in this coral is light stimulated but not light dependent.