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Biological impacts of deep-sea carbon dioxide injection inferred from indices of physiological performance

Brad A. Seibel^1,* and Patrick J. Walsh²

¹ Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039, USA
² Marine Biology and Fisheries, National Institute of Environmental Health Sciences, Marine and Freshwater Biomedical Science Center, Rosenstiel School of Marine and Atmospheric Sciences, 4600 Rickenbacker Causeway, University of Miami, Miami, FL 33149, USA

View larger version (37K): [in a new window]   Fig. 1. Schematic representation of an animal cell with the potential means of regulating intracellular pH. (1), metabolic interconversion of acids and bases. (2), buffering; HA represents a weak acid or base with a dissociation constant in the physiological pH range. (3), transport of acids and bases across cell membranes; carbonic anhydrase (CA) catalyzes the hydration of CO2 to yield H2CO3, which then dissociates to H+, HCO3-, and CO32- (an abbreviated reaction is shown).

View larger version (13K): [in a new window]   Fig. 2. A Davenport diagram, a graphical representation of the Henderson Hasselbalch equation (pH=pK+log[HCO3-]/[CO2]), demonstrating a typical time course for compensation of extracellular (blood) acidosis. Numbers between points represent time (h). Within 1 h of acidotic stress (A—B), extracellular pH generally drops according to the buffering capacity of the plasma. Over the next 12-24 h (B—C), bicarbonate (y-axis; mmol l-1) is transported into the cell (or protons out) in order to shift the equilibrium towards higher pH values. Upon return to normal seawater CO2 tensions, there is a rapid increase in pH (C—D), due again to passive reactions, followed by a slower decompensation phase (D—A) leading to restoration of the original acid—base status. Intracellular pH and bicarbonate concentrations generally follow those in the extracellular fluid. See Cameron (1989) for additional details on acid—base balance.

View larger version (23K): [in a new window]   Fig. 3. Metabolic rates (open blue circles) of fishes, cephalopods and crustaceans as a function of minimum depth of occurrence (the depth below which 90% of the individuals in a population are captured). Also shown is the capacity for buffering of intracellular fluids in cephalopods (green circles) and the pH sensitivity of respiratory proteins (red circles) in crustaceans, fishes and cephalopods. Buffering capacity is measured in `slykes', here equal to the quantity of base that must be added to a homogenate made from a 1 g sample of muscle to titrate the pH from approximately 6 to 7. The Bohr coefficient is the change in the log of respiratory oxygen affinity (P50; defined as the oxygen partial pressure at which the respiratory protein is half-saturated) over the change in pH. Bohr coefficients in these animal groups are negative but are presented here as absolute values. The metabolic rates are normalized to a common body mass of 10 g and measurement temperature of 5°C using measured scaling coefficients and Q10 values where available or assuming a scaling coefficient of -0.25 and a Q10 of 2. Data are from Childress and Seibel (1998) and references therein. Note that the y-axis is a log scale.

View larger version (15K): [in a new window]   Fig. 4. Davenport diagram depicting passive buffering of intracellular pH (black lines) in two cephalopod species. Similar increases in CO2 partial pressure (in mmHg; 1 mmHg=133.3 Pa; represented by the blue isopleths and numbers) will result in dramatically different changes in intracellular pH in shallow-(Stenoteuthis oualaniensis) and deep-living (Japetella heathi) cephalopods. Buffering data are from Seibel et al. (1997).

View larger version (11K): [in a new window]   Fig. 5. Percentage hemocyanin-oxygen saturation as a function of oxygen partial pressure PO2 (mmHg; 1 mmHg=133.3 Pa) at pH 7.61 and 7.36 for Benthoctopus sp. (B. A. Seibel, unpublished data). Ambient PO2 at capture depth (51 mmHg) is indicated by an arrow. At ambient PO2, a drop in blood pH of 0.3 units results in a 40% decrease in hemocyanin saturation. All measurements were on dialyzed hemolymph at 5°C. Changes in pH were achieved by varying CO2 concentrations, thus we cannot distinguish between pH and CO2 effects on oxygen binding.