Physiological responses of Daphnia pulex to acid stress

Journal Article · 20 April 2009 · BMC Physiology

DOI: https://doi.org/10.1186/1472-6793-9-9 · OSTI ID:1629784

Weber, Anna K. ^[1]; Pirow, Ralph ^[2]

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1. Univ. of Munster (Germany). Inst. of Zoophysiology; DOE/OSTI
2. Univ. of Munster (Germany). Inst. of Zoophysiology

Background: Acidity exerts a determining influence on the composition and diversity of freshwater faunas. While the physiological implications of freshwater acidification have been intensively studied in teleost fish and crayfish, much less is known about the acid-stress physiology of ecologically important groups such as cladoceran zooplankton. This study analyzed the extracellular acid-base state and CO₂ partial pressure (P_CO2), circulation and ventilation, as well as the respiration rate of Daphnia pulex acclimated to acidic (pH 5.5 and 6.0) and circumneutral (pH 7.8) conditions. Results: D. pulex had a remarkably high extracellular pH of 8.33 and extracellular P_CO2 of 0.56 kPa under normal ambient conditions (pH 7.8 and normocapnia). The hemolymph had a high bicarbonate concentration of 20.9 mM and a total buffer value of 51.5 meq L^-1 pH^-1. Bicarbonate covered 93% of the total buffer value. Acidic conditions induced a slight acidosis (ΔpH = 0.16–0.23), a 30–65% bicarbonate loss, and elevated systemic activities (tachycardia, hyperventilation, hypermetabolism). pH 6.0 animals partly compensated the bicarbonate loss by increasing the nonbicarbonate buffer value from 2.0 to 5.1 meq L^-1 pH^-1. The extracellular P_CO2 of pH 5.5 animals was significantly reduced to 0.33 kPa, and these animals showed the highest tolerance to a short-term exposure to severe acid stress. Conclusion: Chronic exposure to acidic conditions had a pervasive impact on Daphnia's physiology including acid-base balance, extracellular P_CO2, circulation and ventilation, and energy metabolism. Compensatory changes in extracellular non-bicarbonate buffering capacity and the improved tolerance to severe acid stress indicated the activation of defense mechanisms which may result from gene-expression mediated adjustments in hemolymph buffer proteins and in epithelial properties. Mechanistic analyses of the interdependence between extracellular acid-base balance and CO₂ transport raised the question of whether a carbonic anhydrase (CA) is involved in the catalysis of the CO₂-HCO_{$$\overline{3}$$}-H⁺ reaction, which led to the discovery of 31 CA-genes in the genome of D. pulex.

• Research Organization: Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Biological and Environmental Research (BER). Biological Systems Science Division
• Grant/Contract Number: AC02-05CH11231
• OSTI ID: 1629784
• Journal Information: BMC Physiology, Journal Name: BMC Physiology Journal Issue: 1 Vol. 9; ISSN 1472-6793
• Publisher: BioMed CentralCopyright Statement
• Country of Publication: United States
• Language: English