Daphnia-pH#1 - BMC Physiology BioMed Central Open Access...

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Bio Med Central Page 1 of 25 (page number not for citation purposes) BMC Physiology Open Access Research article Physiological responses of Daphnia pulex to acid stress Anna K Weber and Ralph Pirow* Address: Institute of Zoophysiology, University of Münster, Münster, Germany Email: Anna K Weber - akweber@gmx.de; Ralph Pirow* - pirow@uni-muenster.de * Corresponding author Abstract 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 CO 2 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 ( pH = 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 non- bicarbonate 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 CO 2 transport raised the question of whether a carbonic anhydrase (CA) is involved in the catalysis of the reaction, which led to the discovery of 31 CA-genes in the genome of D. pulex . Published: 21 April 2009 BMC Physiology 2009, 9 :9 doi:10.1186/1472-6793-9-9 Received: 29 February 2008 Accepted: 21 April 2009 This article is available from: http://www.biomedcentral.com/1472-6793/9/9 © 2009 Weber and Pirow; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0 ), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Daphnia-pH#1 - BMC Physiology BioMed Central Open Access...

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