Physiological responses of Daphnia pulex to acid stress

Abstract

BackgroundAcidity 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 (PCO2), 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.ResultsD. pulex had a remarkably high extracellular pH of 8.33 and extracellular PCO2 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 non-bicarbonate buffer value from 2.0 to 5.1 meq L-1 pH-1. The extracellular PCO2 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.ConclusionChronic exposure to acidic conditions had a pervasive impact on Daphnia's physiology including acid-base balance, extracellular PCO2, 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 reaction, which led to the discovery of 31 CA-genes in the genome of D. pulex.

Physiological and ecological responses of crustaceans to ocean acidification

“…Consequently, environmental disruption of haemolymph acid -base status is more likely to be compensated in strong iono-and osmoregulators, where ion exchange mechanisms are well developed. This relationship could well explain why freshwater crustaceans, which are strong iono-and osmoregulators, can survive considerable acidification of their freshwater habits (Abrahamsson 1972, McMahon & Stuart 1989, Felten et al 2008, Weber & Pirow 2009). Likewise, strong iono-and osmoregulators are likely to be less vulnerable to ocean acidification, because they possess the mechanisms that enable them to compensate for haemolymph acid -base disturbances, at least in the shorter term.…”

Cladocera Community Composition as a Function of Physicochemical and Morphological Parameters of Dystrophic Lakes in NE Poland

“…However, their relative abundances were rather low ( Table 2), suggesting that dystrophic environments are not optimal for their development. Daphnia longispina group is also known for its low tolerance to acidic waters, being incapable of surviving at the low pH that characterizes dystrophic lakes; pH 5.5 is considered the lower limit for this taxa (Alibone 1981, Weber andPirow 2009). The occurrence of Daphnia (in 3 lakes) and Eubosmina (in 11 lakes) at very low relative abundances was likely more associated with the morphometry of the studied lakes (small and shallow) than with dystrophic conditions.…”

Combined salinity and acidity stressors alterDaphnia magnapopulation growth and structure under severe absence of photoperiod

“…Under these conditions, daphnids reproduce asexually by parthenogenesis resulting in a female-only population. The ability to endure a range of abiotic stressors or sensitivity to contaminants made daphnids a key model to study tolerance [2,46], phenotypic plasticity [47], and toxicity in aquatic environments [48].…”

“…The genome of Daphnia may express different phenotypes in reaction to different environmental conditions; a phenomenon referred to as phenotypic plasticity [7]. Rapid responses to environmental stressors, such as ionic and osmotic regulation [8], which may be due to transgenerational epigenetic effects [9–10], are associated with high energetic costs [2,11], and may enable Daphnia to occupy a wider niche range compared to other fresh water inhabitants [12]. However, low pH levels have been shown to reduce zooplankton species richness and alter the structure of cladoceran populations [13–14], leading to diminished survival and growth of D. magna below pH 5 [15].…”

“…However, low pH levels have been shown to reduce zooplankton species richness and alter the structure of cladoceran populations [13–14], leading to diminished survival and growth of D. magna below pH 5 [15]. For example, Daphnia species are less abundant in acidified lakes compared to other arthropods ( e.g ., calanoid copepods and insects), which may therefore dominate the ecosystem instead of Daphnia [2,16].…”