Effects of environmental acidification on adult ranid  frogs

The rapid worldwide decline in frog populations is now a well recognized phenomena which has attracted research efforts from scientists all over the planet in a concerted effort to understand the causes for this alarming occurrence. This problem has also attracted the attention of the World Conservation Union (IUCN) and the Species Survival Commission (SSC) which established the Declining Amphibian Populations Task Force (DAPTF) in 1991. The World Conservation Union is an umbrella organization linking many governmental and non-governmental organizations around the world. The SSC is the largest Commission within the IUCN. The DAPTF is a global network of biologists and conservationists concerned with the issue of declining amphibian populations.

There are good reasons for thinking that the disappearance of amphibians from human impacted and pristine environments is particularly alarming. Amphibians may be serving as a measure of the health of the environment; they perform important ecological functions, are a source of biomedicinal supplies (drugs manufactured from frog skins) and have an overall aesthetic appeal.

Several reasons have been proposed for the decline in amphibian populations including:

During the past several years, a number of faculty in the biology department at Widener University have been working to elucidate the effects of environmental stress on adult ranid frogs. In particular, we have been studying the effects of acidification on immune responses of frogs. It turns out that varied species of frogs show very different levels of tolerance upon exposure to mild acid conditions (pH 5.5). Rana pipiens are more susceptible to acid environments than other species tested and their immune response is compromised to a much greater degree than those of other frogs exposed to the same acid conditions.(etc_2006.pdf)

In 1996 we  conducted a study designed to answer the question: Can adult frogs sense the pH of their environment and orient themselves towards a favorable pH? The results of this study, were published  in the September 99 issue of the Journal of Herpetology (see JH_1999.pdf), and described in short below.

We exposed  adult Rana pipiens to mild aci d conditions for a ten day period under controlled laboratory conditions.  Frogs exposed to citrate buffered water at pH 5.5 for 10 days exhibited 72% mortality as compared with 3.5% mortality in the control group held at pH 7.0.  Furthermore, within the pH 5.5 group there was a difference in the acid sensitivity based on the physiological state of the frogs. Frogs that had recently emerged from hibernation exhibited 100% mortality within the first four days of exposure to pH 5.5. This contrasts  to frogs that were post breeding and suffered  58% mortality throughout the ten days of the experiment. Our results suggest that Rana pipiens are sensitive to mild acidic conditions, especially those emerging from hibernation.

We pondered  whether Rana pipiens exposed to pH 5.5 elevate their metabolism as a response to this stress. Our data indicates  that whole body metabolism  of acid-exposed frogs for ten days, is not different from the metabolism of frogs exposed to pH 7.0 during the entire experimental period, in spite of the large differences in their survival (see abstract SICB 1998).

Based on data from several experiments we have developed a theoretical model (See JEB 2003) that provides a possible explanation for the effects of environmental acidification on the natural defense mechanisms of ranid frogs. This model also suggests that different parts of the natural defense systems of Rana pipiens exhibit differential sensitivities to acid exposure. We suggest that the gut epithelia may be compromised when frogs are exposed to an environmental  pH of  6.0. In contrast, frogs exposed to pH 6.0 have the same total number of WBC/spleen as controls (frogs exposed to pH 7.0), the same percent WBC as controls and the same percent viable WBC (Journal of Herpetology 2002).  

We are the first to demonstrate acid sensitivity of adult R. pipiens (Vatnick et al, 1999). Our results fit well with hypotheses by other investigators (Glorioso et al., 1974; Carey, 1993; Maniero and Carey, 1997). These authors suggested that environmental stress is the initiating factor in a cascade of physiological events. These events may start with immunosuppression followed by systemic distribution of opportunistic and virulent bacteria, and may ultimately lead to the death of adult frogs. We have recently published  (Environmental Toxicology and Chemistry 2006) further evidence to our model (See JEB 2003) and suggested that environment pollutants and toxicants serve as immunosupressors (see also SETAC GLOBE 2004 for our first coining of the term).
 
We also examined the interaction of cold and acid exposure on the immune function of ranid frogs and found out that cold exposure by itself does not cause a systemic bacterial infection in adult Rana pipiens, but acid stress following cold exposure does ( Journal of Experimental Zoology 2003). 

Our  most current experiments use peritoneal thyoglycollate injections  to induce an inflammatory response.  This technique is widely used to study immune responses in several species.  We are working on characterization of the acid effects on the amelioration of this response.  We are interested in the  signals involved in the initation and supression of the immune response in the presence and absence of an acidic environment. Specifically, we are studying the role of Corticosterone and C Reactive Proteins (see our SICB 2003 poster)

 

Recently we have investigated the effects of atrazine on the natural defense mechanisms of Rana pipiens.  Atrazine exposure suppresses the innate immune response of these frogs in very similar ways that acid does (see SETAC GLOBE 2004).  Therefore we  coined the term "immune disruptors" to describe environmental pollutants that disrupt immune function to parallel the term endocrine disruptors commonly used to describe the effect of certain environmental pollutants on endocrine function.

References

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Blaustein, A.R.,. Hoffman, P.D,. Hokit, D.G,., Kiesecker , J.M,. Walls, S.C. and Hays, J.B.(1994). UV repair and resistance to solar UV-B in amphibian eggs: a link to population declines. Proceedings of the National Academy of Science, USA 91: 1791-1795.

Blaustein, A.R. and Wake D. B. (1995). The puzzle of declining amphibian populations. Scientific American 272 : 52-57.

Carey, C. (1993). Hypothesis concerning the causes of the disappearance of boreal toads from the mountains of Colorado. Conservation Biology 7, (2)355-362.

Cunningham, A.A., Langton, T.E.S., Bennett, P.M., Drury, S.E.N., Gough, R.E. and Kirkwood, J.K. (1993). Unusual mortality associated withpoxvirus-like particles in frogs (Rana temporaria). Veterinary Record 133, 141-142.

Glorioso, J. C., R. L. Amborski, G. F. Amborski, and D. D. Culley.  1974.  Microbiological studies on septicemic bullfrogs (Rana catesbeiana).  Am. J. Vet. Res. 35:1241-1244.

Harte J. and Hoffman, E. (1989). Possible effects of acidic deposition on a Rocky Mountain population of the tiger salamander Ambystroma tigrinum. Conserv. Biol., 3, 149-58

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Howdeshell K. L. and Smalley K. N. (1996). Effects of estrogen on reproductive parameters of the male plains leopard frog (Rana blairi). Kansas Academy of Science, Annual Meeting, March 22. Division of Biological Sciences, Emporia State University, Emporia, KS 66801.

Lannoo M. (1996). Fish introductions and aquacultural practices are the most serious threat to upper midwestern amphibian populations. Froglog 17, 1.

Laurance, W.F., McDonald, K.R. and Speare, R. (1996). Epidemic disease and the catastrophic decline of Australian rain forest frogs. Conservation Biology 10, (2) 406-413.

Maniero, G. D., and C. Carey.  1997.  Changes in selected aspects of immune function in the leopard frog, Rana pipiens, associated with exposure to cold.  J. Comp. Physiol. B. 167:256-263.

I. Vatnick, M. A. Brodkin, M. P. Simon, B.W. Grant, C. R. Conte, M. Gleave, R. Myers, and M. M. Sadoff. (1999). The effects of exposure to mild acidic conditions on adult frogs (Rana pipiens and Rana clamitans): Mortality rates and pH preferences. Journal of Herpetology : in press
 

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