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Australia鈥檚 scarlet-sided pobblebonk frog thrives in waters so acidic they should be deadly, with its tadpoles tolerating water with a pH within the range of human stomach acid. This may be thanks to the frogs鈥� heightened ability to draw dissolved calcium from the water into their gills, alleviating the harmful effects of the acid.

Freshwater lakes and streams typically fall into the neutral range of 6 to 8 on the pH scale. But naturally occurring highly acidic freshwater ecosystems, such as South America鈥檚 Rio Negro, are found around the world, says at the University of Queensland in Australia.

In eastern Australia, there鈥檚 the Wallum 鈥� a sandy, coastal region whose swamps and lakes can plunge below pH 3.

Many creatures have adapted to such acidic ecosystems, including the Wallum鈥檚 scarlet-sided pobblebonk frogs (Limnodynastes terraereginae), whose tadpoles tolerate water about 32 times more acidic than the level that kills most frog species.

Low pH is normally detrimental for fish and amphibians because it causes cells in their gill lining to pull apart and lose sodium ions.

鈥淭he gills become leaky, and then the animal dies because it loses all its salt,鈥� says at the University of Alberta in Canada, who was not involved with the research.

Calcium is a crucial component of the junctions that hold gill lining cells together, so Cramp and her colleagues investigated the role of calcium ions in granting the pobblebonk its acid-thwarting powers. The team collected clumps of hundreds of pobblebonk eggs from Bribie Island, Queensland, and raised the frogs in the lab.

鈥淸The frogs鈥橾 ability to actually thrive at pH 3.5 is phenomenal and something that we marvel at still,鈥� says Cramp.

The researchers put some of the tadpoles in acidic water and others in acidic water with extra calcium. After 7 days, they measured the amount of sodium ions present in their bodies and the surrounding water. The tadpoles in low pH water with extra calcium lost fewer sodium ions than those in acidic water without calcium added, hinting at calcium鈥檚 protective role in acidic environments. Additionally, when the tadpoles were exposed to ruthenium red, a compound that interferes with channels that allow cells to take up calcium, the frogs lost their acid tolerance, leaking large amounts of sodium.

Thanks to analyses of RNA levels in the tadpoles鈥� gills, the team also found that tadpoles raised in pH 3.5 had greater activity of the gene for calbindin 鈥� a protein that transports calcium 鈥� in their gills than tadpoles raised in pH 6.5 conditions, regardless of the amount of calcium in the water.

The researchers think the tadpoles can supply the junctions between gill lining cells with a reliable stream of calcium by sucking it up from the water and storing it in cells, which stabilises the lining in highly acidic conditions.

鈥淚t鈥檚 a really nice description of yet another example of nature finding a way to live in some of these unique, low pH environments,鈥� says Goss.

Cramp notes that another potential mechanism to surviving low pH could be that the tadpoles evolved to change how their gills absorb sodium, and this is a subject her team鈥檚 continuing research.

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