Life at High Temperatures

by Thomas D. Brock

Life and Death at Low pH

© 1994 Yellowstone Association for Natural Science, History & Education, Inc. Yellowstone National Park, Wyoming 82190.

The numerous acid hot springs of Yellowstone eventually flow into the surrounding rivers and lakes, adding striking amounts of acid to these waters. Even when cool, these acid waters are not felicitous habitats for most plants and animals. Acid rain from burning fossil fuels has caused the acidification of many lakes in the eastern U.S. and acidic lakes and rivers of Yellowstone provide a natural laboratory to study adaptation to acid.

Lower pH Limits for Growth of Various Organisms

Group Lower pH limit Examples
Fish 4 Carp
Insects 2 Ephydrid flies
Mosses 3 Sphagnum
Vascular plants 2.5-3 Heather, Sedges
Eucaryotic microbes
Algae 1-2 Euglena mutabilis

0 Cyanidium caldarium
Fungi 1-2 Dactylaria gallopava
Procaryotes 0.8 Thiobacillus, Sulfolobus
Cyanobacteria 4 Synechococcus

We can see from the above table that fish do not live in very acid waters; sport fish, such as trout, are even more sensitive to low pH, disappearing when the pH drops below 6. Interestingly, when fish are wiped out by low pH, invertebrate animals often increase greatly in numbers, because they are less sensitive to acidification and no longer have any predators feeding on them.

An alga of acid waters called Zygogonium often forms large mats which spread out over the moist acidic basins. This alga provides the base of a food chain in which a few other algae and certain specialized animals thrive. One ephydrid fly, Ephydra thermophila, feeds on the acidophilic algae. Interestingly, this fly of very acid environments can also survive on algae from neutral/alkaline springs, but is not found there apparently because it cannot compete with the pink-egg laying Ephydra bruesi we discussed earlier. Another smaller ephydrid, Scatella paludum, also carries out its life cycle in acid springs. The ephydrid flies of the acid areas are preyed upon by other insects, such as wasps, mites, and tiger beetles. Birds such as killdeer are often seen on the acid alga mats, picking up a dinner of these acidophilic insects. Surprisingly, the birds do not seem to mind the acidic conditions, although they of course can move to less acid waters also. Certain acid lakes in Yellowstone, for instance Nymph Lake, Turbid Lake, and Clear Lake, have quite large populations of Canada geese.

An interesting pair of lakes is North and South Twin Lakes, along the Mammoth/Norris road. North Twin Lake receives most of its water from the very acid run-off from Roaring Mountain, and has a pH of 3.5, whereas South Twin Lake receives its water from nonacid creeks and has a pH of 6. Thus, even though these two lakes are almost touching, they differ strikingly in pH (and also in biological diversity). Obsidian Creek, near Roaring Mountain, also becomes quite acidic during the summer, and this change in pH is reflected in the algae that develop in its waters.

The table also shows that cyanobacteria are rather intolerant of low pH. Thus, although cyanobacteria form the major components of the orange mats we discussed earlier, they are replaced in acid waters by the eucaryotic thermal alga, Cyanidium caldarium, which is dark green.
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