An upper temperature limit near 600 for eukaryotic organisms is documented by results of a systematic search for fungi able to grow at higher temperatures. Samples from hot springs, thermal soils, self-heating coal waste piles, and other natural and man-made heated habitats did not yield fungi when enrichments were done at 620, whereas fungi able to grow at 55-60°c an be readily isolated from such habitats. Earlier work had shown that eukaryotic algae are also absent from environments with temperatures above 56 60°. It is suggested that the failure of eukaryotes to evolve members able to grow at higher temperatures is due to their inability to form organellar membranes that are both thermostable and functional.Although it is now well established that some prokaryotic organisms are able to live at temperatures over 900, and even in boiling water (4-6, 9, 10), there is a definite upper temperature limit for eukaryotic organisms near 600. It is the purpose of this paper to provide documentation for this upper temperature limit, with special attention to the thermophilic fungi. Previous work has dealt with the eukaryotic algae, which have an upper-temperature limit of 56-60' (14,15 in pure culture (13,16,21,23,25). However, it has been customary to enrich for thermophilic fungi at temperatures near 500 (13), and it is conceivable that species able to grow at higher temperatures were missed. In attempting to define the upper temperature limit for eukaryotic organisms, we conducted a systematic search for fungi able to grow at temperatures above those already known.We collected soil, species of thermophilic fungi, the incubation temperature was 500, whereas for the search for species having unprecedentedly high growth temperatures, 62°was used. A temperature of 620 was selected because it is above the temperature range that will enrich for known species of thermophilic fungi. Despite occasional reports of growth of certain species in pure culture at 62 and 630, it is our experience that these same species grow extremely poorly above 61.50.

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SUMMARY Acid spring effluents are often covered with mats of the eucaryotic phycocyanin‐containing alga. Cyanidium caldarium. The primary bacterial component of such mats is an acidophilic strain of Bacillus coagulans, and the primary fungal component is Dactylaria gallopava. Because of the limited species diversity, C. caldarium mats appeared to be an excellent system for studying algal excretion and various microbial interactions in nature. From 2 to 6% of the NaH14CO3 taken up by natural or laboratory populations of the alga was excreted as 14C‐labeled materials. The maximum excretion occurred at temperature, light, and pH values optimum for NaH14CO3 uptake. However, when excretion was expressed as a percentage of NaH14CO3 uptake, a higher percentage of the radioactivity was excreted at nonoptimal conditions for NaH14CO3 uptake. Fungal biomass was directly proportional to algal density, but bacterial numbers varied widely and did not correlate with algal numbers. The bacterial and fungal components could be grown in mixed culture with either growing C. caldarium cultures or in an extract prepared, by healing algal cells.

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