The spores survived and germinated after being exposed to 115 degree C for two hours

Can the mesophilic fungal spores that normally grow at 20 degree C to 35 degree C survive higher temperatures and still be able to germinate when the conditions are right? Believe it or not, laboratory studies have shown that some species of mesophilic spores can indeed survive even when exposed to 115 degree C for two hours! The duration of survival was longer at lower temperatures.

“The fungi are among the most heat-resistant eukaryotes on record and are referred to as ‘Agni's Fungi,' after the Hindu God of Fire,” notes the paper published recently in the Fungal Biology journal.

Serendipitous

This has been a serendipitous finding of a Chennai based scientist T.S. Suryanarayanan and his team. Dr. Suryanarayanan is the Director of the Vivekananda Institute of Tropical Mycology.

“We were studying the fungi for enzymes of pharmaceutical interest and discovered the heat-resistant trait accidentally,” said Dr. Suryanarayanan. “We were very surprised by the find.”

Leaf litter became the natural choice to look for new species as bacteria and fungi facilitate the degradation of the leaves. And the search was further narrowed down to endophyte fungi that normally live inside living leaves and turn into leaf-litter degrading fungi once the leaf dies.

Tropical forests are one of the best habitats to search for new endophyte species. Hence the search for leaf litter naturally took him to a forest adjacent to the Mudumalai wildlife sanctuary.

As Dr. Suryanarayanan recalls, the fungi were isolated from the dead leaves and cultured for extracting the enzymes of interest. In the process, the fungi with the spores had to be heated to 115 degree C for two hours to completely remove the water content.

However, after weighing the fungi along with the spores, for some inexplicable reasons, the scientists did not discard the waste material. Instead they cultured the spores of Bartalinia sp.

What happened next was totally unexpected. “The spores germinated and produced fungus after a few days,” he said. The ability to germinate was sufficient proof that the spores had indeed survived the heat treatment.

The scientists repeated the experiment several times to be sure that the phenomenon was real and not due to any contamination. Having observed this strange phenomenon in one species, the researchers studied a few more species and saw them behaving the same way.

There is something more important than the spores' ability to withstand higher temperatures. “The change in temperature was not gradual. There was a sudden and steep increase in temperature from 20 degree C to 115 degree C,” said Dr. Suryanarayanan. “But the fungal spores still survived.”

Another aspect is the nature of the heat. “It was dry heat, and this type of heat has a very different effect on the fungal cells,” he underlined.

The mechanism of survival may be very different as the temperature shift was very sudden and steep, and the heat was dry in nature.

The temperature inside the oven and the kind of heat that they were subjected to were very different from what even the higher temperature resistant thermophilic fungi can withstand. The optimum temperature at which thermophilic fungi grows is around 50 degree C.

So how did the mesophilic fungi withstand such high temperatures? The answer lies in the original habitat from where the litter was collected.

“The location from where we collected the litter is often subjected to forest fires,” he said. “Our hypothesis is that periodical forest fires have made the spores adapt to and survive high temperatures.”

Dr. Suryanarayanan is leaving this month-end as a Fulbright-Nehru Senior Fellow to the Department of Biochemistry, Ohio State University to study the feasibility of using fungi to produce biofuel from plant waste.