Abstract

Thermophilic microbes are present everywhere around us and their only known natural biotope is far away and most usually associated with geothermal energy. To answer this paradox, we explore the hypothesis that the phyllosphere (surface of leaves), due to its exposition to the sun, could well be a thermophilic habitat for microbes and thus a source of thermophilic microbes growing around 50°C – 60°C. To support this hypothesis, we reviewed the heat sources on earth and associated microbial habitats, as well as the difficult identification of thermophilic microbes. We further present an experiment to show the presence and activity of thermophilic bacteria in the phyllosphere. Leaves were collected from eleven tree species from five locations on three continents belonging to three different biomes. On fresh leaves, 16S rDNA sequencing reveals the presence of 0.2 to 7% of clearly identified thermophilic bacteria. Moreover, after incubation at 55°C under aerobic and anaerobic conditions, 16S rDNA sequencing reveals the presence of 4 to 99% of clearly identified thermophilic bacteria. The accumulation of observations provides coherence to our hypothesis and allows the emergence of a new vision of leaves as a thermophilic biotope. We then propose a life cycle of microbes belonging to the thermophilic biotope associated with leaf surfaces.

Highlights

  • Thermophilic microorganisms are the Holy Grail in Biotechnology

  • The aim of this paper is to provide arguments in support of this hypothesis and to answer the following questions: (i) Can leaf surface provide the main biotope for thermophilic microbes? (ii) Are thermophilic microbes present on leaves? (iii) What are the ecology and the life cycle of microbes living on leaf surface hot biotope?

  • Our observations show the presence of live thermophilic bacteria on the surfaces of many different leaves

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Summary

Introduction

Thermophilic microorganisms are the Holy Grail in Biotechnology. Thermophilic microbes are extensively used in processes such as thermophilic anaerobic digestion, composting as well as other fermentation processes and the cellular components of such microorganisms (i.e. enzymes «thermozymes», proteins and nucleic acids) have considerable potential for many industrial applications (Bruins et al 2001, Haki 2003). The average temperature of the earth is 15°C. The temperature, varies from 7000°C (centre of earth) to −89°C (Vostok Station in Antarctica). Over this huge range of temperatures, microbes can grow between -18°C and 113°C and can encounter four sources of heat: geothermal energy, self-heating (the organisms’ metabolism), human/ animal activity and solar radiation

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