In the hot springs of Yellowstone Park, US; Iceland; New Zealand; and Russia, microbes thrive in temperatures up to 100°C. The volcanically heated waters can also be very acidic or alkaline, and yet despite these harsh conditions, there are single-celled bacteria, and the single-cell archaea, that thrive. Deep underwater at the bottom of the ocean, geothermal vents spew out gases and water up to 400°C, and similar microbes survive there too.

Normally, cells would be expected to die at these temperatures. Heat denatures the proteins that enzymes are made of, stopping them from working; it also breaks the chemical bonds that hold the lipids in cell membranes together, and destroys the DNA double helix.

Yet thermophilic enzymes remain intact and functional, offering several industries the potential to boost reaction rates and improve productivity. Their stability makes them ideal for use in the food, brewing, pulp and paper and feed processing industries. Many of these industries already use enzymes from mesophilic bacteria, which grow at moderate temperatures of between 20 and 45°C, but thermophiles have the added advantage of being resistant to harsh conditions such as chemical denaturing agents, wide pH ranges, and non-aqueous solvents....

...Hot fuels

Meanwhile, other researchers are investigating the use of thermophiles in the production of biofuels.

Liquid biofuels such as bioethanol, biodiesel, biobutanol and biokerosene are formed from the fermentation of starch and lignocellulose from plant dry matter. However, because cellulose is so resistant to being broken down by enzymes, industries must first pretreat plant materials using heat and strong acids and bases to break the chemical bonds, sometimes adding other enzymes, and then cooling the products – a costly process.

Yet many thermophiles produce enzymes that are able to break down carbohydrate polymers, such as starch, cellulose and hemicellulose, at high temperatures.

Professor Lee Lynd

Lee Rybeck Lynd, professor of engineering and of biological sciences at Dartmouth, US, studies the production of energy from plant biomass. He currently leads the biomass deconstruction and conversion research for the US department of energy-funded Bioenergy Science Center (BESC), for which thermophiles are a major focus. Lynd believes that thermophiles could solve this industrial problem. His research group is investigating the possibility of ‘consolidated bioprocessing’ (CBP), ie converting cellulose from plants into ethanol in a single step without the costly pretreatment steps. Although his group is focusing on the production of ethanol, the method could be used to make a broad range of fuels and chemicals.

Recently, Lynd’s team investigated the ability of six thermophilic microorganisms to breakdown switchgrass into soluble products, which could then be fermented and ultimately used as fuel. The researchers showed that Clostridium thermocellum was twice as effective at breaking down switchgrass as the fungal enzymes currently used by industry. They also showed that the thermophiles were able breakdown switchgrass that hadn’t been pretreated, representing a reduction in the costs and a potential boost to the efficiency of biofuel production. ‘In light of rapid recent advances, there are indications that thermophiles are knocking on the door with respect to industrial applications for the production of biofuels,’ says Lynd.

According to Lynd, ‘It is becoming clear that the thermophilic, cellulolytic bacterium Clostridium thermocellum and some other thermophiles are able to achieve several-fold higher solubilisation of grassy cellulosic feedstocks with no pretreatment other than autoclaving. The challenge is to combine this distinctive ability to access cellulosic biomass with production of fuel molecules at industrially-viable yield and titer. Great strides are being made using genetic techniques that have only recently become available.’