Soaking Up the Heat

Humans are happiest in middling temperatures: When things get too cold, we turn to fires, radiators and overcoats; if too hot, to fans and air conditioners, to stay comfortable. Microorganisms do not have this luxury, yet without physical mechanisms to regulate internal body temperature or technology to alter their surroundings, they can be found happily thriving both in frozen Antarctica and in boiling hot springs.   

 

How do these bacteria and algae manage to get by in such extreme environments? If we could learn their secret, we might be able to use it to engineer crops that could grow in different climates or develop enzymes for industry that could work efficiently at different temperatures. Prof. Avigdor Scherz of the Plant Sciences Department and a team that included research students Oksana Shlyk-Kerner, Ilan Samish and Hadar Kless, and postdoctoral fellows Drs. David Kaftan, Neta Holland and P. S. Maruthi Sai, set out to find the mechanism used by organisms that flourish in extreme temperatures. 

 

In their experiment, the scientists worked with two different kinds of bacteria, both of them photosynthetic (that is, using the sun’s energy to create sugars for food). Employing a multidisciplinary approach, the research team focused on one of the key stages of photosynthesis, a reaction that takes place in enzymes in the “reaction center” of the bacterial cell. As they gradually raised the surrounding temperature, they timed the reaction to see how turning up the heat affected the reaction rate. 

 

They expected the change in rate to follow a general rule: As the temperature rises, the reaction should become faster and faster. To their surprise, neither bacterial enzyme obeyed the rule: Instead, they each peaked at a different temperature, after which the rate held steady, even as the temperature continued to rise. The peak for each microorganism occurred right in its optimal “comfort zone.” In other words, say the scientists, the bacterial enzymes aren’t affected by vagaries of the weather but are tuned to work most efficiently at the average temperature of their everyday habitat. This adaptation may protect them from potential ill effects stemming from swings in enzyme activity if things get too cool or too hot.

 

But what gives these enzymes the ability to function at their best in one temperature range or another? One of the bacteria the scientists tested was happiest when temperatures were in the moderate range; the other was a lover of intense heat. The puzzle was that the enzymes in the photosynthetic reaction centers of both are almost completely identical. Nonetheless, the scientists managed to find a tiny difference between them: Just two amino acids (the building blocks of proteins) were changed in the long protein sequence. As the French say: “Viva la difference!” When the scientists genetically engineered these proteins, replacing the two amino acids with their counterparts from the other bacteria, they saw a 10-degree change in the average temperature for peak enzyme activity, about the same as they found in the natural enzymes. This convinced them that the two tiny amino acids play a key role in setting the photosynthesis thermostat to either “lukewarm” or “tropical heat.” 

 

These findings, which appeared in Nature, may have future applications in a number of different fields. Crops, for instance, might be adapted to growing in extremely hot climates such as deserts. Enzymes used in industrial processes might be tweaked to work more efficiently. Scherz envisions another possible future use for the ability to control rates of photosynthesis: Plants grown for biofuel could be altered so as to produce greater biomass. These would absorb more than the usual amount of carbon dioxide from the atmosphere, thereby providing an environmentally friendly, renewable energy source and reducing a greenhouse gas, all at the same time.        
  
Prof. Avigdor Scherz's research is supported by the Charles W. and Tillie K. Lubin Center for Plant Biotechnology; the Avron-Wilstatter Minerva Center for Research in Photosynthesis; the Sylvia and Martin Snow Charitable Foundation; H. Thomas Beck, Toronto, Canada; Samuel T. Cramer, Beverly Hills, CA; Mr. and Mrs. Abraham Kahn, Mexico; and Mrs. Sharon Zuckerman, Toronto, Canada. Prof. Scherz is the incumbent of the Yadelle and Robert N. Sklare Professorial Chair in Biochemistry.  

 

  
Photosynthesis – the process of creating usable energy from sunlight and carbon dioxide – is the basis of all life on Earth. Photosynthetic organisms – plants, algae and some bacteria – feed all of the organisms higher up the food chain, supply the air with oxygen and help regulate the climate. But the real news is that many of these organisms, especially the higher plants, convert light to energy with an efficiency that human engineers can only dream of. 

 

Some of the  photosynthesis research at the Weizmann Institute explores such basic questions as how, even in dim light, the plant cell captures the solar energy it needs to function, while avoiding damage from overexposure in strong light. Meanwhile, the exploration of different aspects of photosynthesis has inspired a host of applications and new research avenues.

 

One of them is a method pioneered by Prof. Scherz and Prof. Yoram Salomon of the Biological Regulation Department that aims to destroy tumors by clotting their vascular supply, using light to activate a chlorophyll-based compound (chlorophyll being the green, light-absorbing substance in plants). This process, which takes advantage of the superb ability of chlorophyll to absorb light and generate radicals, is now in Phase II/III clinical trials for the treatment of localized prostate cancer.