Industrial. The industrial applications of biotechnology are huge. It has found a home in all aspects of industry, from production of non-polluting fuel, new washing powders and biosensors, to the extraction of metal ions in the mining industry.
It is important though to consider what kinds of organisms are used in biotechnology. Obviously organisms that we consume are used, e.g. cattle, but biotechnology would be an extremely limited science if it were not for microbes. Microbes include organisms such as Fungi, Bacteria and Viruses. They not only provide the foundation for much of the basic research involved in biotechnology, they help to create the many of the processes which are integral to this science.
They are the ideal biotechnological tools as they display processes that are common to all living organisms. Microbes’ rapid growth and multiplication rate means they can be produced quickly and enlarge numbers in the laboratory. This gives scientists an opportunity to look at several generations in a short time. They can then study processes like genetics faster than in other organisms. I believe David Perlman’s Laws of Applied Microbiology summarises the advantages of microbes.
⎧always right
The microbe is ⎨your friend
⎩a sensitive partner
There are no stupid microbes
Microbes can and will do anything
⎧smarter ⎫than
Microbes are ⎨wiser ⎬chemists
⎩more energetic ⎭engineers, etc.
David Perlamn 1980
I think that this emphasizes the fact that nature has already discovered the solution to many of today’s problems and that the majority of these solutions are present in the microorganisms. We only know of a minute proportion of the diversity of microbes, and with every new discovery of species it seems another application is found. That is the reason why they are so important for biotechnological research.
An important application of microbes in biotechnology can be seen in the mining industry, Biomining.
Although mining is one of humankind’s oldest activities, the techniques used to extract minerals haven’t changed much for centuries. Ores are dug from the ground, crushed, and then extreme heat or toxic chemicals extract minerals such as copper or gold. The environmental and health effects of traditional mining technologies have been deleterious. As the high-grade reserves of ores become depleted it becomes uneconomical and non-environmentally friendly for companies to continue these processes and so new ones must be found for extracting minerals from low-grade deposits. For example, Chile is the world’s leading copper produce, with an annual production exceeding a million tons. Its copper reserves have been estimated at 150 million tons. However, some 47 million tons of this total are contained in low-grade ores. This has lead them to search for new methods of extraction.
Predictably nature already has the answer. Many minerals of commercial interest are contained within metal sulphides. It just so happens that certain strains of bacteria are able to leach these minerals out of the ore into a form that can easily be extracted. This “bacterial leaching” is becoming increasingly important in the copper industry as it provides a cost effective, pollution free solution to the problem. Currently 25% of all copper worldwide, worth more than $1 billion annually, is produced through bioprocessing. This ranks it as one of the most important applications of biotechnology today.
The process of bioleaching is, at first complex, but here is a summary of the method before we delve into the chemistry behind it. Thiobacillus ferooxidans, which is naturally present in certain sulphur-containing materials, gets energy by oxidizing inorganic materials, such as copper sulphide minerals. This process releases acid and an oxidizing solution of ferric ions, which can wash out metals from crude ore. Poor quality copper ore, which is bound up in a sulphide matrix, is dumped outside a mine and treated with sulphuric acid to encourage the growth of T. ferooxidans. As the bacteria chew up the ore, copper is released and collected in solution. The sulphuric acid is recycled.
The solubilisation by bacteria of metals from ores proceeds by either “direct leaching” or “indirect leaching.” Thiobacillus ferooxidans uses both methods. It is an acidophillic organism (acid loving) and this is due to the environment in which it lives.
Indirect Leaching
Thiobacillus ferooxidans obtains its energy by either oxidising ferrous iron or sulphide, leading to the production of ferric sulphate according to the following equations.
FeS2 + 3.5O2 + H2O → FeSO4 + H2SO4
Pyrite
2FeSO4 + 0.5O2 + H2SO4 → Fe2(SO4)3 + H2O
Ferrous Sulphate Ferric Sulphate
Now Ferric Sulphate is a strong oxidising agent that is able to dissolve several economically important copper sulphide minerals by the reactions.
CuFeS2 + 2Fe2(SO4)3 → CuSO4 + 5FeSO4 + 2S0
Chalcopyrite
Cu2S + 2Fe2(SO4)3 → 2CuSO4 + 4FeSO4 + S0
Chalcocite
Cu5FeS4 + 6Fe2(SO4)3 → 5CuSO4 + 13 FeSO4 + 4S0
Bornite
Leaching by Fe2(SO4)3 is called indirect because it is independent of the presence of oxygen or microbial action. Thiobacillus ferooxidans also derives its energy from the oxidation of elemental sulphur (S0), generated by the reactions above, to sulphuric acid.
2S0 + 3O2 + 2H2O → 2H2SO4
The sulphuric acid maintains the low pH that is optimal for the acidophillic bacteria and also stabilises the intermediate ferric sulphate created by oxidation of the sulphide.
Fe(SO4) + 2H2O → 2Fe(OH)SO4 + H2SO4
But there is an unexpected bonus where the acid also leaches various copper oxide minerals.
Cu3(OH)2(CO3)2 + 3H2SO4 → 3CuSO4 + 2CO2 + 4H2O
Azurite
CuSiO3.2H2O + H2SO4 → CuSO4 + SiO2 + 3H2O
Chrysocolla
Direct Leaching
In this case the bacteria becomes attached to the mineral particles and using oxidative enzymes present in the cell membrane, break down the crystal lattice of the metal sulphide.
The method behind using these processes of bacteria to extract copper in a usable form is called Dump Leaching. Copper is usually obtained from open-cut mining. Material containing an excess of 0.5% of copper is subjected to smelting, while, as stated before the copper in lower grade ores is recovered by heap or dump leaching. This process involves piles of crushed ore, up to 100 feet high, being placed on a relatively impermeable surface and watered. This water is continually flushed through the rock. Over time the pyrite (iron containing compound) oxidises and the solution becomes highly acidic and rich in ferric sulphate. These conditions convert the copper sulphide minerals (as stated above) into a soluble form of copper sulphate. This metal-rich effluent can then be pumped out and the copper extracted via the addition of iron scraps, which precipitate the copper due to its higher E-cell potential:
Cu2+(aq) + Fe0(s) → Cu0(s) + Fe2+
Once the copper has precipitated the Fe2+ rich solution is transferred to shallow oxidation ponds where Thiobacillus ferooxidans rapidly oxidises Fe2+ to Fe3+ and forms some additional sulphuric acid through the oxidation of sulphur compounds. Other Thiobacillus strains also contribute to the leaching process by oxidising sulphur compounds but they are usually found in the top layer of rocks. The newly formed ferric sulphate is then pumped back to the top of the pile and the process repeated. This is a continual process and so is very economical to run.
However there is a problem with the use of bacteria in biomining. The oxidative processes themselves are very exothermic in nature and produce an environment that is detrimental to the bacteria themselves. Expensive cooling towers have to be used which are subject to corrosion from the acid solution and scaling. Also the bioleaching process is affected by the toxicity of metals like Ag, Co, Cu, Hg, Ni and Zn. This obviously effects the efficiency of the process and so scientists are now looking for new types of bacteria to use in this process that can operate under these harsh conditions.
So the search for a rapidly growing multi-metal-tolerant thermoacidophilic bacteria was on. Scientists at the Saurashtra University in India took up this challenge and began screening. They took samples of soil from hot springs around Rajkot city, the probable location of thermophillic bacteria, and upon analysis discovered 72 strains that were able to oxidise FeS2. This was much greater than anticipated. The performance of each of these strains was measured against differing concentrations of heavy metals. From the above 16 strains were selected, their FeS2 solubilisation ranged from 64% to 78% at temperature and pH ranging from 58oC to 65oC and pH 2 to 3 respectively within 8 to 10 days. Then using a process of natural selection and adaptation, a process that is greatly enhanced by the fast multiplication rate of microbes, 3 strains were refined. The so-called Th-VI-2, Th-V-6 and Th-II-26 were capable to complete the oxidation of FeS within five days with 87.8%, 82.6% and 87% efficiency respectively. These values are much greater than the effectiveness of Thiobacillus ferooxidans, which cannot even operate at these temperatures, heralding an even more cost effective solution to the problems of obtaining copper from low-grade ores and reducing pollution. I think that this really emphasizes a point made earlier that “nature has already found the solution to all mans’ problems,” and microbes are the organisms that are giving us the most help.