Scientists are working towards enabling plants create their own nitrogen so that use of chemical fertilisers used to pack soil with nitrogen can be reduced, thus minimising soil contamination and pollution of waterways, writes S Ananthanarayanan.
Plant growth is the heart of food production and also the engine of elimination of atmospheric carbon dioxide. Plants do this by turning the carbon in carbon dioxide into hydrocarbons, which are sources of energy for animals. But the process itself uses energy and the laws of physics say the process is not fully efficient and the energy used is always more than the energy stored. But the energy used is sunlight, which, as long as there is the Sun, comes free.
But plants need more than just carbon, water and sunlight to get along; they need chlorophyll and a host of other agents, all of which need the element nitrogen to be formed. Now nitrogen is abundant; it forms the greatest part of the air in the atmosphere, but this is not nitrogen in the form that plants can use.
Atmospheric nitrogen is ‘inert’ and needs to be ‘fixed’ to become available — another process that takes a lot of energy. This happens regularly and to a great extent by micro-organisms like bacteria. It also happens during lightning flashes in storms. How fast it takes place may be a factor that limits how fast plants can grow.
With increasing population and demands on agriculture, the last century or so has seen great use of synthetic and chemical fertilisers to pack the soil with usable nitrogen, and also with small quantities of other soil nutrients like phosphorus, potassium, calcium, magnesium, some metals, etc. The trouble is that the delivery of synthetic fertiliser to the plant is wasteful and leads to contamination of the soil and pollution of waterways. And synthetic fertiliser is needed in great quantities.
The production, which is energy intensive, has to be through use of fossil fuels which adds to atmospheric pollution. It is in this context that the preliminary work of Prof Himadri Pakrasi and his team at Washington University, St Louis, towards enabling plants to create their own fertiliser, right where it is needed, has been received with great interest.
The reason why this is not obvious is that creating food and creating usable nitrogen are opposing processes. The first extracts carbon from carbon dioxide and releases oxygen. The second needs a strictly oxygen-free environment.
All bacteria of a class called blue-green bacteria or cyanobacteria have the capacity to trap the energy of sunlight and use it for synthesis, usually of hydrocarbons, using carbon dioxide. But only some bacteria use stored energy to fix nitrogen. It is thought that the evolutionary ancestor of all bacteria had this capacity, but it was lost, maybe with the rise in oxygen levels.
And then there are some bacteria that can do both. As opposing processes, they need to be separated, either by being done in different places or at different times. One cyanobacterium, Cyanothece 51142, which Pakrasi and his team have studied now for 10 years, does it by allotting time slots. During the day, it uses photosynthesis to create and store available carbon. And at night, it burns the carbon to use up the oxygen so that it can start creating usable nitrogen.
Both processes are really ways of pulling carbon or nitrogen atoms out of the stable niches of secure bonding – carbon with two oxygen atoms as CO2 and nitrogen as N2, two nitrogen atoms bound to each other. For the second process to occur, a chemical group needs to bond with the tightly held nitrogen atom, a job it cannot do if there are ready-to-mingle oxygen atoms coming in the way.
The St Louis team is now discovering ways to transfer the genetic coding in the bacterium Cyanothece 51142 which makes it able to fix nitrogen on to another suitable bacterium so that the new organism can do this too.
Developing the genetic engineering tools that can do this would be the ‘proof of principle’ that the ability to fix nitrogen can be built into plant cells, the traditional factories of photosynthesis. “That would really revolutionise agriculture,” says Pakrasi, PhD, the Myron and Sonya Glassberg/Albert and Blanche Greensfelder Distinguished University Professor, in Arts & Sciences, and director of the International Center for Advanced Renewable Energy and Sustainability (I-CARES) at Washington University in St Louis.
Cyanothece 51142 happens to be a suitable starting point, because the genetic coding for its nitrogen fixing ability consists of a panel of some 30 genes placed together and activated by common signals – a formation that could be more easily transferred to another genome. The target bacterium Synechocystis 6803 is the best-studied strain of cyanobacteria. “Not only has its genome been sequenced, it is naturally “transformable” and able to integrate foreign DNA into its genome by swapping it with similar native strands of DNA,” says the notice sent out by Washington University.
The (US) National Science Foundation just awarded Pakrasi and his team more than $3.87 million to explore this idea further. The grant will be administered out of I-CARES, a university-wide center that supports collaborative research in the areas of energy, environment and sustainability. This award is one of four grants funded by the foundation jointly with awards funded by the Biotechnology and Biological Sciences Research Council in the United Kingdom.