It seems that bacteria do something called ‘quorum sensing‘: they emit chemicals which allow them to indirectly count how many of their little brethren are present.
Some of these bugs can then be made to do smart stuff in response -like glow. Today’s invention exploits this ability, to provide a simple concentration indicator.
Imagine a cereal plant needs to have the water concentration in its root system monitored in order not to dehydrate. Luminescent bacteria are contained in plastic grids set in the earth (each say 1m^2 in size). This keeps them inside, but allows water molecules to move through its membrane.
A transparent side branch from each grid protrudes vertically up through the soil which allows the onset of local dehydration in that part of the field to be detected as the bacteria inside light up.
In order to maintain the number of bacteria within each grid (since they reproduce exponentially) a robotic microwave emitter roves the field and blasts the side branches at the rate required to keep the number of inhabitants constant.