In any experiment, there are several kinds of variables, explained more closely [here]
The independent variable is whatever you change to produce an effect. It's the variable that you change from trial to trial to produce a change in the dependent variable; by contrast, the dependent variable is what you measure.
In an experiment looking to determine how the amount of salt dissolved in water affects boiling point, for example, the independent variable would be the amount of salt dissolved.
So, in an experiment determining how the amount of nitrogen affects oil production by microalgae, the independent variable would be the amount of nitrogen added, because that is what is varied by the experimenter. The rate of oil production would be the dependent variable, because that is what is measured and analyzed, and all other choices would be controlled variables.
Hope this helped!