What is one element that is essential to life?

In oceanography the Redfield ratio is well known as a measure of which elements comprise phytoplankton (a large fraction of global biomass) and in which ratiOS they occur.

The generalized formula is C(106)H(263)O(110)N(16)P(1).

Sulphur is not mentioned but I guess it is of quite some importance as well. From the above Formula One can deduce, that hydrogen is clearly the most abundant element in living organisms (at least when measured as the number of atoms) and on the second and third rank are oxygen and carbon with very similar amounts.

The first three ranks are always taken by H, C and O but the following might change depending on the type of biomolecule one looks at.