Biological nitrogen fixation is the process by which certain bacteria and archaea convert atmospheric nitrogen gas (N2) into ammonia (NH3), which can be used by plants and other organisms. This process is vital for the nitrogen cycle and plays a crucial role in maintaining the availability of nitrogen for living organisms.

The conversion of one molecule of N2 to two molecules of NH4 requires a series of enzymatic reactions involving nitrogenase, an enzyme complex that catalyzes the conversion. This process requires a significant amount of energy in the form of ATP (adenosine triphosphate) to drive the reactions.

Here's a detailed explanation of why 16 ATP molecules are required for the conversion:

1. Nitrogenase Complex Activation:
- The nitrogenase complex consists of two main components: the MoFe protein and the Fe protein.
- In order to activate the nitrogenase complex, the Fe protein binds to ATP and transfers an electron to the MoFe protein.
- This ATP hydrolysis step requires one ATP molecule.

2. Reduction of Nitrogen:
- The MoFe protein uses the energy from the activated nitrogenase complex to reduce N2 to NH3.
- This process involves the transfer of electrons and protons to the nitrogen molecule.
- Each reduction reaction requires 8 electrons and 8 protons.
- The transfer of each electron requires one ATP molecule, resulting in a total of 8 ATP molecules for the reduction of one N2 molecule.

3. Hydrogenation of Nitrogen:
- After the reduction of N2 to NH3, the nitrogenase complex further catalyzes the hydrogenation of NH3 to NH4.
- This process also requires the transfer of electrons and protons.
- Each hydrogenation reaction requires 8 electrons and 8 protons.
- The transfer of each electron requires one ATP molecule, resulting in a total of 8 ATP molecules for the hydrogenation of one N2 molecule.

Therefore, the total number of ATP molecules required for the conversion of one N2 to 2 NH4 is 1 ATP (activation) + 8 ATP (reduction) + 8 ATP (hydrogenation) = 17 ATP molecules.

However, it is worth noting that during the process, one ATP molecule is generated through the electron transport chain, resulting in a net requirement of 16 ATP molecules.

In conclusion, the correct answer is '16' ATP molecules.