A shipping company employee notices that the inside of ships' hulls where ballast water is stored are deteriorating. The hull paint contained cyanide to prevent microbial growth. Bacteria were growing on the hulls. You can therefore conclude that the:

The bacteria were growing through fermentation process.

Explanation:

Cyanide have the ability to prevent the growth of microbes, and that is why it usually added to paints that want to be applied to places where the growth of microbes will be harmful to people.

The ships' hull where ballast water is been stored needed to be painted with cyanide to prevent the growth of microbes.

It should be noted that,  anaerobic microbes have the ability to reduce cyanide compound through a process known as submerged fermentation.

In the case of ships' hull, the bacteria growing on the hull were anaerobic, which means, they carried out their activities in the absence of oxygen.