Organic sediment on lake bottoms and the ocean floor is broken down by microbes into inorganic forms of phosphorous in a process known as

Mineralization

Explanation:

The phosphates play a very important role and keeps on transferring from the rocks to the soil, plants, water, and organism, maintaining a cycle known as the phosphorous cycle.

During rainfall, the rocks comprised of phosphorous are weathered, as a result of which these phosphate ions are carried away with the sediments and water. Some of them are stored in the soil, which is again absorbed by the plants. The remaining flow with the water and eventually ends up in the oceans and seas.  

In the oceans, the marine organisms then consume this phosphate, which leads to the formation of shells comprised of phosphate, and after their death, these shells are deposited on the bottom of the seafloor.

This process is known as mineralization. It is a process in which the inorganic materials are precipitated into a matrix that is organic in nature.