Which organism is the one that decomposes sea organisms to get petroleum?
Petroleum is not formed by the action of one organism.
It is a fossil fuel derived from very old organic materials like small plants, algae, animals etc. which settle into the sea bottom and get buried in very low or no amount of oxygen. In the same manner, layers of such organisms is formed inthe sae bed. Further after thousand and millions of years by the action of heat and pressure, petroleum is formed by decomposition of this organic material under very high temperature and absence of oxygen.