This is a potential pathway for the oxidation of toluene in Fe(III)-reducing microorganisms, which play important roles in sediments naturally composed of hydrocarbons. The oxidation of toluene, an aromatic hydrocarbon, in these microorganisms is coupled to Fe(III) reduction. GS-15 is the first microorganism discovered to link aromatic compound oxidation to the reduction of Fe(III). The oxidation of p-cresol and phenol in these organisms is also coupled to Fe(III) reduction. Under strict anaerobic conditions in these organisms, GS-15 can completely oxidize toluene to carbon dioxide by utilizing Fe(III) as the only electron acceptor in the reaction.
This mechanism can be used to clean up toxic oil spills or other toluene contaminations by introducing the microorganisms to the site.

First, a little about definitions.
Orbital states can be described with several terms:

• Filled – An orbital that contains the maximum number of electrons it can hold.
• Empty – An orbital that contains no electrons.
• Occupied – An orbital that contains at least one electron.
• Unoccupied – An orbital that contains at least one open space for an electron.

	Filled/Occupied
	Occupied/Unoccupied
	Unoccupied/Empty

With these definitions in mind, we can see that a filled orbital is also occupied, but an occupied orbital is not necessarily filled. Also, an orbital can be both occupied and unoccupied. A good way to remember this is that occupied means that one space is occupied by an electron, and unoccupied measn at least one space is free to accept an electron. Thus, it is better to think of filled and empty in terms of the entire orbital, but of occupied and unoccupied in terms of the spaces for electrons.

So, the best way to remember the terms would be as follows:

• Filled – An orbital that is filled with the maximum number of electrons it can hold.
• Empty – An orbital that is empty of electrons.
• Occupied – An orbital that has at least one space occupied by one electron.
• Unoccupied – An orbital that has at least one unoccupied space for an electron.

So, turning now to Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO), we can understand what is meant by these terms better with our definitions form above. These orbitals are called the frontier orbitals, and determine the way the molecule interacts with other species. The HOMO is the orbital that could act as an electron donor, since it is the outermost(highest energy) orbital containing electrons. The LUMO is the orbital that could act as the electron acceptor, since it is the innermost(lowest energy) orbital that has room to accept electrons. In accordance with the above definitions, a single orbital may be both the LUMO and the HOMO.