There is an official Mars One FAQ page answering exactly this question:
Mars One will take specific steps to ensure that the Mars environment
(which we will study, and on which we will depend) will not be harmed.
The Mars base will be forced to recycle just about everything, and pay
close attention to its energy use and minimize the leakage of
materials and energy. Nutrients are scarce on Mars. They either need
to be imported from Earth, or extracted from the ground or atmosphere.
Solar panels, which will also be launched from Earth, will generate
the settlement's electricity. All of this means that a Mars resident
will have a much smaller ecological footprint than that of the average
person on Earth.
In addition, the development and operation of the settlement itself
can greatly improve our sustainability efforts on Earth. The necessity
to recycle everything on Mars will provide a high-profile boost to our
recycling industry, as will the demand for lightweight solar panel
technology. New methods of cultivating crops and growing plants on
Mars can also teach us on Earth a great deal about how to improve our
environment from experiences on another planet.
Prevention of environmental contamination
Mars One will take the required actions to prevent environmental
contamination caused by the importation of Earth life (humans and
their companion organisms). Mars One has begun discussions with the
ICSU Committee on Space Research’s (COSPAR’s) panel on planetary
protection and the COSPAR panel on exploration to identify the
measures that need to be taken with respect to prevent this
contamination. Dr. Chris McKay of the COSPAR panel on exploration and
Prof. Dr. John D. Rummel of the COSPAR panel of planetary protection
are two of our advisers. Based on discussions with these two panels,
Mars One will acquire the necessary systems and take the required and
necessary actions to protect Mars.
If I remember correctly, this FAQ question is a reply to one of the questions to Bas Lansdorp's AMA on Reddit. Or perhaps Mars One press conference held in Washington DC in December 10, 2013. I'll update, if I find some more info on it. ;)