If we were to terraform a planet I think the use of bacteria is essential. I believe this mainly due to the history of our own planet. Many millions of years ago life of earth was very different and harsh, we certainly would not be able to survive due to the low levels of oxygen in the atmosphere. It was bacteria that started to transform the atmosphere which then allowed some plants like species to develop and then fast forward those millions of years and we get to the situation we are in today.
Today there are still many types of bacteria that can survive extreme environments, such as heat, UV radiation and lack of oxygen which might have a suitable role in aiding planet transformation. I know at the moment Mars is seen as a planet that could in theory undergo terraformation as it has a supply of water and carbon which would be essential to these bacteria. The problem with Mars is that it has very little in the way of an atmosphere. However if you were able to engineer bacteria to have all three traits that I mentioned before then it is possible that they could build this atmosphere back up. With the beginnings of an atmosphere the conditions on Mars would become less extreme and you could start to introduce simple plant like organisms until, in theory, you could replicate Earth on Mars and then allow humans to live there.
This would be a huge undertaking, with a lot of time and money required but maybe that will be something you could get involved in, if you decide to become a scientist.