which kingdom forms a link between prokaryotic and complex eukaryotes?

Kingdom Protista forms a link between prokaryotic and complex eukaryotic organisms. This kingdom consists of unicellular and eukaryotic organisms.
The boundaries of this kingdom are a bit blurred. In this kingdom, what may be a plant to  a biologist, can be an animal to another. These can have mixotrophic nutrition (eg. Euglena).