A scientific journalist needs to be proficient in both areas: as a journalist, writing good enough for magazines and newspapers. And as an amateur scientist, or at least as a dedicated learner capable of follow a potentially complex science and explain it in simple terms.
Many scientific journalists, after years of experience, are expert enough on a subject to publish entire books about a topic, e.g. astronomy, oceanography, geology, etc. These books are a major channel for public education - often a scientist's writing is too terse to be enjoyable, while a book written by a good scientific journalist can interest the generic reader and convey a great amount of information.
Notable scientific journalists: