How do you say "guitar player" in Spanish?

Ecologist (as a job, specialist in ecology, environmentalist) is...

Ecologist (as an ideology, a defender of the ecologism, protector of the environment) is...

Do the jobs ending in -ista change for the masculin or femenin (-isto, -ista), or do we only change the article?

Solo el artículo: el taxista, la taxista

Finally, remember that some jobs ending in -ist in English, also end in -ista in Spanish; but some others don't. 
Usually, the science-related jobs don't (biólogo, químico, científico...).