For someone who specializes in a certain field or who is an expert, we have a suffix : 家, an equivalent to
【-ist】in English， as in 【scientist】.
We have 科学家，企业家，思想家，政治家，文学家，书法家， and, curiously, 野心家 which is funny as it is derogative.
" I am a scientist who specializes in biology" would be 我是生物学家， as biology is 生物学。
Well, some people are so modest they don't feel like calling themselfves XXX家, they might say 我是搞/做生物的。（I do biology for a living.）