This is a harder question to answer than you may imagine! Marine biology is the field of knowledge relating to marine organisms. But what is a marine biologist? To many, it means being a dolphin trainer but to others it means managing a marine wildlife sanctuary. There are many answers to this question and I would say that a marine biologist is someone who works in some way in studying, observing, protecting, or managing marine organisms, be they microbe, plant or animal. If you study marine fish populations you are a marine biologist. If you manage a marine wildlife preserve and are concerned with protection of marine organisms there, then you too are a marine biologist. You know you're a marine biologist if you have a notebook or computer that you record information often about marine organisms. But you may also be a marine biologist if you are collecting sponges, looking for bioactive drugs. You may be counting them, doing DNA sequencing of them, observing them in the laboratory or making theoretical models predicting their abundance once fishing is decreased. So marine biologists do many things, but what they have in common is working with marine organisms.