Final answer:
An ecologist focuses on the interactions at the organism, population, community, ecosystem, biome, and biosphere levels. Molecular or cell biology levels, which deal with internal processes of organisms, do not fall under the ecologist's study.
Step-by-step explanation:
A biologist studying interactions among the bacteria in an ecosystem could not be working at the level of molecular biology or cell biology. In ecology, the focus is on the interactions between organisms and their environment at the level of organisms, populations, communities, ecosystems, biomes, and the biosphere. Molecular biology and cell biology are concerned with the biochemical and cellular processes within the organisms themselves, rather than their interactions with other organisms and the environment.
In the context of biology's hierarchy, the biologist in the question would be studying at the levels of population, community, or ecosystem ecology, given that bacteria interact with each other and other species within these levels. The least inclusive level mentioned that is not part of a biologist's ecological study is that of an individual organism, and the levels continue to broaden up to ecosystems and the biosphere.