Why is it difficult, using the traditional definition of species to classify some bacteria and archaea at the species level?

since the vast majority have never been studied in the laboratory and have only been detected by analysis of their nucleic acids in samples from the environment. Although archaea have, in the past, been classed with bacteria as prokaryotes, this classification has been described as outdated, since it fails to distinguish between the three very distinct domains of life.

The answer is ozone layer.