Within the field of microbial ecology, oligonucleotide probes are used in order to determine the presence of microbial species, genera, or microorganisms classified on a more broad level, such as bacteria, archaea, and eukaryotes via fluorescence in situ hybridization (FISH). rRNA probes have enabled scientists to visualize microorganisms, yet to be cultured in laboratory settings, by retrieval of rRNA sequences directly from the environment. Examples of these types of microorganisms include:
*Nevskia ramosa: N. ramosa is a neuston bacterium that forms typical, dichotomically-branching rosettes on the surface of shallow freshwater habitats.
*Achromatium oxaliferum: This huge bacterium (cell length up to >100 µm, diameter up to 50 µm) contains sulfur globules and massive calcite inclusions and inhabits the upper layers of freshwater sediments. It is visible to the naked eye and has, by its resistance to cultivation, puzzled generations of microbiologists.