The biological world is divided into two foundational cell types: prokaryotes and eukaryotes. This division centers on the presence or absence of a nucleus. Prokaryotic cells are structurally simple, lacking a membrane-bound nucleus and other specialized compartments known as organelles. These organisms, which include all bacteria and archaea, store their genetic material in a region called the nucleoid. Eukaryotic cells, by contrast, are far more complex, housing their DNA within a membrane-enclosed nucleus. They also contain an array of organelles that compartmentalize and organize cellular functions.
Slime Molds are Eukaryotes
Slime molds are definitively eukaryotic organisms. They are not prokaryotes, and their cells possess the defined internal architecture that separates them from this simpler group. Slime molds are currently classified under the Kingdom Protista, a diverse collection of eukaryotes that are not animals, plants, or fungi. This classification reflects characteristics similar to both protozoa and fungi. Slime molds were historically grouped with fungi due to their shared ability to produce spores. However, they lack the chitin in their cell walls and the absorptive mode of nutrition characteristic of true fungi.
The Internal Cell Structure
The internal structure of a slime mold cell provides evidence of its eukaryotic nature. Like all eukaryotes, the genetic material of a slime mold is contained within a well-defined nucleus, surrounded by a nuclear envelope. This membrane-bound organization is the distinguishing feature separating them from prokaryotes. Beyond the nucleus, slime mold cells are rich in complex, membrane-bound organelles that perform specialized tasks. They possess mitochondria, which are the powerhouses of the cell, generating energy through aerobic respiration. The cells also contain an endoplasmic reticulum and a Golgi apparatus, networks of membranes involved in the synthesis, modification, and transport of proteins and lipids. These components are entirely absent in prokaryotic cells, confirming that slime molds belong in the Eukarya domain.
Unique Life Cycle and Behavior
Confusion regarding slime mold classification often stems from their unusual life cycle and complex behaviors. Slime molds are broadly separated into acellular (or plasmodial) types and cellular types.
Acellular slime molds, such as Physarum polycephalum, form a large, single-celled mass called a plasmodium. This plasmodium is essentially one gigantic cell that can grow to be several square meters in size, containing millions of nuclei without internal cell walls separating them. The complexity of this multinucleated mass, which streams across surfaces to find food, can mistakenly suggest a primitive structure.
Cellular slime molds, like Dictyostelium discoideum, spend most of their lives as individual amoebae. When food becomes scarce, thousands of these individual cells aggregate together in response to a chemical signal, often cyclic adenosine monophosphate (cAMP). This aggregation forms a mobile, multicellular structure known as a pseudoplasmodium or ‘slug’. This temporary, coordinated multicellularity moves as a unit before forming a spore-producing fruiting body. These complex behaviors are all carried out using the sophisticated internal cellular machinery of eukaryotic cells.